During the Eocene epoch (55 million to 38 million years ago), several ancestors direct evolutionary modern animals appeared. Among these animals-all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of several modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were fundamentally forward-moving in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, and now-extinct species such as the giant saber-toothed tigers. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
Earth is one of nine planets in the solar system, the only planet known to harbor life, and the ‘home’ of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly as continents that rise above the oceans.
Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.
Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets-Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right degree of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth, not either too far or nearer from the Sun. Nevertheless, because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot-462°C’s (864°F)-hot enough to melt lead and too hot for life to exist.
Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, suggesting that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.
For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas, for example, that the Earth was a sphere and that it travelled around the Sun were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. With the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.
In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings can study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.
From this recent space exploration, we now know that Earth is one of the most geological activities unbound of all the planets and moons in the solar system. Earth is constantly changing. Over long periods land is built up and worn away, oceans are formed and re-formed. Continents move around, break up, and merge.
Life itself contributes to changes on Earth, especially in, and the way living things can alter Earth’s atmosphere. For example, Earth at once had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.
A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.
Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km. (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km./h. (about 67,000 mph.) as it orbits the Sun. All but one planet orbit the Sun in the same plane-that is, if an imaginary line were extended from the centre of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.
Earth’s orbital path is not quite a perfect circle but instead is elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km. (about 95 million mi.) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi.) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.
The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has arm-like discs of stars that spiral out from its centre. The solar system is found in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the centre of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.
Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is in a measure less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets-Mercury, Venus, Earth, and Mars-that makes up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system-Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.
Earth has one natural satellite, the Moon. The Moon orbits the Earth, undivided and compelling of one revolution in an elliptical path in 27 days 7 hr 43 min 11.5-sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than four billion years ago when a large meteorite or small planet struck the Earth.
As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min’s 4.1-sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr. 48 min.’s 46-sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It indirectly takes by more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s centre.
Earth’s axis of rotation is inclined (tilted) 23.5° on its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur between these orientations.
The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km. (6,000 mi.) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consist of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapour; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.
The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi.) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi.) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C’s, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.
Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life. Most life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Water vapour, carbon dioxide, methane, and nitrous oxide in the atmosphere act as ‘greenhouse gases’. Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and by that help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.
Other gases in the atmosphere are also essential to life. The trace amount of ozone based in Earth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth’s atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle.
The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is always shifting on the Earth’s surface and changing back and forth between ice, liquid water, and water vapour.
The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapour. Some of this water vapour falls as precipitation directly back into the oceans, completing a short cycle. Some water vapour, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventual melts or is warmed by the Sun and turned into water vapour, entering the atmosphere and falling again as precipitation. All water that falls on land eventually return to the ocean, completing the water cycle.
The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which hold more than 97 percent of all water on Earth. Glaciers and the polar ice caps encircle just more than 2 percent of Earth’s water as solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapour. Only 0.017 percent of all the water on Earth is found in lakes and rivers. A mere 0.001 percent is found in the atmosphere as water vapour. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.
The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (nine to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is five to 10 km (three to 6 mi). The crust has a definite boundary called the Mohorovicic discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.
Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They have a tendency to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.
Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter coloured, less dense igneous rock, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.
The biosphere can support life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend deeply into the Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.
Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge super-continents, and the breakup of super-continents into smaller continents.
The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface. Another principal factor is the movement of Earth’s continents and sea-floors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all these changes. Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new sea-floor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.
Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes make up the process of weathering. There are two types of weathering: physical weathering and chemical weathering.
Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.
Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical feed weathering also causes the exit of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface and groundwater water. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.
Erosion is the process that removes lose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.
Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can lend support to lose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The forces of the running water removes lose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens ever more material so the process continues.
Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds of a functional dynamic spread of ice. For major glaciers to form, this lack of snowmelt has to occur for many years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), Arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the results of glacial erosion. Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock. Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetic water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.
A landscape’s geologic age-that is, how long current conditions of weathering and erosion have affected the area-determines its overall appearance. Younger landscapes tend to be more rugged and angular in appearance. Older landscapes have a tendency to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.
Opposing this tendency toward a levelling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km. (40 to 60 mi.) thick. Compared with the interior of the Earth, however, this region is moderately thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.
Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth’s surface over long periods. Tectonics comes from the Greek word, tektonikos, which means ‘builder’.
According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Coco’s plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.
These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions and mountain building and the formation of the oceans and continents.
When the plates pull apart, two types of phenomena come about, depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new sea-floor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.
Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new sea-floor. According to this explanation, molten rock known as magma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appear and forms the mid-ocean ridges. These ridges exist in all the worlds’ ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new sea-floor.
When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below is known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called Subduction.
If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be sub-ducted. If both are continental crust, Subduction can continue for a brief while but will eventually ends because continental crust is not dense enough to be forced very far into the upper mantle.
The results of this Subduction process are readily visible on a map showing that 80 percent of the world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. The Subduction zone created by the collision of two oceanic plates-the Pacific plate and the Philippine plate-can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m’s (36,198 ft) below sea level.
On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m’s (29,035 ft) above sea level.
Finally, some of Earth’s plates neither collide nor pull apart yet slips past each other. These regions are convened by the transforming margins. Few volcanoes occur in these areas because neither plate is forced down into Earth’s interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transformed margin.
The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (one in) per year. Still, over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.
The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth’s magnetic field. This field is vital to life because it shields the planet’s surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.
Earth’s interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the centre of the core is about 6,400 km (about 4,000 mi).
Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes send geophysicists information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth as waves. The way these waves proceed through the interior of Earth reveals the nature of materials inside the planet.
The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means ‘a weak layer’. The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.
The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a ‘sticky’ liquid when it is subjected to enough heat and pressure.
The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one source of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C’s (12,000°F).
Scientists believe that Earth’s liquid iron core aids to make over a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun’s solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass point north. According to Gilbert, Earth’s magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territories with Nunavut. The magnetic south poles lies just off the coast of Wilkes Land, Antarctica.
Not only do the magnetic poles wander, but they also reverse their polarity-that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth’s rotation and changes in the flow of liquid iron in the outer core.
Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory may be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped in some respects like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.
Inside the magnetosphere are the Van Allen’s radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths resembling Earth’s magnetic field. The radiation belts by that shield Earth’s surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth’s magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth’s atmosphere, creating the phenomenon known. Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system took form of about 4.6 billion years. Originating endurably in some lengthily endurance from dust and giant gaseous particles-wave substances known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.
Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense of departing-that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn-are by and large made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.
Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.
Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.
Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon came to an end 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.
Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Residing until the late 1800s scientists could only determine the relative ages of rock strata. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.
The study of fossils also enabled geologists to set the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could detect an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists can assemble Earth’s geologic time scale from the Archaean Eon to the present.
The Precambrian is a time span that includes the Archaean and Proterozoic eons began roughly four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.
The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.
By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.
Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, by 3.5 billion years ago, and where anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue
-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.
For billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The inordinate branch of Precambrian time occurred some 560 million to 545 million years ago and seeing an appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils may be the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.
At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Most of the major groups of animals we know today made their first appearance during the Cambrian explosion. Most of the different ‘body plans’ found in animals today-that is, the way of an animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.
Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species can flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.
Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built several mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth-like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.
The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.
The Mesozoic Era, beginning 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, as the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth’s history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first birds and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
The Mesozoic was an eventful era geologically with many changes to Earth’s surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subduction zone extended along North America’s western margin. This Subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this featured characteristic is called the American Cordillera, and exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today shows that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey through and beyond the solar system and begin to colonize other planets in our galaxy, and find yet of another place to call ‘home’.
The Cenozoic era (65 million years ago to the present time) is divided into the Tertiary period (65 million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they tend to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared
Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic era.
The Paleocene epoch (65 million to 55 million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents
During the Eocene epoch (55 million to 38 million years ago), most direct evolutionary ancestors of modern animals appeared. Among these animals-all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern birds as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (38 million to 24 million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of many of modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (24 million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were basically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, as well as now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the musk-ox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today suggests that sea-floor spreading be still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two or 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years.
Most biologists agree that animals evolved from simpler single-celled organisms. Exactly how this happened is unclear, because few fossils have been left to record the sequence of events. Faced with this lack of fossil evidence, researchers have attempted to piece together animal origins by examining the single-celled organisms alive today.
Modern single-celled organisms are classified into two kingdoms: the Prokaryotes and protists. Prokaryotes, which include bacteria, are very simple organisms, and lack many features seen in animal cells. Protists, on the other hand, are more complex, and their cells contain all the specialized structures, or organelles, found in the cells of animals. One protist group, the choanoflagellates or collar flagellates, contains organisms that bear a striking resemblance to cells that are found in sponges. Most choanoflagellates live on their own, but significantly, some form permanent groups or colonies.
This tendency to form colonies are widely believed to have been an important stepping stone on the path to animal life. The next step in evolution would have involved a transition from colonies of independent cells to colonies containing specialized cells that were dependent on each other for survival. Once this development had occurred, such colonies would have effectively become single organisms. Increasing specialization among groups of cells could then have created tissues, triggering the long and complex evolution of animal bodies.
This conjectural sequence of events probably occurred along several parallel paths. One path led to the sponges, which retain a collection of primitive features that set them apart from all animals. Another path led to two major subdivisions of the animal kingdom: the Protostomes, which include arthropods, annelid worms, mollusks, and cnidarians; and the deuterostomes, which include echinoderms and chordates. Protostomes and deuterostomes differ fundamentally in the way they develop as embryos, strongly suggesting that they split from each other a long time ago.
Animal life first appeared perhaps a billion years ago, but for a long time after this, the fossil record remains almost blank. Fossils exist that seem to show burrows and other indirect evidence for animal life, but the first direct evidence of animals themselves appears about 650 million years ago, toward the end of the Precambrian period. At this time, the animal kingdom stood on the threshold of a great explosion in diversity. By the end of the Cambrian Period, 150 million years later, all of the main types of animal life existing today had become established.
When the first animals evolved, dry land was probably without any kind of life, except possibly bacteria. Without terrestrial plants, land-based animals would have had nothing to eat. Nevertheless, when plants took up life on land more than 400 million years ago, that situation changed, and animals evolved that could use this new source of food. The first land animals included primitive wingless insects and probably a range of soft-bodied invertebrates that have not left fossil remains. The first vertebrates to move onto land were the amphibians, which appeared about 370 million years ago.
For all animals, life on land involved meeting some major challenges. Foremost among these was the need to conserve water and the need to extract oxygen from the air. Another problem concerned the effects of gravity. Water buoys of living things, but air, which is 750 times less dense than water, generates almost no buoyancy at all. To function effectively on land, animals needed support.
In soft-bodied land animals such as earthworms, this support is provided by a hydrostatic skeleton, which works by internal pressure. The animal's body fluids press out against its skin, giving the animal its shape. In insects and other arthropods, support is provided by the exoskeleton (external skeletons), while in vertebrates it is provided by bones. Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move. This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.
Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process could use resources in a different way. Some of these species are surviving today, but these are a minority; an even greater number are extinct, having lost the struggle for survival
Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind. Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period-typically thousand of the years-unique features were to mean that they can no longer breed within the former circle of relative relations. At this point, a new species comes into being.
In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become ever more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces several different species. In the Hawaiian Islands, for example, 22 species of honey-creepers have evolved from a single pioneering species of a finch-like bird.
Another type of isolation is thought to occur where there is no physical separation. Here, differences in behaviour, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a some duration, in that they live long enough new species are created.
The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. Nevertheless, if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species change, or it eventually becomes extinct.
During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.
Compared with plants, animals make up only a small part of the total mass of living matter on earth. Despite this, they play an important part in shaping and maintaining natural environments.
Many habitats are directly influenced by the way animals live. Grasslands, for example, exist partly because grasses and grazing animals have evolved a close partnership, which prevents other plants from taking hold. Tropical forests also owe their existence to animals, because most of their trees rely on animals to distribute their pollen and seeds. Soil is partly the result of animal activity, because earthworms and other invertebrates help to break down dead remains and recycle the nutrients that they contain. Without its animal life, the soil would soon become compacted and infertile.
By preying on each other, animals also help to keep their own numbers in check. This prevents abrupt population peaks and crashes and helps to give living systems a built-in stability. On a global scale, animals also influence some of the nutrient cycles on which almost all life depends. They distribute essential mineral elements in their waste, and they help to replenish the atmosphere's carbon dioxide when they breathe. This carbon dioxide is then used by plants as they grow.
Until relatively recently in human history, people existed as nomadic hunter-gatherers. They used animals primarily as a source of food and for raw materials that could be used for making tools and clothes. By today's standards, hunter-gatherers were equipped with rudimentary weapons, but they still had a major impact on the numbers of some species. Many scientists believe, for example, that humans were involved in a cluster of extinctions that occurred about 12,000 years ago in North America. In less than a millennium, two-thirds of the continent's large mammal species disappeared.
This simple relationship between people and animals changed with domestication, which also began about 12,000 years ago. Instead of being actively hunted, domesticated animals were slowly brought under human control. Some were kept for food or for clothing, others for muscle power, and some simply for companionship.
The first animal to be domesticated was almost certainly the dog, which was bred from wolves. It was followed by species such as the cat, horse, camel, llama, and aurochs (a species of wild cattle), and by the Asian jungle fowl, which is the ancestor of today's chickens. Through selective breeding, each of these animals has been turned into forms that are particularly suitable for human use. Today, many domesticated animals, including chickens, vastly outnumber their wild counterparts. Sometimes, such as the horse, the original wild species has died out together.
Over the centuries, many domesticated animals have been introduced into different parts of the world only to escape and establish themselves in the wild. With stowaway pests such as rats, these ‘feral’ animals have often affected native wildlife. Cats, for example, have inflicted great damage on Australia's smaller marsupials, and feral pigs and goats continue to be serious problems for the native wildlife of the Galápagos Islands.
