The first humans

The first humans:Modern humans belong to the mammalian order of primates, which originated about 55 million years ago; primates included the only extant hominid species. The earliest hominid was Ardipithecus (“ground ape”) and Australopithecus (“southern ape”), both small-brained intermediates between apes and humans that were capable of standing and walking upright. Homo habilis, the earliest member of the genus Homo, appeared at least 2 million years ago. This larger-brained “handy man” began making tools for hunting. Homo ergaster first appeared in Africa about 1.8 million years ago and spread into Asia about 800,000 years later. Smaller-toothed than Homo habilis, H. ergaster—followed by Homo erectus—developed fire as a tool, which enabled it to cook food. Neanderthals, a near relative of modern humans, originated about 200,000 years ago, and Homo sapiens (modern humans) appeared in Africa about 100,000 years later. The two coexisted for thousands of years, but by 30,000 years ago, Homo sapiens had become dominant and the Neanderthals had died out. Classification of Homo sapiens in relation to its ancestors is enormously problematic: modern humans must be classified not only by bone structure, but also by specific
behavior—the ability to plan future action; to follow traditions; and to use symbolic communication, including complex language and the ability to use and recognize symbols.

Mammals 1

Mammals 1:Since the extinction of most of the dinosaurs 65 million years ago, mammals (along with birds) have been the dominant vertebrates on land. This class includes terrestrial, aerial, and aquatic forms. Having developed from the therapsids, the first true mammals— small, nocturnal, shrewlike creatures, such as Megazostrodonappeared over 200 million years ago during the Triassic period
(250–200 million years ago). Mammals had several features that differed from those of their ancestors: an efficient four-chambered heart allowed these warm-blooded animals to sustain high levels of activity; a covering of hair helped them maintain a constant body temperature; an improved limb structure gave them more efficient locomotion; and the birth of live young and the immediate supply of food from the mother’s milk aided their rapid growth. Since the end of the Mesozoic era (65 million years ago), the number of major mammal groups and the abundance of species in each
have varied dramatically. For example, the Perissodactyla (the group that includes Coelodonta and modern horses) was a common group during the Early Tertiary period (about 54 million years ago).
Today, the mammalian groups with the most species include the Rodentia (rats and mice), the Chiroptera (bats), the Primates (monkeys and apes), the Carnivora (bears, cats, and dogs), and the Artiodactyla (cattle, deer, and pigs), while the Proboscidea group, which formerly included many genera, such as Phiomia, Moeritherium, Tetralophodon, and Mammuthus, now has only three species of elephant. In Australia and South America, millions of years of continental isolation led to increased diversity of the marsupials, a group of mammals distinct from the placentals that existed elsewhere.

Marginocephalians 1

Marginocephalians 1: were a group of bipedal and quadrupedal ornithischian dinosaurs with a narrow
shelf or deep, bony frill at the back of the skull. Marginocephalians were probably descended from the same ancestor as the ornithopods and lived in what are now North America, Africa, Asia, and Europe during the Cretaceous period (145–65 million years ago). They were divided into two groups: Pachycephalosauria (“thick -headed lizards”), such as Pachycephalosaurus and Stegoceras, and Ceratopsia (“horned faces”), such as Triceratops and Psittacosaurus. The thick skulls of Pachycephalosauria may have protected their brains during possible head-butting contests fought to win territory and mates; their hips and spines may also have been strengthened to withstand the shock. The bony frill of Ceratopsia would have added to their frightening appearance when charging; the neck was strengthened for impact and to support the huge head, with its snipping beak and powerful slicing toothed jaws. A charging ceratopsian would have been a formidable opponent for even the largest predators. Ceratopsians were among the most abundant herbivorous dinosaurs of the
Late Cretaceous period (97–65 million years ago).

