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.
No comments:
Post a Comment