Despite the growth of domestication, humans continue to hunt some wild animals. Some forms of hunting are carried out mainly for sport, but others provide food or animal products. Until recently, one of the most significant of these forms of hunting was whaling, which reduced many whale stocks to the brink of extinction. Today, highly efficient sea fishing threatens some species of fish with the same fate since the beginning of agriculture. The human population has increased by more than two thousand times. To provide the land needed for growing food and housing people, large areas of the earth's landscapes have been completely transformed. Forests have been cut down, wetlands drained, and deserts irrigated, reducing these natural habitats to a fraction of their former extent.
Some species of animals have managed to adapt to these changes. A few, such as the brown rat, raccoon, and house sparrow, have benefited by exploiting the new opportunities that have opened and have successfully taken up life on farms, or in towns and cities. Nonetheless, most animals have specialized ways of life that make them dependent on a particular kind of habitat. With the destruction of their habitats, their number inevitably declines.
In the 20th century, animals have also had to face additional threats from human activities. Foremost among these are environmental pollution and the increasing demand for resources such as timber and fresh water. For some animals, the combination of these changes has proved so damaging that their numbers are now below the level needed to guarantee survival.
Across the world, efforts are currently underway to address this urgent problem. In the most extreme cases, gravely threatened animals can be helped by taking them into captivity and then releasing them once breeding programs have increased their number. One species saved in this way is the Hawaiian mountain goose or nē? nē? . In 1951, its population had been reduced to just 33. Captive breeding has since increased the population to more than 2500, removing the immediate threat of extinction.
While captive breeding is a useful emergency measure, it cannot assure the long-term survival of a species. Today animal protection focuses primarily on the preservation of entire habitats, an approach that maintains the necessary links between the different species the habitats support. With the continued growth in the world's human population, habitat preservation will require a sustained reduction in our use of the world's resources to minimize our impact on the natural world.
Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.
Fossils provide for most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period of time. The best index fossils tend to be marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.
The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.
At first glance, the sudden jump from 8000 Bc to 10,000 years ago looks peculiar. On reflection, however, the time-line has clearly not lost 2,000 years. Rather, the time-line has merely shifted from one convention of measuring time to another. To understand the reasons for this shift, it will help to understand some of the different conventions used to measure time.
All human societies have faced the need to measure time. Today, for most practical purposes, we keep track of time with the aid of calendars, which are widely and readily available in printed and computerized forms throughout the world. However, long before humans developed any formal calendar, they measured time based on natural cycles: the seasons of the year, the waxing and waning of the moon, the rising and setting of the sun. Understanding these rhythms of nature was necessary for humans so they could be successful in hunting animals, catching fish, and collecting edible nuts, berries, roots, and vegetable matter. The availability of these animals and plants varied with the seasons, so early humans needed at least a practical working knowledge of the seasons to eat. When humans eventually developed agricultural societies, it became crucial for farmers to know when to plant their seeds and harvest their crops. To ensure that farmers had access to reliable knowledge of the seasons, early agricultural societies in Mesopotamia, Egypt, China, and other lands supported specialists who kept track of the seasons and created the world’s first calendars. The earliest surviving calendars date from around 2400 Bc.
As societies became more complex, they required increasingly precise ways to measure and record increments of time. For example, some of the earliest written documents recorded tax payments and sales transactions, and indicating when they took place was important. Otherwise, anyone reviewing the documents later would find it impossible to determine the status of an individual account. Without any general convention for measuring time, scribes (persons who wrote documents) often dated events by the reigns of local rulers. In other words, a scribe might indicate that an individual’s tax payment arrived in the third year of the reign (or third regnal years) of the Assyrian ruler Tiglath-Pileser. By consulting and comparing such records, authorities could determine if the individual were up to date in tax payments.
These days, scholars and the public alike refer to time on many different levels, and they consider events and processes that took place at any times, from the big bang to the present. Meaningful discussion of the past depends on some generally observed frames of reference that organize time coherently and allow us to understand the chronological relationships between historical events and processes.
For contemporary events, the most common frame of reference is the Gregorian calendar, which organizes time around the supposed birth date of Jesus of Nazareth. This calendar refers to dates before Jesus’ birth as Bc (‘before Christ’) and those afterwards as ad (anno Domini, Latin for ‘in the year of the Lord’). Scholars now believe that Jesus was born four to six years before the year recognized as ad one in the Gregorian calendar, so this division of time is probably off its intended mark by a few years. Nonetheless, even overlooking this point, the Gregorian calendar is not meaningful or useful for references to events in the so-called deep past, a period so long ago that to be very precise about dates is impossible. Saying that the big bang took place in the year 15,000,000,000 Bc would be misleading, for example. No one knows exactly when the big bang took place, and even if someone did, there would be little point in dating that moment and everything that followed from it according to an event that took place some 14,999,998,000 years later. For purposes of dating events and processes in the deep past and remote prehistory, then, scientists and historians have adopted different principles of measuring time.
In conventional usage, prehistory refers to the period before humans developed systems for writing, while the historical era refers to the period after written documents became available. This usage became common in the 19th century, when professional historians began to base their studies of the past largely on written documentation. Historians regarded written source materials as more reliable than the artistic and artifactual evidence studied by archaeologists working on prehistoric times. Recently, however, the distinction between prehistory and the historical era has become much more blurred than it was in the 19th century. Archaeologists have unearthed rich collections of artifacts that throw considerable light on so-called prehistoric societies. When, contemporary historians realize much better than did their predecessors that written documentary evidence raises as many questions as it does answers. In any case, written documents illuminate only selected dimensions of experience. Despite these nuances of historical scholarship, for purposes of dating events and processes in times past, the distinction between the term’s prehistory and the historical era remains useful. For the deep past and prehistory, establishing precise dates is rarely possible: Only in the cases of a few natural and celestial phenomena, such as eclipses and appearances of comets, are scientists able to infer relatively precise dates. For the historical era, on the other hand, precise dates can be established for many events and processes, although certainly not for all.
Since the Gregorian calendar is not especially useful for dating events in the distant period long before the historical era, many scientists who study the deep past refer not to years ‘Bc’ or AD’ but to years ‘before the present’. Astronomers and physicists, for example, believe the big bang took place between 10 billion and 20 billion years ago, and that planet Earth came into being about 4.65 billion years ago. When dealing with Earth’s physical history and life forms, geologists often dispense with year references together and divide time into alternate spans of time. These time spans are conventionally called eons (the longest span), eras, periods, and epochs (the shortest span). Since obtaining precise dates for distant times is impossible, they simply refer to the Proterozoic Eon (2.5 billion to 570 million years ago), the Mesozoic Era (240 million to 65 million years ago), the Jurassic Period (205 million to 138 million years ago), or the Pleistocene Epoch
(1.6 million to 10,000 years ago).
Because the Pleistocene Epoch is a comparatively recent time span, archaeologists and pre historians are frequently able to assign at least approximate year dates to artifacts from that period. As with all dates in the distant past, however, it would be misleading to follow the principles of the Gregorian calendar and refer to dates’ Bc. As a result, archaeologists and pre-historians often call these dates’ bp (‘before the present’), with the understanding that all dates bp are approximate. Thus, scholars date the evolution of The Homo sapiens to about 130,000 bp and the famous cave paintings at Lascaux in southern France to about 15,000 Bc.
The Dynamic Timeline, of which all date before 8000 Bc refers to dates before the present, and all dates since 8000 Bc categorizes time according to the Gregorian calendar. Thus, a backward scroll in the time-line will take users from 7700 Bc to 7800 Bc, 7900 Bc, and 8000 Bc to 10,000 years ago. Note that the time-line has not lost 2,000 years! To date events this far back in time, the Dynamic Timeline has simply switched to a different convention of designating the dates of historical events.
Written documentation enables historians to establish relatively precise dates of events in the historical era. However, placing these events in chronological order requires some agreed upon starting points for a frame of reference. For purposes of maintaining proper tax accounts in a Mesopotamian city-state, dating an event in relation to the first year of a king’s reign might be sufficient. For purposes of understanding the development of entire peoples or societies or regions, however, a collection of dates according to the regnal years of many different local rulers would quickly become confusing. Within a given region there might be many different local rulers, so efforts to establish the chronological relationship between events may entail an extremely tedious collation of all the rulers’ regnal years. Thus, to facilitate the understanding of chronological relationships between events in different jurisdictions, some larger frame of reference is necessary. Most commonly these larger frames of reference take the form of calendars, which not only make it possible to predict changes in the seasons but also enable users to organize their understanding of time and appreciate the relationships between datable events.
Different civilizations have devised thousands of different calendars. Of the 40 or so calendars employed in the world today, the most widely used is the Gregorian calendar, introduced in 1582 by Pope Gregory XIII. The Gregorian calendar revised the Julian calendar, instituted by Julius Caesar in 45 Bc, to bring it closer in line with the seasons. Most Roman Catholic lands accepted the Gregorian calendar upon its promulgation by Gregory in 1582, but other lands adopted it much later: Britain in 1752, Russia in 1918, and Greece in 1923. During the 20th century it became the dominant calendar throughout the world, especially for purposes of international business and diplomacy.
Despite the prominence of the Gregorian calendar in the modern world, millions of people use other calendars as well. The oldest calendar still in use is the Jewish calendar, which dates’ time from the creation of the world in the (Gregorian) year 3761 Bc, according to the Hebrew scriptures. The year 2000Bc. in the Gregorian calendar thus corresponding to the year am 5761 in the Jewish calendar (am stands for anno Mundi, Latin for ‘the year of the world’). The Jewish calendar is the official calendar of Israel, and it also serves as a religious calendar for Jews worldwide.
The Chinese use another calendar, which, as tradition holds, takes its point of departure in the year 2697 Bc in honour of a beneficent ruler’s work. The year AD 2000 of the Gregorian calendar, and with that it corresponds to the year 4697 in the Chinese calendar. The Maya calendar began even earlier than the Chinese-August 11, 3114 Bc. Maya scribes calculated that this is when the cycle of time began. The Maya actually used two interlocking calendars-one a 365-day calendar based on the cycles of the sun, the other a sacred almanac used to calculate auspicious or unlucky days. Despite the importance of these calendars to the Maya civilization, the calendars passed out of general use after the Spanish conquest of Mexico in the 16th century AD.
The youngest calendar in widespread use today is the Islamic lunar calendar, which begins the day after the Hegira, Muhammad’s migration from Mecca to Medina in ad 622. The Islamic calendar is the official calendar in many Muslim lands, and it governs religious observances for Muslims worldwide. Since it reckons time according too lunar rather than solar cycles, the Islamic calendar does not neatly correspond to the Gregorian and other solar calendars. For example, although there were 1,378 solar years between Muhammad’s Hegira and AD 2000, that year corresponds to the year 1420 in the Islamic calendar. Like the Gregorian calendar and despite their many differences, the Jewish, Chinese, and Islamic calendars all make it possible to place individual datable events in proper chronological order.
Recently, controversies have arisen concerning the Gregorian calendar’s designation of Bc and ad to indicate years before and after the birth of Jesus Christ. This practice originated in the 6th century ad with a Christian monk named Dionysius Exiguus. Like other devout Christians, Dionysius regarded the birth of Jesus as the singular turning point of history. Accordingly, he introduced a system that referred to events in time based on the number of years they occurred before or after Jesus’ birth. The system caught on very slowly. Saint Bede the Venerable, a prominent English monk and historian, employed the system in his own works in the 8th century ad, but the system came into general use only about AD 1400. (Until then, Christians generally calculated time according to regnal years of prominent rulers.) When Pope Gregory XIII ordered the preparation of a new calendar in the 16th century, he intended it to serve as a religious calendar as well as a tool for predicting seasonal changes. As leader of the Roman Catholic Church, Pope Gregory considered it proper to continue recognizing Jesus’ birth as the turning point of history.
As lands throughout the world adopted the Gregorian calendar, however, the specifically Christian implications of the term’s Bc and ad did not seem appropriate for use by non-Christians. Really, they did not even seem appropriate to many Christians when dates referred to events in non-Christian societies. Why should Buddhists, Hindus, Muslims, or others date time according to the birth of Jesus? In saving the Gregorian calendar as a widely observed international standard for reckoning time, while also avoiding the specifically Christian implications of the qualification’s Bc and ad, scholars replaced the birth of Jesus with the notion of ‘the common era’ and began to qualify dates as BCE (‘before the common era’) or Ce (“in the common era”). For the practical purpose of organizing time, BCE is the exact equivalent of Bc, and Ce is the exact equivalent of AD, but the term’s BCE and Ce have very different connotations than do Bc and AD.
The qualification’s BCE and Ce first came into general use after World War II (1939-1945) among biblical scholars, particularly those who studied Judaism and early Christianity in the period from the 1st century Bc (or BCE) and the 1st century ad (or Ce). From their viewpoint, this “common era” was an age when proponents of Jewish, Christian, and other religious faiths intensively interacted and debated with one another. Using the designations, BCE and Ce enabled them to continue employing a calendar familiar to them all while avoiding the suggestion that all historical time revolved around the birth of Jesus Christ. As the Gregorian calendar became prominent throughout the world in the 20th century, many peoples were eager to find terms more appealing to them than Bc and ad, and accordingly, the BCE and Ce usage became increasingly popular. This usage represents only the most recent of many efforts by the world’s peoples to devise meaningful frameworks of time.
Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in an explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the s of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen s in the Sun is the source of its energy.
Many scientists favour the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
According to this theory, temperatures decreased with increasing distance from the centre of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out-that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets-Jupiter and Saturn-are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.
Within the planetesimal Earth, heavier matter sank to the centre and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the centre contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.
Not long after they formed, more than four billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By four billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so existing was impossible for life.
Geologists divide the history of the Earth into three eons: the Archaean Eon, which lasted from around four billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
The Archaean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archaean is generally dated as the age of the oldest terrestrial rocks, which are about four billion years old. The Archaean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.
Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative age of rock strata, or layering. They knew that overall the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.
The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth’s geologic time scale from the Archaean Eon to the present.
The Precambrian is a time span that includes the Archaean and Proterozoic eons and began about four billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about four billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to
4.2 billion years.