Ornithopods 1

Ornithopods 1 were a group of ornithischian (“bird-hipped”) dinosaurs. These bipedal and quadrupedal herbivores had a horny beak, plant-cutting or grinding cheek teeth, and a pelvic and tail region stiffened by bony tendons. They evolved teeth and jaws adapted to pulping vegetation and flourished from the Middle Jurassic to the Late Cretaceous period (165–65 million years ago) in North America, Europe, Africa, China, Australia, and Antarctica. Some ornithopods were no larger than a dog, while others were immense creatures up to 49 ft (15 m) long. Iguanodonts, an ornithopod group, had a broad, toothless beak at the end of a long snout, large jaws with long rows of ridged,
closely packed teeth for grinding vegetation, a bulky body, and a heavy tail. Iguanodon and some other iguanodonts had large thumb-spikes that were strong enough to stab attackers. Another group, the hadrosaurs, such as Gryposaurus and Hadrosaurus, lived in Late Cretaceous times (97–65 million years ago) and with their broad beaks are sometimes known as “duckbills.” They were characterized by their deep skulls and closely packed rows of teeth, while some, such as Corythosaurus and
Lambeosaurus, had tall, hollow, bony head crests.

Thyreophorans 1

Thyreophorans 1: SHIELD BEARERS were a group of quadrupedal armored dinosaurs. They were one clade among several within the Ornithischia (bird-hipped dinosaurs), they were characterized by rows of bony studs, plates, or spikes along the back, which protected some from predators and may have helped others regulate body temperature. Up to 30 ft (9 m) long, with a small head and small cheek teeth, thyreophorans had shorter forelimbs than hind limbs and probably browsed on low-level vegetation. The earliest thyreophorans were small and lived in Early Jurassic times (about 200 million years ago) in Europe, North America, and China. Stegosaurs, such as Stegosaurus and Kentrosaurus, replaced these older forms. The earliest stegosaur remains come mainly from China. Several genera of stegosaurs survived into the Early Cretaceous period (145–100 million years ago).
Ankylosaurs, with a combination of beak and teeth in close proximity, and cheek teeth adapted for
cropping vegetation, appeared at the same time as stegosaurs. They originated in the Late Jurassic period (155 million years ago) and in North America survived until 65 million years ago.

Sauropodomorphs 1

Sauropodomorphs 1: (“lizard-feet forms”) were herbivorous, usually quadrupedal (four-footed)
dinosaurs. A suborder of the Saurischia, they were characterized by small heads, bulky bodies, and
long necks and tails. Sauropodomorphs have often been split into two groups: prosauropods and
sauropods. Prosauropods lived from Late Triassic to Early Jurassic times (225–180 million years ago) and included beasts such as the small Anchisaurus and one of the first very large dinosaurs, Plateosaurus. By Middle Jurassic times (about 165 million years ago), sauropods had replaced prosauropods and spread worldwide. They included the heaviest and longest land animals ever, such as Diplodocus and Brachiosaurus. Sauropods persisted to the end of the Cretaceous period (65 million years ago). Many of these dinosaurs moved in herds, protected from predatory theropods by their huge bulk and powerful tails, which they could use to lash out at attackers. Sauropodomorphs
were the most common large herbivores until Late Jurassic times (about 145 million years ago), and appear to have survived in both southern and northern continents until the end of the Cretaceous period.

Theropods 1

Theropods 1:An enormously successful subgroup of the Saurischia, the bipedal (two-footed) theropods (“beast feet”) emerged 230 million years ago in Late Triassic times; the oldest known example comes from South America. Theropods spanned the age of most dinosaurs (230–65 million years ago) and beyond, and included most of the known predatory dinosaurs. The typical theropod had smallish arms with sharp, clawed fingers; powerful jaws lined with sharp teeth; an S-shaped neck; long, muscular hind limbs; and clawed, usually four-toed feet. Many theropods may have been warm-blooded; most were exclusively carnivorous. Theropods ranged from animals no larger than
a chicken to huge creatures, such as Tyrannosaurus and Baryonyx. The group also included ostrichlike omnivores and herbivores with toothless beaks, such as Struthiomimus and Gallimimus. Birds are dinosaurs and evolved from within a group of tetanuran theropods called maniraptorans.
Archaeopteryx, small and feathered, was the first known bird and lived alongside other dinosaurs.