The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this time frame, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet because of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapour. As Earth continued to cool, the water vapour condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapour originally came from comets containing frozen water that struck Earth during meteorite bombardment.
By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were entered around the South Pole and formed a super-continent called Rodinia. Slowly, pieces of this super-continent broke away from the central continent and travelled north, forming smaller continents.
Life originated during the Precambrian. The earliest fossil evidence of life consists of Prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first Prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by 3.5 billion years ago, and where anaerobic-that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as Cyanobacteria or blue-green algae. These Cyanobacteria gradually introduced oxygen in the atmosphere because of photosynthesis. In shallow tropical waters, Cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old
The colonization of Australia/New Guinea was not achieved until the time to which took off around 50,000 years ago. Another extension of human range that soon followed as the one into th coldest parts of Eurasia. While Neanderthals lived in glacial times and were adapted to the cold, they penetrated no farther north than Germany and Kiev. That’s not surprising, since Neanderthals apparently lacked needles, sewn clothing, warm houses, and other technological essentials of survival in the coldest climates. Anatomically modern peoples who possess such technology had expanded into Siberia by around 20,000 years ago (there are the usual much olde disputed claims). That expansion may have been responsible for the extinctions of Eurasia’s wooly mammoth and wooly rhinoceroses.
With the settlement of Austral/New Guinea, humans now occupied three of the five habitable continents. However, Antartica because it was not reached by humans until the 19th century and has two continents, North America and South America. That left only two continents, North America and South America. They were surely the last ones settled, for the obvious reason tat reaching the Americas fro the Old World required either boats (for which there is no evidence even in Indonesia until 40,000 years ago and nine in Europe until much later) in order to cross by sea, or else it required the occupation of Siberia (unoccupied until about 20,000 years ago) ib order to cross the Bering land bridge.
However, it is uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized. The oldest unquestionable human remains in the Americas are at sites in Alaska dated around 12,000 Bc., followed by a profusion of sites in the United States south of the Canadian border and in Mexico in the centuries just before 11,000 Bc. The latter sites are called Clovis sites, named just after the type site near the town of Clovis, New Mexico, where there characteristic large stone spearpoints were first recognized. Hundreds of Clovis sites are now known, blanketing all 48 of the lower U.S. states south into Mexico. Unquestioned and in Patagonia. These facts suggest the interpretation that Clovis sites document the America’s first colonized by people, who quickly multiplied, expanded, and filled the two continents.
Nevertheless, it may be all the same, that differences between the long-term histories of peoples of the different continents have been due not to innate differences in the people themselves but to differences in their environments. That is to say, that if the populations of Aboriginal Australia and Eurasia could have been interchanged during the Late Pleistocene, the original Aboriginal Australia would no be the ones occupying most of the Americas and Australia, we well as Eurasia, while the original Aboriginal and Australia, as well as Eurasia, while the original Aboriginal Eurasians would be the ones now reduced to a downtrodden population fragment in Australia. One might at first be inclined to dismiss this assertion as meaningless, because the excrement is imaginary and claims itself its outcome that cannot be verified, but historians are nonetheless able to evaluate related hypotheses by retrospective tests. For instance, one can examine what did happen when European farmers were transplanted to Greenland or the U.S. Great Plains, and when farmers stemming ultimately from China emigrated to the Chatha Islands, the rain forests of Borneo, or the volcanic soil o Java or Hawaii. These tests confirm that the same ancestral peoples either ended up extinct, or returned to living as hunter-gatherers, or went on to build complex states, depending on their environments., similarly, Aboriginal Australian hunter-gatherers, variously transplanted to Finders Island, Tasmania, or southeastern Australia, ended up extinct, or as canal builders intensively managing a productive fishery, depending on their continents.
Of course, the continents differ in innumerable environmental features affecting trajectories of human societies. But merely a laundry list of ever possible difference does not constitute any one answer. Just four sets of differences appear as considered being the most important ones.
The fist set consists of continental difference in the wild plant and anal species available as starting materials for domestication. That’s because food production was critical for the accumulation of food surpluses that could feed non-food producing specialists, and for the buildup of large populations enjoying a military advantage though mere numbers even before they had developed any technological or political advantage.
On each continent, animal and plant domestication was concentrated in a few especially favourable homelands’ accounting for only a small fraction of the continent’s total area. In the case of technical innovations and political institutions as well, most societies acquire much more from other societies than they invent themselves. Thus diffusion and migration within a continent contribute importantly the development of its societies, which tend in the log run to share each other’s development (insofar as environments permit) because of the processes illustrated in much more form by Maori New Zealand’s Musket Wars. That is, societies initially lacking an advantage ether acquire it from societies possessing it or (if they fail to do so) are replaced by those other society.
Even so, for billions of years, life existed only in the simple form of Prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduces by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils are the precursors of many fossil groups that were to explode in Earth's oceans in the Paleozoic Era.
At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different ‘body plans’ found in animals today-that is, the way and animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae-also originated during this period.
Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.
Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the super-continent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built many mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. They’re slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the super-continent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented more than 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouth like opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.
The Paleozoic ended with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.
The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m’s (15 ft) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.
The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small in size and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and was one of the most successful animals in Earth’s history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
The Mesozoic was an eventful era geologically with many changes to Earth’s surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America begun splitting from what is now Africa, and in the process the South Atlantic Ocean formed? As the landmass that became North America drifted away from Pangaea and moved westward, a long Subductions zone extended along North America’s western margin. This Subductions zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. A great deal of this feature, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends largely on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder.
Or, perhaps, that a single catastrophic event had been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continued to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller micro-continents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that sea-floor spreading is still causing the country to grow.
Late in the Tertiary Period, about six million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between two million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that Anically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years.
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid 1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends mainly on the behaviour of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the sea-floor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about three billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about seven billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find among other different places to call ‘home’.
The dinosaurs were one of a group of extinct reptiles that lived from about 230 million to about sixty-five million years ago. British anist Sir Richard Owen coined the word dinosaur in 1842, derived from the Greek words’ deinos, meaning ‘marvellous’ or ‘terrible’, and sauros, meaning ‘lizard’. For more than 140 million years, dinosaurs reigned as the dominant on land.
Owen distinguished dinosaurs from other prehistoric reptiles by their upright rather than sprawling legs and by the presence of three or more vertebrae supporting the pelvis, or hipbone. They classify dinosaurs into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or bird-hipped dinosaurs. Dinosaur bones occur in sediments deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (240 million to 205 million years ago), the Jurassic (205 million to 138 million years ago), and the Cretaceous (138 million to sixty-five million years ago).
Historical references to dinosaur bones may extend as far back as the 5th century Bc. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins—legendary beasts that were part eagle and part lions-guarding nests in central Asia. ‘Dragon bones’ mentioned in a 3rd century ad text from China are thought to refer to bones of dinosaurs.
The first dinosaurs studied by paleontologists (scientists who study prehistoric life) were Megalosaurus and Iguanodon, whose partial bones were discovered early in the 19th century in England. The shape of their bones shows that these animals resembled large, land-dwelling reptiles. The teeth of Megalosaurus, which are pointed and have serrated edges, suggest that this animal was a flesh eater, while the flattened, grinding surfaces of Iguanodon teeth suggest that it was a plant eater. Megalosaurus lived during the Jurassic Period, and Iguanodon lived during the early part of the Cretaceous Period. Later in the 19th century, paleontologists collected and studied more comprehensive skeletons of related dinosaurs found in New Jersey. From these finds they learned that Megalosaurus and Iguanodon walked on two legs, not four, as had been thought.
Some ornithischians quickly became quadrupedal (four-legged) and relied on body armour and other physical defences rather than fleetness for protection. Plated dinosaurs, such as the massive Stegosaurus of the late Jurassic Period, bore a double row of triangular bony plates along their backs. These narrow plates contained tunnels through which blood vessels passed, allowing the animals to radiate excess body heat or to warm themselves in the sun. Many also bore a large spined plate over each shoulder. Stegosaurs resembled gigantic porcupines, and they probably defended themselves by turning their spined tails toward aggressors.
During the Cretaceous Period, stegosaurs were supplanted by armoured dinosaurs such as Ankylosaurus. These animals were similar in size to stegosaurs but otherwise resembled giant horned toads. Some even possessed a bony plate in each eyelid and large tail clubs. Their necks were protected by heavy, bony rings and spines, showing that these areas needed protection from the attacks of carnivorous dinosaurs.
The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to 65 million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about 65 million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m’s (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous have been discovered, including Hesperornis, a diving bird about 1.8 m’s (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved. The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation. Becoming the most abundant plant-eating dinosaurs. They ranged in size from small runners that were two m’s (6 ft) long and weighed 15 kg (33 lb), such as Hypsilophodon, to elephantine cows that were (32 ft) long and weighed 4 metric tons, such as Edmontosaurus. These animals had flexible jaws and grinding teeth, which eventually surpassed those of the modern cows in their suitability for chewing fibrous plants. The beaks of ornithopods became broader, earning them the name duck-billed dinosaur. Their tooth batteries became larger, their backs became stronger, and their forelimbs lengthened until their arms became elongated walking sticks, although ornithopods remained bipedal. The nose supported cartilaginous sacks or bony tubes, suggesting that these dinosaurs may have communicated by trumpeting. Fossil evidence from the late Cretaceous Period includes extensive accumulations of bones from ornithopods drowned in floods, indicating that duck-billed dinosaurs often migrated in herds of thousands. A few superbly preserved Edmontosaurus skeletons encased within impressions of skin have been discovered in southeastern Wyoming.
Pachycephalosaurs were small bipedal ornithischians with thickened skulls, flattened bodies, and tails surrounded by a latticework of bony rods. In many of these dinosaurs, such as the Pachycephalosaurus, -a large specimen up to eight m’s (26 ft) a long-the skull was capped by a rounded dome of solid bone. Some paleontologists suggest that males may have borne the thickest domes and butted heads during mating contests. Eroded pachycephalosaur domes are often found in stream deposits from late in the Cretaceous Period.
The quadrupedal ceratopsians, or horned dinosaurs, typically bore horns over the nose and eyes, and had a saddle-shaped bony frill that extended from the skull over the neck. These bony frills were well developed in the late Cretaceous Triceratops, of which is a dinosaur that could reach lengths of up to eight m’s (26 ft) and weighing more than 12 metric tons. The frill served two purposes: It protected the vulnerable neck, and it contained a network of blood vessels on its undersurface to radiate excess heat. Large accumulations of fossil bones suggest that ceratopsians lived in herds.
Controversy surrounds the extinction of the dinosaurs. According to one theory, dinosaurs were slowly driven to extinction by environmental changes linked to the gradual withdrawal of shallow seas from the continents at the end of the dinosaurian era. Proponents of this theory postulate that dinosaurs dwindled in number and variety over several million years.
An opposing theory proposes that the impact of an asteroid or comet caused catastrophic destruction of the environment, leading to the extinction of the dinosaurs. Evidence to support this theory includes the discovery of a buried impact crater (thought to be the result of a large comet striking the earth) that is 200 km (124 mi) in diameter in the Yucatán Peninsula of Mexico. A spray of debris, called an ejecta sheet, which was blown from the edge of the crater, has been found over vast regions of North America. Comet-enriched material from the impact’s fiery explosion was distributed all over the world. With radiometric dating (Radiometric Dating), scientists have used the decay rates of certain s to date the crater, ejecta sheet, and fireball layer. Using similar techniques to date the dramatic changes in the record of microscopic fossils, they have found that the impact and the dinosaur extinction occurred nearly simultaneously.
Although large amounts of ash suggest that most of North and South America was devastated by fire from the impact, the longer-term planetwide environmental effects of the impact were ultimately more lethal to life than the fire. Dust blocked sunlight from the earth’s surface for many months. Scorched sulfur from the impact site, water vapour and chlorine from the oceans, and nitrogen from the air combined to produce a worldwide fallout of intensely acidic rain. Scientists postulate that darkness and acid rain caused plant growth to cease. As a result, both the herbivorous dinosaurs, which were dependent on plants for food, and the carnivorous dinosaurs, which fed on the herbivores, were exterminated. On the other hand, animals such as frogs, lizards, and small insect-eating turtles and mammals, which were dependent on organisms that fed on decaying plant material, were more likely to survive. Their survival indicates that, in most areas, the surface of Earth did not freeze.
Fossilized dinosaur remains are usually buried in sediments deposited on land. These remains are likely to be found in regions where the silt and sands spread by rivers of the Mesozoic Era are exposed. Fossils are easier to find in arid badlands-rugged, rocky areas with little vegetation, where the sediments are not covered by soil. The excavation of large skeletal fossils involves painstaking procedures to protect the fossils from damage. Fewer than 3,000 dinosaur specimens have been collected to date, and only fifty skeletons of the 350 known varieties of dinosaurs are completely known. Probably less than 10 percent of the varieties of dinosaurs that once lived have been identified.
The shape of dinosaur bones provides clues to how these animals interacted with each other. These bones also reveal information about body form, weight, and posture. Surface ridges and hollows on bones indicate the strength and orientation of muscles, and rings within the bones indicate growth rates. Diseased, broken, and bitten bones bear witnesses to the hazards of life during the dinosaurian age. Cavities in bones reflect the shape of the brain, spinal cord, and blood vessels. Delicate ossicles, or small bony structures in the skull, reveal the shape of the eyeball and its pupil. The structure of the skull and fossilized contents of the abdominal region provide clues to diet.
Organic molecules are also preserved within bones in trace quantities. By studying isotopes of s within these molecules, scientists can gather evidence about body-heat flow and about the food and water consumed by dinosaurs. Impressions in sediment depict skin texture and foot shape, and trackways provide evidence about speed and walking habits.
A 113-million-year-old fossil called Scipionyx samniticus, discovered in southern Italy in the late 1980s, is the first fossil identified that clearly shows the structure and placement of internal organs, including the intestines, colon, liver, and muscles. The fossilized internal organs of Scipionyx samniticus give paleontologists information about how dinosaurs metabolized their food, and other general information about dinosaurs.
Beginning in the late 19th century, the field of paleontology grew as scientific expeditions to find fossil remains became more frequent. American paleontologist Othniel Charles Marsh and his collectors explored the western United States for dinosaurian remains. They identified many genera that have since become household names, including Stegosaurus and Triceratops. In the early part of the 20th century, American paleontologist’s Barnum Brown and Charles Sternberg demonstrated that the area now known as Dinosaur Provincial Park in Alberta, Canada, is the richest site for dinosaur remains in the world. Philanthropist Andrew Carnegie-sponsored excavations in the great Jurassic quarry-pits in Utah, which subsequently turned into Dinosaur National Monument. Beginning in 1922, explorer Roy Chapman Andrews led expeditions to Mongolia that resulted in the discovery of dinosaur eggs. More recently, Luis Alvarez, a particle physicist and Nobel laureate, and his son, geologist Walter Alvarez, discovered evidence of the impact of an asteroid or comet debris that coincided with the extinction of the dinosaurs. Among foreign scholars, German paleontologist Werner Janensch, beginning in 1909, led well-organized dinosaur collecting expeditions to German East Africa (modern Tanzania), where the complete skeletal anatomy of the gigantic Brachiosaurus was documented.
One of the most important fossil-rich sites is located in China. In a small town about 400 km (about 200 mi) northeast of Beijing, there is a fossil formation, called the Yixian formation. Of which have yielded many fossilized specimens of primitive and bird-like dinosaurs, and soft parts such as feathers and fur. Some scientists believe these fossils provide evidence that may have evolved from dinosaurs. Among the recent finds in the Yixian formation is an eagle-sized animal with barracuda-like teeth and very long claws named Sinornithosaurus millenii. Although this dinosaur could not fly, it did have a shoulder blade structure in which allowed a wide range of arm motion similar to flapping. Featherlike structures covered most of the animal’s body.
Another important dinosaur discovery made in 1993 strengthens the evolutionary relationship between dinosaurs and, A 14-year-old boy who was hunting for fossils near Glacier National Park in northern Montana found a fossil of a nearly complete skeleton of a small dinosaur, later named Bambiraptor feinbergi. The fossil is of a juvenile dinosaur only one m (3 ft) long with a body that resembles that of a roadrunner. It has several physical features similar to those of early, including long, winglike arms, bird-like shoulders, and a wishbone. Some scientists propose that Bambiraptor feinbergi may be a type of dinosaur similar to those from which evolved. Other scientists believe that the animal lived too late in time to be ancestral to, while still other scientists hypothesize that dinosaurs may have led to flying ancestral dinosaurs, from which more than once in evolutionary time.
Argentina is another area rich in fossils. In 1995 a local auto mechanic in Nequén, a province on the eastern slopes of the Andes in Argentina, found the fossils of Giganotosaurus, a meat-eating dinosaur that may have reached a length of more than 13 m’s (43 ft). Five years later, in a nearby location, a team of researchers unearthed the bones of what could be the largest meat-eating dinosaur. The newly discovered species is related to the Giganotosaurus, but it was larger, reaching a length of 14 m’s (45 ft). This dinosaur was heavier and had shorter legs than the Tyrannosaurus rex. The fossilized bones indicate that the dinosaur’s jaw was shaped like scissors, suggesting it used its teeth to dissect prey.
In early 2000 AD., scientists used X-rays to view the chest cavity of a dinosaur fossil found in South Dakota. Computerized three-dimensional imaging revealed the remains of what is thought to be the first example of a dinosaur heart ever discovered. The heart appears to contain four chambers with a single aorta, a structure that more closely resembles the heart of a bird or mammal than the heart of any living reptile. The structure of the heart suggests that the dinosaur may have had a high metabolic rate that is more like that of an active warm-blooded animal than that of a cold-blooded reptile.
Many unusual dinosaur fossils found in the Sahara in northern Africa might be related to dinosaur fossils discovered in South America, indicating that the two continents were connected through most of the dinosaurian period. These findings, along with other studies of the environments of dinosaurs and the plants and animals in their habitats, help scientists learn how the world of dinosaurs resembled and differed from the modern world.
The ancestors of dinosaurs were crocodile-like-creatures called archosaurs. They appeared early in the Triassic Period and diversified into a variety of forms that are popularly known as the thecodont group of reptiles. Many of these creatures resembled later Cretaceous dinosaurs. Some archosaurs led to true crocodiles. Others produced pterosaurs, flying reptiles that possessed slender wings supported by a single spar-like finger. Still other archosaurs adopted a bipedal (two-legged) posture and developed S-shaped necks, and it was certain species of these reptiles that eventually evolved into dinosaurs.
Fossil evidence of the earliest dinosaurs dates from about 230 million years ago. This evidence, found in Madagascar in 1999, consists of bones of an animal about the size of a kangaroo. This dinosaur was a type of saurischian and was a member of the plant-eating prosauropods, which were related to ancestors of the giant, long-necked sauropods that included the Apatosaurus. Before this discovery, the earliest known dinosaur on record was the Eoraptor, which lived 227 million years ago. Discovered in Argentina in 1992, the Eoraptor was an early saurischian, one m (3 ft) long, with a primitive skull.
Scientists have identified the isolated bones and teeth of a few tiny dinosaurs representing ornithischians dating from the beginning of the Jurassic Period, around 205 million years ago. By the middle of the Jurassic Period, around 180 million years ago, most of the basic varieties of saurischian and ornithischian dinosaurs had appeared, including some that far surpassed modern elephants in size. Dinosaurs had evolved into the most abundant large animals on land, and the dinosaurian age had begun.
Earth’s environment during the dinosaurian era was far different from it is today. The days were several proceeding moments shorter than they are today because the gravitational pull of the sun and the moon have over time had a braking influence on Earth’s rotation. Radiation from the Sun was not as strong as it is today because the Sun has been slowly brightening over time.
Other changes in the environment may be linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in Earth’s atmosphere-the so-called greenhouse effect-was several times more abundant in the air during the dinosaurian age. As a result, surface temperatures were warmer and no polar ice caps could form.
The pattern of continents and oceans was also very different during the age of dinosaurs. At the beginning of the dinosaurian era, the continents were united into a gigantic super-continent called Pangaea (all lands), and the oceans formed a vast world ocean called Panthalassa (all seas). About 200 million years ago, movements of Earth’s crust caused the super-continent to begin slowly separating into northern and southern continental blocks, which broke apart further into the modern continents by the end of the dinosaurian era.
Because of these movements of Earth’s crust, there was less land in equatorial regions than there is at present. Deserts, possibly produced by the warm, greenhouse atmosphere, were widespread across equatorial land, and the tropics were not as rich an environment for life forms as they are today. Plants and animals may have flourished instead in the temperate zones north and south of the equator.
The most obvious differences between dinosaurian and modern environments are the types of life forms present. There were fewer than half as many species of plants and animals on land during the Mesozoic Era than there are today. Bushes and trees appear to have provided the most abundant sources of food for dinosaurs, rather than the rich grasslands that feed most animals today. Although flowering plants appeared during the dinosaurian era, few of them bore nuts or fruit.
The animals of the period had slower metabolisms and smaller brains, suggesting that the pace of life was relatively languid and the behaviour were simple. The more active animals-such as ants, wasps, and mammals-first made their appearance during the dinosaurian era but was not as abundant as they are now.
The behaviour of dinosaurs was governed by their metabolism and by their central nervous system. The dinosaurs’ metabolism-the internal activities that supply the body’s energy needs-affected their activity level. It is unclear whether dinosaurs were purely endothermic (warm-blooded), like modern mammals, or ectothermic (cold-blooded), like modern reptiles. Endotherms regulate their body temperature internally by means of their metabolism, rather than by using the temperature of their surroundings. As a result, they have higher activity levels and higher energy needs than ectotherms. Ectotherms have a slower metabolism and regulate their body temperature by means of their behaviour, taking advantage of external temperature variations by sunning themselves to stay warm and resting in the shade to cool down. By determining whether dinosaurs were warm or cold-blooded, paleontologists could discover whether dinosaurs behaved more like modern mammals or more like modern reptiles.
Gradual changes in dinosaur anatomy suggest that the metabolic rates and activity levels of dinosaurs increased as they evolved, and some scientists believe this indicates that dinosaurs became progressively more endothermic. Overall, dinosaur body size decreased throughout the latter half the dinosaurian era, increasing the dinosaurs’ need for activity and a higher metabolism to maintain warmth. Smaller animals have more surface area in proportion to their volume, which causes them to lose more heat as it radiates from their skin. Well-preserved fossils show that many small dinosaurs were probably covered with hair or feather-like fibres. Dinosaurs’ tooth batteries (many small teeth packed together) became larger, enabling them to chew their food more efficiently, their breathing passages became separated from their mouth cavity, allowing them to chew and breathe while, and their nostrils became larger, making their breathing more efficient. These changes may have helped the dinosaurs digest their food and change it into energy more quickly and efficiently, thereby helping them maintain a higher metabolism.
The central nervous system of dinosaurs affected their behavioural flexibility-how much they could adapt their behaviours to deal with changing situations. Scientists believe that the ratio of dinosaurs’ brain size to their body weight increased as the animals evolved. As a result, their behavioural flexibility increased from a comparable level to that of modern crocodiles, in the primitive dinosaurs, to a level that is comparable to that of modern chickens and opossums, in some small Cretaceous dinosaurs.
Imprints of the skin of large dinosaurs show that the skin had a textured surface without hair or feathers. The eyes of dinosaurs were about twice the diameter of those of modern mammals. The skeleton of one small dinosaur was found preserved in windblown sand. Its head was tucked next to its forelimbs, resembling the posture of a modern bird, and its tail was wrapped around its body, resembling the posture of a cat.
Many, if not all, dinosaurs laid eggs, and extensive deposits of whole and fragmented shells have been found in China, India, and Argentina, suggesting that large nesting colonies were common. A very few eggs have been identified from the skeletons of embryos contained within them. In proportion to the body weight of the mother, dinosaurs laid smaller eggs in greater numbers than do. Scientists have found what they believe is a typical nest dug into Cretaceous streamside clays in Montana. The nest is a craterlike structure about two m’s (6.6 ft) in diameter-thought to be about the diameter of the mother’s body.
The large number of bones of small dinosaurs that have been found in nesting colonies indicates that the mortality rate of juveniles was very high. The growth rings preserved in dinosaur bones suggest that primitive dinosaurs grew more slowly than later dinosaurs. The growth rings in some giant dinosaurs suggest that these dinosaurs may have grown to adulthood rapidly and had shorter life spans than some large modern turtles, such as the giant tortoise, which can live 200 years in captivity.
Saurischian dinosaurs were characterized by a primitive pelvis, with a single bone projecting down and back from each side of the hips. This pelvis construction was similar to that of other ancient reptiles but, unlike other reptiles, saurischians had stronger backbones, no claws on their outer front digits, and forelimbs that were usually much shorter than the hind limbs. There were three basic kinds of saurischians: theropods, prosauropods, and sauropods.
Nearly all theropods were bipedal flesh eaters. Some theropods, such as Tyrannosaurus of the late part of the Cretaceous Period, reached lengths of twelve m’s (39 ft) and weights of 5 metric tons. In large theropods the huge jaws and teeth were adapted to tearing prey apart. Fossil trackways reveal that these large theropods walked more swiftly than large plant-eating dinosaurs and were more direct and purposeful in their movements. Other theropods, such as The Compsognathus, were small and gracefully built, resembling modernly running such as the roadrunner. Their heads were slender and often beaked, suggesting that these theropods fed on small animals such as lizards and infant dinosaurs. Some of them possessed brains as large as those of modern chickens and opossums.
Other theropods, called raptors, bore powerful claws, like those of an eagle, on their hands and feet and used their flexible tails as balancing devices to increase their agility when turning. These animals appear to have hunted in packs. Many paleontologists believe that may have arisen from small, primitive theropods that were also ancestors of the raptors. Evidence for this theory has been augmented by the discovery of an Oviraptor nest in the Gobi Desert. The nest contains the fossil bones of an Oviraptor sitting on its brood of about fifteen eggs, exhibiting behaviours remarkably similar to that of modern.
Unlike the primitive theropods, the prosauropods had relatively small skulls and spoon-shaped, rather than blade-shaped, teeth. Their necks were long and slender and, because they were bipedal, the prosauropods could browse easily on the foliage of bushes and trees that were well beyond the reach of other herbivores. A large clawed, a hook-like thumb was probably used to grasp limbs while feeding. The feet were broad and heavily clawed. When prosauropods appeared in the fossil record along with the earliest known theropods, they had already reached lengths of three m’s (10 ft). By the end of the Triassic Period, the well-known Plateosaurus had attained a length of nine m’s (30 ft) and a weight of 1.8 metric tons. During the late Triassic and early Jurassic periods, prosauropods were the largest plant-eating dinosaurs.
Sauropods, which include giants such as Apatosaurus (formerly known as Brontosaurus) and Diplodocus, descended from prosauropods. By the middle of the Jurassic Period they had far surpassed all other dinosaurs in size and weight. Some sauropods probably reached lengths of more than twenty-five m’s (82 ft) and weighed about 90 metric tons. These dinosaurs walked on four pillar-like legs. Their feet usually bore claws on the inner toes, although they otherwise resembled the feet of an elephant. The sauropod backbone was hollow and filled with air sacks similar to those in a bird’s vertebrae, and the skull was small in proportion to the animals’ size. The food they ate was ground by stones in their gizzard, a part of their digestive tract. Indeed, sauropods may be compared with gigantic elephants, with the sauropods’ long necks performing the function of an elephant’s trunk, and their gizzard stones acting as the strong teeth of an elephant. Some sauropods, such as the late Jurassic Apatosaurus, used their long, thin tails as a whip for defence, while others used their tails as clubs.