The dinosaurs

The dinosaurs:The dinosaurs were a large group of reptiles that were the dominant land vertebrates (animals with backbones) for most of the Mesozoic era (245–65 million years ago). They appeared some 230 million years ago and were distinguished from other scaly, egg-laying reptiles by an important feature: dinosaurs had an erect limb stance. This enabled them to keep their bodies well above the ground, unlike the sprawling and semisprawling stance of other reptiles. The head of the
dinosaur’s femur (thighbone) fit into a socket in its pelvis (hip-bone), producing efficient and mobile
locomotion. Dinosaurs are categorized into two groups according to the structure of their pelvis: saurischian (lizard-hipped) and ornithischian (bird-hipped) dinosaurs. In the case of most saurischians, the pubis (part of the pelvis) jutted forward, while in ornithischians it slanted back, parallel to the ischium (another part of the pelvis). Dinosaurs ranged in size from smaller than a domestic cat to the biggest land animals ever known. The Dinosauria were the most successful land
vertebrates ever, and survived for 165 million years, until most became extinct 65 million years ago.

Amphibians and reptiles

Amphibians and reptiles: The earliest known amphibians, such as Acanthostega and Ichthyostega, lived about 363 million years ago at the end of the Devonian period (409–363 million years ago). Their limbs may have evolved from the muscular fins of lungfishlike creatures. These fish can use their fins to push themselves along the bottom of lakes and some can breathe at the water’s surface. While amphibians can exist on land, they are dependent on a wet environment because their skin does
not retain moisture and most species must return to the water to lay their eggs. Evolving from amphibians, reptiles first appeared during the Carboniferous period (363–290 million years ago): Westlothiana, a possible early reptile, lived on land 338 million years ago. The development of the amniotic egg, with an embryo enclosed in its own wet environment (the amnion) and protected by a waterproof shell, freed reptiles from the amphibian’s dependence on a wet habitat. A scaly skin
protected the reptile from desiccation on land and enabled it to exploit ways of life closed to its amphibian ancestors. Reptiles include the dinosaurs, which came to dominate life on land during the Mesozoic era (245–65 million years ago).

Early signs of life

Early signs of life:For almost a thousand million years after its formation, there was no known life on Earth. The first simple, sea-dwelling organic structures appeared about 3.5 billion years ago; they may have formed when certain chemical molecules joined together. Prokaryotes, single-celled microorganisms such as blue-green algae, were able to photosynthesize, and thus produce oxygen. A thousand million years later, sufficient oxygen had built up in the Earth’s atmosphere to allow
multicellular organisms to proliferate in the Precambrian seas (before 570 million years ago). Soft-bodied jellyfish, corals, and seaworms flourished about 700 million years ago. Trilobites, the first animals with hard body frames, developed during the Cambrian period (570–510 million years ago). However, it was not until the beginning of the Devonian period (409–363 million years ago) that early land plants, such as Asteroxylon, formed a waterretaining cuticle, which ended their dependence on an aquatic environment. About 360 million years ago, the first amphibians crawled onto the land, although they probably still returned to the water to lay their soft eggs. By the time the first reptiles and synapsids appeared late in the Carboniferous, animals with backbones had become fully independent of water.

Quaternary period

Quaternary period (1.6 million years ago–present) forms the second part of the Cenozoic era (65 million years ago–present): it has been characterized by alternating cold (glacial) and warm (interglacial) periods. During cold periods, ice sheets and glaciers have formed repeatedly on northern and southern continents. The cold environments in North America and Eurasia, and to a lesser extent in southern South America and parts of Australia, have caused the migration of many life-forms toward the Equator. Only the specialized ice age mammals such as Mammuthus and Coelodonta, with their thick wool and fat insulation, were suited to life in very cold climates. Humans developed throughout the Pleistocene period (1.6 million–10,000 years ago) in Africa and migrated northward into Europe and Asia. Modern humans, Homo sapiens, lived on the cold European continent 30,000 years ago and hunted other mammals. The end of the last ice age and the climatic changes that occurred about 10,000 years ago brought extinction to many Pleistocene mammals, but enabled humans to flourish.