In ancestral ornithischians the bony structure projecting down and back from each side of the hips was composed of two bones, so that their hips superficially resembled the hips of. Early ornithischians were small bipedal plant eaters, about one m (3 ft) in length. These animals led to five kinds of descendants: stegosaurs, ankylosaurs, ornithopods, pachycephalosaurs, and ceratopsians.
Paleontology helps to study the prehistoric animal and plant life through the analysis of fossil remains. The study of these remains enables scientists to trace the evolutionary history of extinct and living organisms. Paleontologists also play a major role in unravelling the mysteries of the earth's rock strata (layers). Using detailed information on how fossils are distributed in these layers of rock, paleontologists help prepare accurate geologic maps, which are essential in the search for oil, water, and minerals.
Most people did not understand the true nature of fossils until the beginning of the 19th century, when the basic principles of modern geology were established. Since about 1500, scholars had engaged in a bitter controversy over the origin of fossils. One group held that fossils are the remains of prehistoric plants and animals. This group was opposed by another, which declared that fossils were either freaks of nature or creations of the devil. During the 18th century, many people believed that all fossils were relics of the great flood recorded in the Bible.
Paleontologists gain most of their information by studying deposits of sedimentary rocks that formed in strata over millions of years. Most fossils are found in sedimentary rock. Paleontologists use fossils and other qualities of the rock to compare strata around the world. By comparing, they can determine whether strata developed during the same time or in the same type of environment. This helps them assemble a general picture of how the earth evolved. The study and comparison of different strata are called stratigraphy.
Fossils provide most of the data on which strata are compared. Some fossils, called index fossils, are especially useful because they have a broad geographic range but a narrow temporal one-that is, they represent a species that was widespread but existed for a brief period. The best index fossils have a tendency leaning toward being marine creatures. These animals evolved rapidly and spread over large areas of the world. Paleontologists divide the last 570 million years of the earth's history into eras, periods, and epochs. The part of the earth's history before about 570 million years ago is called Precambrian time, which began with the earth's birth, probably more than four billion years ago.
The earliest evidence of life consists of microscopic fossils of bacteria that lived as early as 3.6 billion years ago. Most Precambrian fossils are very tiny. Most species of larger animals that lived in later Precambrian time had soft bodies, without shells or other hard body parts that would create lasting fossils. The first abundant fossils of larger animals date from about 600 million years ago.
Coming to be, is the Paleozoic era, of which it lasted for about 330 million years. It includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Index fossils of the first half of the Paleozoic era are those of the invertebrates, such as trilobites, graptolites, and crinoids. Remains of plants and such vertebrates as fish and reptiles make up the index fossils of the second half of this era.
At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except vertebrates. The characteristic animals of the Cambrian period were the trilobites, a primitive form of arthropod, which reached their fullest development in this period and became extinct by the end of the Paleozoic era. The earliest snails appeared in this period, as did the cephalopod mollusks. Other groups represented in the Cambrian period were brachiopods, bryozoans, and Foraminifera. Plants of the Cambrian period included seaweeds in the oceans and lichens on land.
The most characteristic animals of the Ordovician period (500 million to 435 million years ago) were the graptolites, which were small, colonial hemichordates (animals possessing an anatomical structure suggesting part of a spinal cord). The first vertebrates-primitive fish-and the earliest corals emerged during the Ordovician period. The largest animal of this period was a cephalopod mollusk that had a shell about three m’s (about 10 ft) in length. Plants of this period resembled those of the Cambrian periods.
The most important evolutionary development of the Silurian period (435 million to 410 million years ago) was that of the first air-breathing animal, a scorpion. Fossils of this creature have been found in Scandinavia and Great Britain. The first fossil records of vascular plants-that are, land plants with tissue that carries food-appeared in the Silurian period. They were simple plants that had not developed separate stems and leaves.
The dominant forms of animal life in the Devonian period (410 million to 360 million years ago) were fish of various types, including sharks, lungfish, armoured fish, and primitive forms of ganoid (hard-scaled) fish that were probably the evolutionary ancestors of amphibians. Fossil remains found in Pennsylvania and Greenland indicate that early forms of amphibia may already have existed during the Devonian period. Early animal forms included corals, starfish, sponges, and trilobites. The earliest known insect was found in Devonian rock.
The Devonian is the first period from which any considerable number of fossilized plants have been preserved. During this period, the first woody plants developed, and by the end of the period, land-growing forms included seed ferns, ferns, scouring rushes, and scale trees, the modern relative of club moss. Although the present-day equivalents of these groups are mostly small plants, they developed into treelike forms in the Devonian period. Fossil evidence indicates that forests existed in Devonian times, and petrified stumps of some larger plants from the period measure about 60 cm (about twenty-four in) in diameter.
The Carboniferous period lasted from 360 million to 290 million years ago. During the first part of this period, sometimes called the Mississippian period (360 million to 330 million years ago), the seas contained a variety of echinoderms and foraminifer, with most forms of animal life that appeared in the Devonian. A group of sharks, the Cestraciontes-or shell-crushers-were dominant among the larger marine animals. The predominant group of land animals was the Stegocephalia, an order of primitive, lizard-like amphibians that developed from the lungfish. The various forms of land plants became diversified and grew larger, particularly those that grew in low-laying swampy areas.
The second part of the Carboniferous, sometimes called the Pennsylvanian period (330 million to 290 million years ago), saw the evolution of the first reptiles, a group that developed from the amphibians and lived entirely on land. Other land animals included spiders, snails, scorpions, more than 800 species of cockroaches, and the largest insect ever evolved, a species resembling the dragonfly, with a wingspread of about 74 cm (about twenty-nine in.). The largest plants were the scale trees, which had tapered trunks that measured as much as 1.8 m’s (6 ft) in diameter at the base and thirty m’s (100 ft) in height. Primitive gymnosperms known as cordaites, which had pithy stems surrounded by a woody shell, were more slender but even taller. The first true conifers, forms of advanced gymnosperms, also developed during the Pennsylvanian period.
The chief events of the Permian period (290 million to 240 million years ago) were the disappearance of many forms of marine animals and the rapid spread and evolution of the reptiles. Usually, Permian reptiles were of two types: lizard-like reptiles that lived entirely on land, and sluggish, semiaquatic types. A comparatively small group of reptiles that evolved in this period, the Theriodontia, were the ancestors of mammals. Most vegetation of the Permian period was composed of ferns and conifers.
The Mesozoic era is often called the Age of Reptiles, because the reptile class was dominant on land throughout the age. The Mesozoic era lasted about 175 million years, and included the Triassic, Jurassic, and Cretaceous periods. Index fossils from this era include a group of extinct cephalopods called ammonites, and extinct forms of sand dollars and sea urchins
The most notable of the Mesozoic reptiles, the dinosaur, first evolved in the Triassic period (240 million to 205 million years ago). The Triassic dinosaurs were not as large as their descendants in later Mesozoic times. They were comparatively slender animals that ran on their hind feet, balancing their bodies with heavy, fleshy tails, and seldom exceeded 4.5 m’s (15 ft) in length. Other reptiles of the Triassic period included such aquatic creatures as the ichthyosaurs, and a group of flying reptiles, the pterosaurs.
The first mammals also appeared during this period. The fossil remains of these animals are fragmentary, but the animals were apparently small and reptilian in appearance. In the sea, Teleostei, the first ancestors of the modern bony fishes, made their appearance. The plant life of the Triassic seas included a large variety of marine algae. On land, the dominant vegetation included various evergreens, such as ginkgos, conifers, and palms. Small scouring rushes and ferns still existed, but the larger members of these groups had become extinct.
During the Jurassic period (205 million to 138 million years ago), dinosaurs continued to evolve in a wide range of size and diversity. Types included heavy four-footed sauropods, such as Apatosaurus (formerly Brontosaurus); two-footed carnivorous dinosaurs, such as Allosaurus; Two-footed vegetarian dinosaurs, such as Camptosaurus, and four-footed armoured dinosaurs, such as Stegosaurus. Winged reptiles included the pterodactyl, which, during this period, ranged in size from extremely small species to those with wingspreads of 1.2 m’s (4 ft). Marine reptiles included plesiosaurs, a group that had broad, flat bodies like those of turtles, with long necks and large flippers for swimming; Ichthyosauria, which resembled dolphins, and primitive crocodiles, as the mammals of the Jurassic period consisted of four orders, all of which were smaller than small modern dogs. Many insects of the modern orders, including moths, flies, beetles, grasshoppers, and termites appeared during the Jurassic period. Shellfish included lobsters, shrimp, and ammonites, and the extinct group of belemnites, which resembled squid and had cigar-shaped internal shells. Plant life of the Jurassic period was dominated by the cycads, which resembled thick-stemmed palms. Fossils of most species of Jurassic plants are widely distributed in temperate zones and polar regions, indicating that the climate was uniformly mild.
The reptiles were still the dominant form of animal life in the Cretaceous period (138 million to sixty-five million years ago). The four types of dinosaurs found in the Jurassic also lived during this period, and a fifth type, the horned dinosaurs, also appeared. By the end of the Cretaceous, about sixty-five million years ago, all these creatures had become extinct. The largest of the pterodactyls lived during this period. Pterodactyl fossils discovered in Texas have wingspreads of up to 15.5 m’s (50 ft). Other reptiles of the period include the first snakes and lizards. Several types of Cretaceous have been discovered, including Hesperornis, a diving bird about 1.8 m’s (about 6 ft) in length, which had only vestigial wings and was unable to fly. Mammals of the period included the first marsupials, which strongly resembled the modern opossum, and the first placental mammals, which belonged to the group of insectivores. The first crabs developed during this period, and several modern varieties of fish also evolved.
The most important evolutionary advance in the plant kingdom during the Cretaceous period was the development of deciduous plants, the earliest fossils of which appear in early Cretaceous rock formations. By the end of the period, many modern varieties of trees and shrubs had made their appearance. They represented more than 90 percent of the known plants of the period. Mid-Cretaceous fossils include remains of beech, holly, laurel, maple, oak, plane tree, and walnut. Some paleontologists believe that these deciduous woody plants first evolved in Jurassic times but grew only in upland areas, where conditions were unfavourable for fossil preservation.
The Cenozoic era (sixty-five million years ago to the present time) is divided into the Tertiary period (sixty-five million to 1.6 million years ago) and the Quaternary period (1.6 million years ago to the present). However, because scientists have so much more information about this era, they have an aptitude to focus on the epochs that make up each period. During the first part of the Cenozoic era, an abrupt transition from the Age of Reptiles to the Age of Mammals occurred, when the large dinosaurs and other reptiles that had dominated the life of the Mesozoic era disappeared.
Index fossils of the Cenozoic tend to be microscopic, such as the tiny shells of Foraminifera. They are commonly used, along with varieties of pollen fossils, to date the different rock strata of the Cenozoic era.
The Paleocene epoch (sixty-five million to fifty-five million years ago) marks the beginning of the Cenozoic era. Seven groups of Paleocene mammals are known. All of them appear to have developed in northern Asia and to have migrated to other parts of the world. These primitive mammals had many features in common. They were small, with no species exceeding the size of a small modern bear. They were four-footed, with five toes on each foot, and they walked on the soles of their feet. Most of them had slim heads with narrow muzzles and small brain cavities. The predominant mammals of the period were members of three groups that are now extinct. They were the creodonts, which were the ancestors of modern carnivores; the amblypods, which were small, heavy-bodied animals; and the condylarths, which were light-bodied herbivorous animals with small brains. The Paleocene groups that have survived are the marsupials, the insectivores, the primates, and the rodents.
During the Eocene epoch (fifty-five million to thirty-eight million years ago), several direct evolutionary ancestors of modern animals appeared. Among these animals-all of which were small in stature-were the horse, rhinoceros, camel, rodent, and monkey. The creodonts and amblypods continued to develop during the epoch, but the condylarths became extinct before it ended. The first aquatic mammals, ancestors of modern whales, also appeared in Eocene times, as did such modern as eagles, pelicans, quail, and vultures. Changes in vegetation during the Eocene epoch were limited chiefly to the migration of types of plants in response to climate changes.
During the Oligocene epoch (thirty-eight million to twenty-four million years ago), most of the archaic mammals from earlier epochs of the Cenozoic era disappeared. In their place appeared representatives of many modern mammalian groups. The creodonts became extinct, and the first true carnivores, resembling dogs and cats, evolved. The first anthropoid apes also lived during this time, but they became extinct in North America by the end of the epoch. Two groups of animals that are now extinct flourished during the Oligocene epoch: the titanotheres, which are related to the rhinoceros and the horse; and the oreodonts, which were small, dog-like, grazing animals.
The development of mammals during the Miocene epoch (twenty-four million to five million years ago) was influenced by an important evolutionary development in the plant kingdom: the first appearance of grasses. These plants, which were ideally suited for forage, encouraged the growth and development of grazing animals such as horses, camels, and rhinoceroses, which were abundant during the epoch. During the Miocene epoch, the mastodon evolved, and in Europe and Asia a gorilla-like ape, Dryopithecus, was common. Various types of carnivores, including cats and wolflike dogs, ranged over many parts of the world.
The Paleontology of the Pliocene epoch (five million to 1.6 million years ago) does not differ much from that of the Miocene, although the period is regarded by many zoologists as the climax of the Age of Mammals. The Pleistocene Epoch (1.6 million to 10,000 years ago) in both Europe and North America was marked by an abundance of large mammals, most of which were practically modern in type. Among them were buffalo, elephants, mammoths, and mastodons. Mammoths and mastodons became extinct before the end of the epoch. In Europe, antelope, lions, and hippopotamuses also appeared. Carnivores included badgers, foxes, lynx, otters, pumas, and skunks, and now-extinct species such as the giant saber-toothed tiger. In North America, the first bears made their appearance as migrants from Asia. The armadillo and ground sloth migrated from South America to North America, and the muskox ranged southward from the Arctic regions. Modern human beings also emerged during this epoch.
Cave Paint in at Lascaux France, are expressive portions of the cave painting in Lascaux, and was carried out by Palaeolithic artists in or around 13,000 Bc. At the end of the Pleistocene Epoch, the cow and other groups of small horses were painted with red and yellow. Whereas, ochre colours were either blown through reeds onto the wall or mixed with animal fat to apply in the squirted by reeds or thistles. It is believed that prehistoric hunters painted these to gain magical powers that would ensure a successful hunt.
The remains of simple animals provide additional information about climate and climatic change. Because different beetle species are especially well suited for either warm or cool climates, the presence of fossils of a particular type of beetle can give scientists clues to the climate of the region. Fossil algae reveal much about water acidity or alkalinity, water temperature, and the speed of water movement. The Ocean Drilling Program, which collects samples from the sea-floor, has collected enough data to show that the distribution of marine organisms changed significantly during the Pleistocene Epoch.
Invertebrates-animals without backbones, such as shellfish and insects-and plant communities survived the glacial cycles of the Pleistocene Epoch as moderately unscathed. Some animal and plant groups, such as the beetles, moved vast distances but underwent little evolution. Pleistocene mammals, on the other hand, underwent important changes, probably because climate changes affected mammals more than they did invertebrates. Many mammals have evolved significantly since the Pleistocene. Some changes in familiar animals include greater numbers of species of mice and rats and the appearance of modern species of the dog family.
Many mammalian species have become extinct since the Pleistocene. A few of the spectacular mammals that disappeared during the last 20,000 years include the woolly rhinoceros, the giant ground sloth, the saber-toothed tigers, the giant cave bear, the mastodon, and the woolly mammoth. These animals existed just when early humans, and drawings of them exist on cave walls in Europe. Recent theories suggest that these huge mammals could not reproduce quickly enough to replace the number of animals that humans killed for food, and were therefore driven to extinction by human hunting.
Humans continued to evolve during the Pleistocene Epoch. Two genera, Australopithecus and Homo, existed during the early Pleistocene. The last Australopithecines disappeared about one million years before present. Several species of the genus Homo existed during the Pleistocene. Modern humans (Homo sapiens sapient) probably arose from The Homo erectus, which are thought to have evolved from Homo habilis. Paleontologists have found fossils that support the transition between Homo erectus and Homo sapiens dating from about 500,000 years before present to about 200,000 years before present. Anatomically modern humans (Homo sapiens) arose from an earlier human species that lived in Africa. A likely ancestor, known as Homo ergaster, evolved from around 1.9 million years ago. This ancestor arose from an earlier Pleistocene species in Africa, perhaps one known as Homo rudolfensis. Anatomically a modern Homo sapiens appears to have evolved by 130,000 years ago, if not earlier. For a time our species also coexisted in parts of Eurasia with another species of The Homo, Homo neanderthalensis, until between 35,000 and 30,000 years ago. Since then only, our species has survived.
Evidence from both lands and sea environments shows that, at least before the human-induced global warming of the last two centuries, the worldwide climate has been cooling naturally for several thousand years. Ten thousand years have already passed since the end of the last glaciation, and 18,000 years have passed since the last maximum. This may suggest that Earth have entered the beginning of the next worldwide glaciation.
Several possible causes of ice ages exist. Scientists have proposed many theories to explain their occurrence. In the 1920s Yugoslav scientist Milutin Milankovitch proposed the Milankovitch Astronomical Theory, variations in Earth’s position can cause which state’s climatic fluctuations and the onset of glaciation compared with the Sun. Milankovitch calculated that this deviation of Earth’s orbit from its almost circular path occurs every 93,408 years. We have also linked the movement of Earth’s crustal plates, called plate tectonics, to the occurrence of ice ages. The positions of the plates in polar regions may contribute to ice ages. Changes in global sea level may affect the average temperature of the planet and lead to cooling that may cause ice ages. Separate theories explaining the causes of ice ages such as the substantial variations of heat output of the Sun, or the bearing of interplanetary dust cloud, that absorb the sun’s heat for reaching the Earth, and, perhaps from a meteorite affect-have not yet been supported by any solid evidence.
The Milankovitch Astronomical Theory best explains regular climatic fluctuations. The theory is based on three variations in the position of Earth compared with the Sun: the eccentricity (elongation or circularity of the shape) of Earth's orbit, the tilt of Earth's axis toward or away from the Sun, and the degree of wobble of Earth's axis of rotation. The total effect of these changes causes one region of Earth-latitude 60° to seventy north, near the Arctic Circle-to receive low amounts of summer radiation about once every 100,000 years. These cool summer periods last several hundred to several thousand years and thus provide sufficient time to allow snowfields to expand and merge into glaciers in this area, signalling the beginning of glaciation.
When glaciers expand during an ice age, the sea level drops because the water that forms glaciers ultimately comes from the oceans. Global sea level affects the overall temperature of the planet because solar radiation, or heat, is better absorbed by water than by land. When sea levels are low, more land surfaces becomes exposed. Since the land is not able to absorb as much solar radiation as the water can, the overall average temperature of the planet decreases, or cools, and may contribute to the onset of an ice age.
A map showing Earth during an ice age would look very different from a map of contemporaneousness resulting to our world divergence. During the Wisconsin glaciation of 115,000 to 10,000 years ago, two ice sheets, the Laurentides and the Cordilleran, covered the northern two-thirds of North America, including most of Canada, with ice. Other parts of the world, including Eurasia and parts of the North Atlantic Ocean, were also blanketed in sheets of ice
The Laurentides continental ice sheet extended from the eastern edge of the Rocky Mountains to Greenland. The separate Cordilleran Ice Sheet was composed of mountains and ice cap valley glaciers. Flowing onto the surrounding lowlands, in as much as these partial reservoirs were equally part of northern Alaska, and of the Sierra Nevada. The Cascade Range and the Rocky Mountains and as far south as New Mexico contains the whole from which begins their feeding waters from the Cordilleran Ice sheet. Where the continental shelf between Alaska and Siberia was uncovered, the Bering land bridge formed. In northern Eurasia, continental ice extended from Great Britain eastward to Scandinavia and Siberia. Separate mountains, and glacial systems covered the Alps, the Himalayas, and the Andes. The extensive ice sheets on Antarctica and Greenland did not expand very much during to each of glaciation. Sea ice grew worldwide, particularly in the North Atlantic Ocean.
Years of investigation and research, coupled with resolution and courage to follow wherever truth might lead, have established the certainty of a future world cataclysm during which most of the earth's population will be destroyed in the same manner as the mammoths of prehistoric times were destroyed. Such an event has occurred each time that one or two polar ice caps grew to maturity; a recurrent event in global history is clearly written in the rocks of a very old earth.
The earth is approximately four ½ billion years old. Human beings have been living on it for at least 500,000 years and perhaps even one million years. To appreciate the immensity of these figures, one might imagine the age of the earth represented by the period of about one week; the duration of our own epoch, 7,000 years, is then but one second! By a similar analogy, men have lived on an earth that is one week old for just two minutes. Evidently, our own epoch is but a very short and insignificant period in the life of our planet and our species.
In past epochs there have been ice caps at one or both of the geographical poles. The heat of the sun caused these ice caps to grow larger. As the sun heats the air of the hemisphere, the heated air expands, becomes lighter, and rises. The updrafts are greatest in the tropics. As the earth is virtually spherical, the currents of warm air converge at the poles. Meeting head-on from every direction, they create areas of air pressure, become colder and heavier, turn downward, reversing the direction of their flow, and pour back toward the Equator from the polar centres with high velocities. Thus, there is a continuous circulation of raising humid warm air journeying poll ward and a down draft of cold dehumidified air returning from the poles at low or ground altitudes. Air acts like a sponge. When warm, it absorbs water, when cold it cannot hold much water, and in cooling releases any surplus moisture to fall as rain or snow.
Most of the snow that falls in the polar regions does not melt; the air temperature is too low. Instead, the snow is stored, changing to glacial ice. As this process continues through time, the ice masses at the poles constantly grow in volume.
As the prehistoric ice caps grew larger, they’re lent of an ectomorphic drift turning the rotating planet off balance because of the wobble of the earth, causing the earth to roll around sideways to its direction of rotation. Another analogy will make this clear. When you place a weight at the end of a string and then rotate the string in a circle, the weighted end of the string rises to a horizontal plane. Now, imagine yourself and the string as the earth, the weight at the end of the string as the weight of a growing ice cap, and imagine that, instead of intentionally swinging the weighted string, the rotational motion encompasses you, the string and the weight, as though you were standing on a rotating platform. In this depiction, then, your body represents both the present Axis of Spin and Axis of Figure of the earth. Your body does not move; the Axis of Spin remains the same. However, your arm and the weighted extend like a string, here representing a radius of the earth, rise from the vertical (directed toward the pole) to the horizontal (directed toward the Equator). The sphere of which your arm and the weighted string are some radiuses is rolled sideways; the weight, representing the imbalance of an ice cap, rotates from a polar position to an equatorial position. The Axis of Figure, previously represented by your vertical arm, is now changed; the old Axis of Figure is now perpendicular to the Axis of Spin.
The rotating equilibrium thrown off balance by the weight of the growing ice caps, causes the spinning globe to roll over on its side. Yet such an event does not occur lightly. The oceans, like water in a bowl that is suddenly moved, are cast from their basins to flood the land. The winds, previously settled into patterns dependent upon a stable globe, are whipped asunder by the sudden shifting of the globe. The sudden meeting of warm and cold air creates great pressure zones that spawn new rains and hurricanes to sweep across the earth. The forces of nature, loosed from their equilibrium, range wildly in search of new equilibrium. Bringing us to lay upon the Stone Age, of which this period of human technological development characterized by using stone as the principles raw material for tools. In a given geographic region, the Stone Age normally predated the invention or spread of metalworking technology. Human groups in different parts of the world began using stone tools at different times and abandoned stone for metal tools at different times. Broadly speaking, however, the Stone Age generated by times generations estimate the concurrent evidence around 2.5 million years ago. Ending of a partial differentiation that globally extends by 5,000 years ago, moreover, the world’s regional intervals became intermittently more recent. Today only a few isolated human populations rely largely on stone for their technologies, and that reliance is rapidly vanishing with the introduction of tools from the modern industrialized world.
Human ancestors living before the Stone Age likely used objects as tools, a behaviour that scientists find today among chimpanzees. Wild chimpanzees in Africa exhibit a range of tool-using behaviours. For example, they used bent twigs to fish for termites, chewed wads of leaves to soak up liquid, and branches and stones as hammers, anvils, missiles, or clubs. However, when prehistoric humans began to make stone tools they became dramatically distinct from the rest of the animal world. Although other animals may use stone objects as simple tools, the intentional modification of stone into tools, and using tools to make other tools, is behaviourally unique to humans. Although those Africans of 100,000 years ago had more modern skeletons than did their Neanderthal contemporaries, they made essentially the same crude stone tools as Neanderthals, still lacking standardized shapes. This stone Toolmaking and tool-uses were a haling behaviour that became indispensable to the way early humans adapted to their environment and partially affected human evolution
Human technology developed from the first stone tools, in use by two and a half million years ago, to the 1996 laser printer that replaced the outdated 1992 laser printer and used to print out manuscripts depicted of these pages. The rate of development was undetectably slow at the beginning, when hundreds of thousands of years passed with no discernible change in our stone tools and with no surviving evidence for artifacts made of other materials. Today, technology advances so rapidly that it is reported in the daily newspaper.
Archaeologists believe the Stone Age began in the vicinity to 2.5 million years ago because that marks the age of the earliest stone tool remnants ever discovered. The earliest recognizable stone artifacts mark the beginnings of the archaeological record-that is, material remnants of ancient human activities. As recently as 5,000 years ago all human societies on the face of the earth were essentially still living in the Stone Age. Therefore, more than 99.8 percent of humans’ time as Toolmaker-from 2.5 million years ago to 5,000 years ago-took places during the Stone Age. During the Stone Age our ancestors went through many different stages of biological and cultural evolution. It was long after our lineage became anatomically modern that we began to experiment with innovations such as metallurgy, heralding the end of the Stone Age.
The term Stone Age has been used since the early 1800s as a designation for an earlier, prehistoric stage of human culture, one in which stone rather than metal tools were used. By the early 1800s different archaeological sites had equalled uncovered those European involvements embedded by some mysterious components from apparently foregoing of prehistoric intervals. Christian Thomsen, curator of the National Museum in Copenhagen, Denmark, developed a classification scheme to organize the museum’s growing collections into three successive technological stages in the human past: Stone Age, Bronze Age, and Iron Age. The three set -value-class were quickly adopted and spread throughout the museums in Europe. In addition, among excavators, who were to their finding results of remnant categories, that each set class was of a constant basis as justified freely by these three set -stages. The fact that Stone Age remnants were found at the bottom layers showed that they were the oldest
The study of the Stone Age falls under the fields of anthropology, which is the study of human life and cultural origins of human life up to the present, and Archaeology, which is the study of the material remains of humans and human ancestors. Archaeologists seek out, explore, and study archaeological sites, locations around the world where historic or prehistoric people left behind traces of their activities. Archaeologists use the data collected to make theories about how human ancestors lived.
Archaeologists normally use the term artifact to refer to objects modified by human action, either intentionally or unintentionally. The term tool is used to refer to something used by a human or a human ancestor for some purpose and may be modified or not. For instance, a thrown rock is a tool, even if it were not modified. Giving a demonstration of a particular stone artifact is usually difficult as once used as a prehistorical tool, so in practice, archaeologists prefer to use the term artifact instead. In relation to the earlier stages of the Stone Age the unused debris or waste from the manufacture of stone tools is also considered artifactual.
Stone artifacts are important to archaeologists who study prehistoric humans, because they can yield a wide range of information about ancient peoples and their activities. Stone artifacts are, in fact, often the principle archaeological remnants that persist after the passage of time and as such can give important clues as to the presence or absence of ancient human populations in any given region or environment. Careful analysis of Stone Age sites can yield crucial information regarding the technology of prehistoric Toolmaker. Yet leaving to no doubt that we are dealing with biologically and behaviourally modern humans, and, in turn are we to give anthropologists insight into the levels of cognitive (thinking) ability at different stages of human evolution.