Tertiary period

Tertiary period:Following the demise of the dinosaurs at the end of the Cretaceous period, the Tertiary period (65–1.6 million years ago), which formed the first part of the Cenozoic era (65 million years ago–present), was characterized by a huge expansion of mammal life. Placental mammals nourish and maintain the young in the mother’s uterus; only a few groups of placental mammals existed during Cretaceous times, compared with a few dozen during the Tertiary period. One of these included the first hominid, Ardipithecus, which appeared in Africa. By the beginning of
the Tertiary period, the continents had almost reached their present position. The Tethys Sea, which had separated the northern continents from Africa and India, began to close up, forming the Mediterranean Sea and allowing the migration of terrestrial animals between Africa and western Europe. India’s collision with Asia led to the formation of the Himalayas. During the middle part of the Tertiary period, the forest-dwelling and browsing mammals were replaced by mammals such as the horses, better suited to grazing the open savannahs that began to dominate. Repeated cool periods throughout the Tertiary period established the Antarctic as an icy island continent.

Cretaceous period

Cretaceous period:The mesozoic era ended with the Cretaceous period, which lasted from 146 to 65 million years ago. During this period, Gondwana and Laurasia were breaking up into smaller landmasses that more closely resembled the modern continents. The climate remained mild and
moist but the seasons became more marked. Flowering plants, including deciduous trees, replaced many cycads, seed ferns,and conifers. Animal species became more varied, with the
evolution of new mammals, insects, fish, crustaceans, and turtles. Dinosaurs evolved into a wide variety of species during Cretaceous times; more than half of all known dinosaurs—including Iguanodon, Deinonychus, Tyrannosaurus, and Hypsilophodon—lived during this period. At the end
of the Cretaceous period, however, most dinosaurs became extinct. The reason for this mass extinction is unknown but it is thought to have been caused by climatic changes due to either a catastrophic meteor impact with the Earth or extensive volcanic eruptions.

Jurassic period

Jurassic period: The middle part of the Mesozoicera, lasted from 199 to 145 million years ago.During Jurassic times, the landmass of Pangaea broke up into the continents of Gondwana and Laurasia, and sea levels rose, flooding areas of lower land. The Jurassic climate was warm and moist. Plants such as ginkgos, horsetails, and conifers thrived, and giant redwood trees appeared, as did the first flowering plants. The abundance of plant food coincided with the proliferation of herbivorous (plant-eating) dinosaurs, such as the large sauropods (e.g., Diplodocus) and stegosaurs (e.g., Stegosaurus). Carnivorous (flesh eating) dinosaurs, such as Compsognathus and Allosaurus, also flourished by hunting the many animals that existed—among them other dinosaurs. Further Jurassic animals included shrewlike mammals, and pterosaurs (flying reptiles), as well as plesiosaurs and ichthyosaurs (both marine reptiles).

Triassic period

Triassic period:Triassic period  (250–200 million years ago) marked the beginning of what is known as the Age of the Dinosaurs (the Mesozoic era). During this period, the present-day continents were massed together, forming one huge continent known as Pangaea. This landmass experienced extremes of climate, with lush green areas around the coast or by lakes and rivers, and arid deserts in the interior. The only forms of plant life were nonflowering plants, such as conifers, ferns, cycads, and ginkgos; flowering plants had not yet evolved. The principal forms of animal life included diverse, often gigantic, amphibians, rhynchosaurs (“beaked lizards”), and primitive crocodilians. Dinosaurs first appeared about 230 million years ago, at the beginning of the Late Triassic period. Among the earliest dinosaurs were the carnivorous (flesh-eating) herrerasaurids, such as Herrerasaurus and Staurikosaurus. Early herbivorous (plant-eating) dinosaurs first appeared in Late Triassic times and included Plateosaurus and Technosaurus. By the end of the Triassic period, dinosaurs dominated Pangaea, possibly contributing to the extinction of many other reptiles.

Carboniferous to Permian periods

Carboniferous to Permian periods:(363–290 million years ago) takes its name from the thick, carbon-rich layers—now coal—that were produced during this period as swampy tropical forests were repeatedly drowned by shallow seas. The humid climate across northern and equatorial continents throughout Carboniferous times produced the first dense plant cover on Earth. During the early
part of this period, the first reptiles appeared. Their development of a waterproof egg with a protective internal structure ended animal life’s dependence on an aquatic environment. Toward the end of Carboniferous times, the earth’s continents Laurasia and Gondwana collided, resulting in the huge landmass of Pangaea. Glaciers smothered much of the southern hemisphere during the Permian period (290–245 million years ago), covering Antarctica, parts of Australia, and much of South America, Africa, and India. Ice locked up much of the world’s water and large areas of the northern hemisphere experienced a drop in sea level. Away from the poles, deserts and a hot dry climate predominated. As a result of these conditions, the Permian period ended with the greatest mass extinction of life on Earth ever.