Cro-Magnon garbage heaps yield not only stone tools but also tools of bone, whose suitability for shaping, for instance, into fish hooks has apparently gone unrecognized by previous humans. Tools were produced for their adaptivity and distinctive adjustive measures of shapes, their modernity for functions as needles, awls, engraving tools, and so on obviously tell their own story. Instead of only single-piece tools such as hand-held scrapers, Multi-piece tools made their appearance. Recognizable Multi-piece weapons at Cro-Magnon suites include harpoons, spear-throwers, and eventually the bow and arrows, the precursors of rifles and other Multi-piece modern weapons. Those efficient means of killing at a safe distance permitted the hunting of such dangerous prey as rhinos and elephants, while the invention of rope for nets, lines, and snares allowed the addition of fish and to our dirt. Remains of houses and sewn clothing testify to a greater improved ability to survive in cold climates.
During the Stone Age, Earth experienced the most recent in a succession of ice ages, in which glaciers and sea ice covered a large portion of Earth’s surface. The most recent ice age period lasted from 1.6 million to 10,000 years ago, a period of glacial and warmer interglacial stages known as the Pleistocene Epoch. The Holocene Epoch began at the end of the ice age 10,000 years ago and continued to the present time.
Early hominids made stone artifacts either by smashing rocks between a hammer and anvil (known as the bipolar technique) to produce usable pieces or by acceding to a greater extent the controlled process termed flaking, in which stone chips were fractured away from a larger rock by striking it with a hammer of stone or other hard material. Subsequently, throughout the last 10,000 years, additional techniques of producing stone artifacts were to include pecking, grinding, sawing, and boring, in so that it turns into other traditional standards. The most excellent rock for flaking lean of a hard, fine-grained, or amorphous (having no crystal structure) rocks, including lava, obsidian, ignimbrites, flint, chert, quartz, silicified limestone, quartzite, and indurated shale. Ground stone tools could be made on a wider range of raw material types, including coarser grained rock such as granite.
Flaking produces several different types of stone artifacts, which archaeologists look forward to at prehistoric sites. The parent pieces of rock from which chips have been detached are called cores, and the chips removed from cores are called flakes. A flake that has had yet smaller flakes removed from one or more edges to become sharper or form the contours of the known of a retouched piece. The stone used to knock flakes from cores is called a hammerstone or a precursor. Other flaking artifacts include fragments and chunks, most of which are broken cores and flakes.
The terms culture and industries both refer to a system of technology (Toolmaking technique, for example) shared by different Stone Age sites of the same broad time. Experts now prefer to use the term industry instead of culture to refer to these shared Stone Age systems.
Archaeologists have divided the Stone Age into different stages, each characterized by different types of tools or tool-manufacturing techniques. The stages also imply broad time frames and are perceived as stages of human cultural development. The most widely used designations for the successive stages are Palaeolithic (Old Stone Age), Mesolithic (Middle Stone Age), and Neolithic (New Stone Age). British naturalist Sir John Lubbock in 1865 defined the Palaeolithic stage as the period in which stone tools were chipped or flaked. He defined the Neolithic as the stage in which ground and polished stone axes became prevalent. These two stages also were associated with different economic and subsistence strategies: Palaeolithic peoples were hunters-gatherers while Neolithic peoples were farmers. Archaeologists subsequently identified a separate stage of stone tool working in Eurasia and Africa between the Palaeolithic and the Neolithic, called the Mesolithic. This period is characterized by the creation of microliths, small, geometric-shaped stone artifacts attached to wood, antler, or bone to form tools such as arrows, spears, or scythes. Microliths began appearing between 15,000 and 10,000 years ago at the end of the Pleistocene Ice Age.
The Palaeolithic/Mesolithic/Neolithic division system was first applied only to sites in Europe, but is now widely used (with some modification) to refer to prehistoric human development in much of Asia, Africa, and Australasia. Different terminology is often used to describe the cultural-historical chronology of the Americas, which humans did not reach until some point between 20,000 and 12,000 years ago. However, there is a general similarity, the transitional form of flaked stone tools are associated with prehistoric hunters-gatherers to both flaked and ground stone tools associated with the rise of early farming communities. The period in the Americas up to the end of the Pleistocene Ice Age about 10,000 years ago, when most humans were hunters-gatherers, is convened as Paleo-Indian and the subsequent, post-glacial period is known as Archaic.
Archaeologists subdivide the Palaeolithic into the Lower Palaeolithic (the earliest phase), Middle Palaeolithic, and Upper Palaeolithic (the later phase), based upon the presence or absence of certain classes of stone artifacts.
The Lower Palaeolithic dates from approximately 2.5 million years ago until about 200,000 years ago and include the earliest record of human Toolmaking and documents much of the evolutionary history of the genus Homo from its origins in Africa to its spread into Eurasia. Two successive Toolmaking industries characterize the Lower Palaeolithic: the Oldowan and the Acheulean.
The Oldowan industry was named by British Kenyan anthropologists Louis Leakey and Mary Leakey for early archaeological sites found at Olduvai Gorge in northern Tanzania. It is also sometimes called the chopper-core or pebble tool industry. Simple stone artifacts made from small stones or blocks of stone characterize the Oldowan industry. Mary Leakey classified Oldowan artifacts as either heavy-duty tools or light-duty tools, as both their classifications deemed to be heavy-duty tools, which include core types such as choppers, discoids, polyhedrons, and heavy-duty scrapers. Many of these cores may have been produced to generate sharp-edged flakes, but some could have been used for chopping or scraping activities as well. Light-duty tools include retouched forms such as smaller scrapers, awls (sharp, pointed tools for punching holes in animal hides or wood), and burins (chisel like flint tools used for engraving and cutting). Oldowan techniques of manufacturing included hard hammer percussion, or detaching flakes from cores with a stone hammer; the anvil technique, striking a core on a stationary anvil to detach flakes; and bipolar technique, detaching flakes by placing the core between an anvil and the hammerstone.
Early humans probably also made tools from a wide range of materials other than stone. For example, they probably used wood for simple digging sticks, spears, clubs, or probes, and they probably used shell, hide, bark, or horn to fashion containers. Unfortunately, organic materials such as these do not normally survive from earlier Stone Age times, so archaeologists can only speculate about whether such tools were used.
Two of the antiquated Oldowan settings are in Ethiopia, overcoming (as, arrested nearly 2.5 million years ago) and formerly (2.3 million years ago), as this differential studies of the Oldowan localities include Lokalalei (2.3 million years ago), Koobi Fora (1.9 million to 1.4 million years ago) in Kenya, Olduvai Gorge (1.9 million to 1.2 million years ago) in Tanzania, and Ain Hanech (possibly about 1.7 million years ago) in Algeria. The cave deposits at Sterkfontein and Swartkrans (estimated to be from 2.0 million to 1.5 million years ago), in South Africa.
Theories about the intelligence and culture of prehistoric man are beginning to be drastically revised. Accumulated evidence now depicts European men living between 100,000 and 10,000 years ago as communal men who were skilled hunters and Toolmaker, who had developed formal burial rites for members of their tribes and perhaps the orienting initiation among those of whom gainfully employed them as the first religiousities to express a newer beginning of beliefs with the ritual burials for an animals and fellow tribesman alike. Who had some belief in an afterlife, who took excellent care of they’re sick and elderly, and who, in their heyday, carried around pocket sized calendars of their own making.
A ten-member international expedition, led by Ralph S. Solecki of Columbia University, found the bones of a dismembered deer ritually buried by Neanderthal men about 50,000 years ago. The bones of the deer's foot, jaw, and back, its shoulder blades, and the top of its skull were found buried 5 feet deep in the Nahr Ibrahim Cave, north of Beirut, Lebanon. The presence of the skull, the bed of stones on which the bones were placed, and the red earth colouring of the bones, which was not native to the cave, said that a ritual known as hunters' magic was involved in the burial. Solecki interpreted the burial as an attempt “to ensure a successful hunt by the ceremonial treatment of an animal.” Although evidence existed show that bears were ritually treated by Neanderthal men, this was the first discovery of a lone deer buried in this manner.
An American expedition, also led by Solecki, excavated a mountain cave near Shanidar in Iraqui Kurdistan and discovered evidence that Neanderthals practiced a form of religious burial suggesting a belief in an afterlife: at least one actualized in totality from nine skeletons uncovered in the cave was buried with flowers. Also found in the cave was the skeleton of a man of about forty, comparable to a modern age of eighty, who had been born with a deformed right arm. A Neanderthal doctor had skilfully amputated the arm above the elbow, and judging by his death at a ripe old age, the man was carefully cared for from his boyhood until he died because of a rock fall inside the cave, a common peril then.
Recent pale ontological examinations of skeletons suggest that the Neanderthals' stooped posture was the result of a vitamin D deficiency. Lack of sunlight during the Ice Age might have caused their upright posture to become deformed by rickets.
In January it was revealed that a sophisticated system of notation charting the phases of the moon was used throughout most of Europe during the last Ice Age, beginning around 34,000 years ago. Convincingly between such as scribbles and gouges on pieces of bones, antlers, and stone, may have previously seemed regarded as decorations, but were manifested to be representations of the lunar calender. Alexander Marshack, a research associate at the Peabody Museum of Archaeology and Ethnology at Harvard University, began investigating the markings in 1964 and published the results of his study this year in France. The inscribed objects he studied represented all cultural levels from 34,000 to 10,000 years ago. All were pocket sized, and as many as twenty-four tools were used to cut a single sequence, some covering a year or more. This system of notation seems to anticipate the development of a calendar, the idea of number, and the use of abstract symbols. It had been thought that such cognitive abilities developed only after the start of an agricultural society, less than 10,000 years ago.
A tribe of about twenty-four people living a Stone Age way of life was found in the Tasaday Forest on the southern Philippines' Mindanao Island in July. Anthropologists speculate that the tribe has been cut off from the rest of the world for at least 400 years and maybe as much as 2,000 years.
The tribe was first discovered five years ago by an official conducting a census survey. He described the finding a tribe of "jungle people so mysterious that they were known only as the bird who walks the forest like the wind." A long search led to the Tasaday. Interpreters at first had trouble understanding the tribe's language, which is related to Manubo, a native Filipino tongue in the Malayo-Polynesian family.
As communication became easier, it was found that the tribe calls itself the Tasaday because "the man who owns the forest in which they live told their ancestors in a dream to call themselves Tasadays, after a mountain." When asked whether they had ever been off the island, the Tasadays replied that they did not know leaving was possible; in fact, it was found that they had never even seen the ocean. The Tasadays are monogamous in mating but communal in all other ways, have no leader, know no other tribe, have known no unfriendly people, and have never heard of fighting.
The Tasadays committing not to encourage the famished foods but endeavouring to venture as afar from their clearing, consisting of the food, which subsists easily, and founded in the flourishing vegetation of the forest, through which they dwell. The staple of their diet is the pith of the wild palm. To supplement this, they catch tadpoles and small fish with their hands from the nearby streams. Monkey meat is considered a delicacy. After the monkey's hair is singed in a fire and cut away with bamboo blades sharpened by small stones, the meat is roasted.
The group includes six families with thirteen children, nine of whom are boys. All matters of mutual concern, such as food gathering, are decided in an open meeting.
New information about the Mayan civilization, the most highly developed civilization in the New World before the arrival of the white man, was gained from the discovery of an 11 -page codex fragment of a Mayan calendar book. (A codex is a manuscript copy of an ancient text.) The fragment is said to be part of a larger book about twenty pages long. The three other known codices were brought to Europe during the Spanish conquest but did not emerge as important historical material until the 1900's. The newly discovered codex is the first to be found in over a century.
Composed of bark cloth, like the other three, the 11- page codex is expected to reveal "pictorial information on the Venus calendar and its influence on Mayan religion and astrology," according to Michael D. Co., professor of anthropology at Yale University. The fragment dates to the late Mayan period, between 1400 AD. and 1500 AD. The new fragment reveals that the Mayans viewed all four phases of the Venus cycle as threatening. Previously, only the first phase was thought to have been considered sinister.
All four cycles of Venus as seen from the earth were measured by Mayan priests, who calculated that each cycle took 584 days to be completed. Modern astronomers calculate 583.92 days for each complete cycle. The complete 20- page codex would have covered sixty-five Venus cycles.
The co. believes the fragment to be authentic "because it is on bark cloth, [because of] the condition of the fragment. In addition, none of the applicative material duplicates or imitate anything we know about, being identical to the Venus calendar. Lastly, because no forger could be unscrupulous enough to invent material displaying so much knowledge of Mayan life."
Early Slavic tribes formed an organized state in the fourth to sixth centuries, about 500 years earlier than was believed, according to evidence reported in Tass, the Soviet press agency. Arkady Bugai, the Ukrainian archaeologist accredited with the discovery, predicated his determination on radiocarbon dating of charred wood detected in the remains of so named Serpentine Wall, a 500-mile network of defensive earthen works that once encircled the present site of Kiev, the Ukrainian capital. The charred wood used in the radiocarbon tests was from what is believed to be the remains of trees burned to clear ground for the wall. Bugai reasoned that a highly organized state was required to move the seven billion cubic feet of earth that made up the wall, which rises to a height of thirty to 35 feet and is 50 feet wide at its base.
The Serpentine Wall, which enclosed a roughly triangular area, was assumed to have been built to defend the Kiev area from hostile tribes. Ukrainian scholars now believe that the area must have had a population of approximately one million people of whom all was constructed. It was formerly believed that the first consolidation of Russian tribes occurred around the tenth century, during the rise of Kievan Russia.
An expedition bent on disproving the theory that the American man came to North America by crossing a land bridge over what is now the Bering Strait began in September. Gene Savoy, the American explorer who is known for his 1964 discovery of the ruined Inca city of Vilcabamba in Peru, believes that American man originated in the jungles east of the Andes Mountains in South America, where he thinks advanced civilizations flourished as long ago as 1500 Bc. The discovery of a new species of human ancestors and of fossils of the oldest human beings yet to be found in Europe dominated the news in anthropology in 1995.
The discovery of fossils of a new species of a human ancestor-Australopithecus Anamensis-at sites near Lake Turkana in Kenya was announced in August. Anamensis, a small-scale brained upright walker resembling the famous Lucy skeleton (identified with the species’ Australopithecus afarensis), weighed about 110 pounds. The complete upper and lower jaws, a set of lower teeth, a skull fragment, the teeth of several individuals, and a shinbone were dated to between 4.1 million and 3.9 million years ago, according to Meave Leakey (wife of Richard Leakey), one of the accorded archeological researchers.
Anamensis: investigations point to what may seem directly ancestral too consequential afarensis (dated at 3.6 million years old). The shinbone abides of the oldest overseer was evidentially discovered for uprightness: a bipedal locomotion with the ability to walk on two legs: a defining trait of humans. The earliest known evidence before this was the track (3.7 million years old) of three humanlike individuals, probably australopithecines, who strolled across a bed of fresh volcanic ash in what is now Laetoli, Tanzania.
The relationship between Anamensis (from anam, a native Kenyan term for ‘lake’) and an even older species whose discovery was announced in 1994 was unclear. The older species, found in the Middle Awash region of Ethiopia, was first named Australopithecus ramidus. The genus name was later changed to Ardipithecus (‘ground apes’). The teeth and scanty bone fragments of Ardipithecus ramidus were dated at 4.4 million years old.
Fragmentary fossil remains of at least four humans thought to be intermediate between Homo erectus and archaic forms of The Homo sapiens, the later species to which all modern humans belong, were found in caves in Atapuerca in northern Spain, according to a report published in August. Dated as at least 780,000 years old by means of a Paleomagnetic dating technique, the stone tools and skeletal fragments -including some from an adolescent and some from a child-of skulls, hands, and feet represent the oldest humans yet discovered in Europe. The researchers who found the fossils said they could possibly be distant ancestors of the Neanderthals who appeared in Europe hundreds of thousands of years later.
The Spanish fossils partly fill a gap in the history of human evolution and expansion around the world. Previously, the oldest human fossils found in Europe, dating back 500,000 years, belonged to Heidelberg man, a likely ancestor of the Neanderthals, found at the Mauer site in Germany near the French border. It is known, however, that descendants of the earliest humans had spread from Africa to Asia well more than a million years ago. Among reasons given by anthropologists for the late occupation of Europe by Homo is the harshness of Europe's Ice Age climate.
Finds of Neanderthaloid skulls and skeletons continue to be reported from widely separated areas. Digging in a cave at Mount Circeo on the Tyrrhenian sea, 50 miles south of Rome, Italy, Alberto Carlo Blanc uncovered an almost perfectly preserved Neanderthal skull, perfect except a fracture in the right temporal region. It is the third of this type found in Italy. The two skulls previously reported were found in 1929 and 1935 in the Sacopastore region, near Rome, but in not nearly so well preserved a condition as the present find. No other human bones were found here, but the skull was accompanied by fossilized bones of elephants, rhinoceri, and giant horses, all fractured, thus giving some evidence of the mode of life of Neanderthal man. Professor Sergio Sergi, of the Institute of Anthropology at the Royal University of Rome, who has studied this skull in detail believes it to be 70,000 to 80,000 years old. He concludes also that Neanderthal man walked comparably as modern man and not with head thrust forward as had previously been assumed.
Another Neanderthal skeleton is reported to have been found in a cave in Middle Asia by A. P. Okladnikoff of the Anthropological Institute of Moscow University and the Leningrad Institute of Anthropology. The bones of the skeleton were badly shattered, but the jaw and teeth of the skull, it crushed at the back, were almost complete
Hominids that were contemporary with Oldowan sites included two major lineages. Its first functional lines are the robust australopithecines (so called because their cheek teeth were larger than those of other australopithecines). This robust australopithecines-such as Australopithecus aethiopicus and Australopithecus boisei in East Africa, and Australopithecus robustus in South Africa-were bipedal and had small brains, large jaws, and large molars. The other lineage is made up of bipedal, larger brain size, and smaller toothed early members of the genus Homo, such as the Homo habilis, Homo’s rudolfensis, and early Homo erectus. The oldest fossils of The Homo erectus (sometimes called Homo ergaster) found in Africa dates back to about 1.85 million years ago. This species is characterized by an even larger brain and smaller teeth than earlier hominids and by a larger body size. (In 1984 anthropologists in Kenya found a nearly complete skeleton of an adolescent Homo erectus who would have been 1.8 m.’s
(6 ft.) tall as an adult.)
Experts are intuitively certain that these species were responsible for individual Oldowan sites. These species may have made and used Oldowan style stone tools to varying degrees. However, anthropologists have long suspected that the larger-brained and smaller-toothed Homo was probably a more habitual toolmaker. It is likely that Homo erectus was responsible for many Oldowan sites more recent than 1.85 million years ago. In any case, by one million years ago, all these species but The Homo erectus had gone extinct, so researchers can be certain that at least the Homo’s lineage was involved in using and making stone tools. The Homo erectus appears to have moved out of Africa and into Eurasia sometime before one million years ago, although some anthropologists think this geographic spread of hominids may have occurred nearly two million years ago.
The everyday life of Oldowan hominids is largely a matter of archaeological conjecture. Most sites in East Africa are found near lakes or along streams, suggesting that they preferred to live near water sources. Studies of rock sources call to mind that Oldowan hominids sometimes transported stone several kilometres to the sites where stone artifacts are found. Well-preserved sites often have collections of stone artifacts and fragmented fossil animal bones associated together, often in dense concentrations of several thousand specimens. Scholars disagree regarding the nature of these sites. Some archaeologists interpret them as camps, home bases, or central foraging places, similar to those formed by modern hunter-gatherers during their daily activities. Others think that such sites represent scavenging stations where hominids were primarily involved in processing and consuming animal carcasses. Still others view these accumulations as stone caches where hominids collected stone in areas where such raw materials did not occur naturally.
Fossil remains from some Oldowan sites suggest that Oldowan hominids used stone tools to process meat and marrow from animal carcasses, some weighing several hundred pounds. Although some archaeologists have argued that large game hunting may have occurred in the Oldowan, many Oldowan specialists believe these early Stone Age hominids likely obtained most of their meat from large animals primarily through scavenging. The early hominids may have hunted smaller animals opportunistically, however. Modern experiments have shown that sharp Oldowan flakes are especially useful for the processing of animal carcasses -for example, skinning, dismembering, and non-anaesthetized defleshing. The bulk of early hominid diet likely consisted of a variety of plant foods, such as berries, fruits, nuts, leaves, flowers, roots, and tubers, but there are little archaeological records of such perishable foodstuffs.
The term Acheulean was first used by 19th- century French archaeologist Gabriel de Mortillet to refer to remnants of a prehistoric industry found near the town of Saint Acheul in northern France. The distinguishing feature of this site is an abundance of stone hand axes, tools more sophisticated than those found at Oldowan sites. The term Acheulean is now used to refer to hand axe industries in Africa, the Near East, Europe, and Asia dating near 1.5 million years ago to 200,000 years ago and spanning human evolution from A Homo erectus to early archaic Homo sapiens.
The characteristic Acheulean hand axe is a large, pointed or oval shaped form. These hand axes were often made by striking a blank (a rough chunk of rock) from a larger stone and then shaping the blank by carefully removing flakes around its perimeter. Usually, both sides, or faces, of the blank were flaking, a process called bifacial flaking. Later Acheulean hand axes may have been produced by the soft hammer technique, in which a softer hammer of stone, bone, or antler produced thinner, more carefully shaped forms. Other associated forms include cleavers, bifacial artifacts with a sharp, guillotine like bit at one end, its profound thickness and directed artifacts known as picks. Simpler, typical Oldowan artifacts are usually also found at Acheulean sites, and a range of retouched flake tools such as scrapers. Experiments have implicated that Acheulean hand axe and cleavers are excellent tools for heavy-duty slaughtering activities, such as severing animal limbs. Some archaeologists, however, believe they may have served other functions, or perhaps were general, all purpose tools.
Acheulean tools did not entirely replace Oldowan tools. Archaeologists have discovered many sites where Oldowan tools were used throughout the Acheulean time, sometimes in the same geographic region as Acheulean industries. Interestingly, the Acheulean might be especially restricted to Africa, Europe, and western Asia, with few sites in East Asia of stone industries with typical Acheulean hand axes and cleavers during the Lower Palaeolithic. Most of the industries found in East Asia have tendencies toward being simpler, Oldowan like technologies that can be seen at sites at Nihewan and the cave of Zhoukoudian in northern China.
Well-studied Acheulean sites include those at Olduvai Gorge and Isimila, in Tanzania, Olorgesailie, in Kenya, Konso Gardula and Melka Kunture, in Ethiopia: Kalambo Falls, in Zambia, Montagu Cave in South Africa, and Tabun and Gesher Benot Ya'aqov in Israel, Abbeville and Saint Acheul in France, and Swanscombe and Boxgrove, in England, Torralba and Ambrona, of Spain. Most anthropologists think that Acheulean populations of The Homo erectus and early Homo sapiens were probably more efficient hunters than Oldowan hominids. Recently discovered wooden spears from about 400,000 years ago at Schöningen, Germany, and a 300,000-year old wooden spear tips from Clacton, England, suggest that the hominids who made applicable may have hunted game extensively.
Experts disagree about whether Acheulean hominids and their contemporaries harnessed the use of fire. Archaeologists have found evidence such as apparent burnt bone and stone, discoloured sediment, and the presence of charcoal or ash at most sites, including Cave of Hearths, in South Africa, Zhoukoudian, in China, and Terra Amata in France. Discrete fireplaces (hearths), however, may be quite rare. Similarly, there is only questionable evidence of huts or other architectural features.
The Middle Palaeolithic Epoch extends from around 200,000 years ago until about 30,000 years ago. It is also called the Mousterian Industry in Europe, the Near East, and North Africa and called the Middle Stone Age in sub-Saharan Africa.
Toolmaker in the Middle Palaeolithic used a range of retouched flake tools, especially sided scrapers, serrated scrapers, backed knives (blade tools with the non-blade and very much as to be dull sided in the apparatuses to fit comfortably in the hand), and points. Experts believe these tools were used to work animal hides, to shape wood tools, and as projectile points. This period is also characterized by specially prepared cores. Using the disc core method, a circular core could produce many flakes to serve as blanks for retouched tools. With the Levallois method (named after a suburb of Paris, France, where the first such artifacts were discovered), flakes of a predetermined shape were removed from specially prepared cores. This process resulted in ovally-shaped flakes or large, triangular points, depending on the type of Levallois core. Levallois core and flakes are first seen at some late Acheulean sites but become much more common in the Middle Palaeolithic/Middle Stone Age.
Some regional variation can be seen among Middle Palaeolithic industries. A North African variant known as African produced tools and point characterized by tangs (stems projecting from the base of the tool or point, to allow the tool to be attached to a handle or shaft). In Eastern Europe, a variant called Szeletian produced two-sided, leaf-shaped points, a style not usually seen elsewhere until the Upper Palaeolithic Age. In Central Africa, a variant called the Sangoan produced a range of heavy-duty picks and axes.
Middle Palaeolithic/Middle Stone Age archaeological sites are often found in the deposits of caves and rock shelters. Well-studied caves include Pech de l'Aze, Combe Grenal, La Ferrassie, La Quina, and Combe Capelle, in France. Tabun, Kebara, Qafzeh, and Skhul, in Israel, Shanidar, in Iraq, Haua Fteah, in Libya and Klasies River Mouth, in South Africa, also in East Asia, site that are contemporary with the Middle Palaeolithic Ages, are often too, exhibit a simpler Toolmaking technology, without as much standardization of the flake tool forms as in much of the rest of Eurasia and Africa.
Hominids associated with the Middle Palaeolithic Epoch is to include Neanderthals and other archaic Homo sapiens (Homo sapiens predating anatomically modern humans, who lived from about 200,000 to 35,000 years ago). In Europe, the Middle Palaeolithic Age is associated with Homo sapiens neanderthalensis, or Neanderthals, who lived from about 200,000 to 35,000 years ago. Neanderthals were short, robust humans with fully modern cranial capacity. They had more jutting faces, more prominent brow ridges, thicker cranial bones, and larger nose cavities than modern humans. Skeletal remains show that Neanderthals were very robust and muscular. Healed injuries to some skeletons suggest that Neanderthals led stressful, rigorous lives. Famous Neanderthal discoveries include Neander Valley, in Germany, La Chapelle-aux-Saints and La Ferrassie, in France, Krapina, in Croatia, Monte Circeo and Saccopastore, in Italy, Shanidar, in Iraq and Tabun and Amud, in Israel. Fossils of an archaic Homo sapiens from this time have been found at sites such as Dali and Maba, in China and at Florisbad, in South Africa, and Ngaloba, in Tanzania. In addition, fossils interpreted as early anatomically modern humans have been found at some Middle Palaeolithic/Middle Stone Age sites in parts of Africa and the Near East, such as at Qafzeh and Skhul, in Israel, and Klasies River Mouth, in South Africa.
Middle Palaeolithic hominids appear to have been more successful hunters than their predecessors. Abundant animal remains suggest that these hominids ate many kinds of large mammals. It is unknown, however, how much of the meat consumed was obtained through hunting, as opposed to scavenging. Accumulations of remains at some sites show that some animals were of a common species and were adults in their prime, which some researchers suggest is an indication of efficient hunting behaviour. Several sites in Europe that contain the carcass of one or more large animals are believed to be butchery sites, where early humans processed the spoils of kills. Some archaeologists have also argued that some Middle Palaeolithic stone points were probably attached to spears, a development in hunting technology. At Klasies River Mouth Cave in South Africa, archaeologists discovered a buffalo vertebra with a broken tip of what was probably a spearhead embedded in it, which could be evidence that the large mammal was hunted or trapped by hominids.
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