Precambrian to Devonian periods

Precambrian to Devonian periods:When the earth formed about 4.6 billion years ago, its atmosphere consisted of volcanic gases with little oxygen, making it hostile to most forms of life. One large supercontinent, Gondwana, was situated over the southern polar region, while other smaller continents were spread over the rest of the world. Constant movement of the Earth’s crustal plates carried continents across the earth’s surface. The first primitive life-forms emerged around 3.4 billion years ago in shallow, warm seas. The build up of oxygen began to form a shield of ozone around the Earth, protecting living organisms from the Sun’s harmful rays and helping to establish an atmosphere in which life could sustain itself. The first vertebrates appeared about 470 million years ago, during the Ordovician period (510–439 million years ago), the first land plants appeared around 400 million years ago during the Devonian period (409–363 million years ago), and the first land animals about 30 million years later.

Mountain building

Mountain building:The processes involved in mountain building—termed orogenesis—occur as a result of the movement of the Earth’s crustal plates.There are three main types of mountains: volcanic mountains, fold mountains, and block mountains. Most volcanic mountains have been formed along plate boundaries where plates have come together or moved apart and lava and other debris have been ejected onto the Earth’s surface. The lava and debris may have built up to form a dome around the vent of a volcano. Fold mountains are formed where plates push together and cause the rock to buckle upward. Where oceanic crust meets less dense continental crust, the oceanic crust is forced under the continental crust. The continental crust is buckled by the impact. This is how folded mountain ranges, such as the Appalachian Mountains in North America, were formed. Fold mountains are also formed where two areas of continental crust meet. The Himalayas, for example, began to form when India collided with Asia, buckling the sediments and parts of the oceanic crust between them. Block mountains are formed when a block of land is uplifted between two faults as a result of compression or tension in the Earth’s crust. Often, the movement along faults has taken place gradually over millions of years. However, two plates may cause an earthquake by suddenly sliding past each other along a faultline.

Faults and folds

Faults and folds:The continuous movement of the Earth’s crustal plates can squeeze, stretch, or break rock strata, deforming them and producing faults and folds. A fault is a fracture in a rock along which
there is movement of one side relative to the other. The movement can be vertical, horizontal, or oblique (vertical and horizontal). Faults develop when rocks are subjected to compression or tension. They tend to occur in hard, rigid rocks, which are more likely to break than bend. The smallest faults occur in single mineral crystals and are microscopically small, whereas the largest—the Great Rift Valley in Africa, which formed between 5 million and 100,000 years ago—is more than 6,000 miles (9,000 km) long. A fold is a bend in a rock layer caused by compression. Folds occur in elastic rocks, which tend to bend rather than break. The two main types of fold are anticlines (upfolds) and synclines (downfolds). Folds vary in size from a few millimeters long to folded mountain ranges hundreds of miles long, such as the Himalayas and the Alps, which are repeatedly folding. In addition
to faults and folds, other features associated with rock deformations include boudins, mullions, and en échelon fractures.

The Earth’s crust

The earth's crust is the solid outer shell of the Earth. It includes continental crust (about 25 miles/40 km thick) and oceanic crust (about four miles/6 km thick). The crust and the topmost layer of the mantle form the lithosphere. The lithosphere consists of semirigid plates that move relative to each other on the underlying asthenosphere (a partly molten layer of the mantle). This process is known as plate tectonics and helps explain continental drift. Where two plates move apart, there are rifts in the crust. In mid-ocean, this movement results in seafloor spreading and the formation of ocean ridges; on continents, crustal spreading can form rift valleys. When plates move toward each other, one may be subducted beneath (forced under) the other. In mid-ocean, this causes ocean trenches, seismic activity, and arcs of volcanic islands. Where oceanic crust is subducted beneath continental crust or where continents collide, land may be uplifted and mountains formed (see pp. 62–65). Plates may also slide past each other—along the San Andreas fault, for example. Crustal movement on continents may result in earthquakes, while movement under the seabed can lead to tidal waves.