History of life

HISTORY
OF LIFE

The map shows the configuration of the continents at the beginning of the Cambrian, the point in time from which the history of life can be traced with some certainty. In Precambrian times most animals were soft-bodied and are only rarely preserved.

The earth has existed for about 5000 million years and has been populated by organisms of one sort or another for between 3500 - 4000 million years of that time. However, an accurate fossil record of the earth's life can only be traced back some 620 million years to the time when hard skeletons first came into existence. At that time life was present only in the sea and the land was barren. The distribution of land and sea was not as it is today. The configuration of the continents and oceans is constantly changing due to a mechanism called plate tectonics. The earth's crust is made up of a number of plates, like the panels of a football. These plates are formed continually along one edge, where material wells up from the earth's interior,
and are destroyed at another, where one plate slides beneath a neighbouring one and is lost. The upwelling takes place along mid-oceanic ridges and the destruction occurs along deep-sea troughs. The material involved consists of oceanic crust, rich in silica and magnesia. The continents are made of a different sort of crust, rich in silica and aluminium, which, being lighter, remains on top so that the continents are carried here and there over the globe by plate-tectonic activity. This process has continued throughout geological time and will continue until the end of the world. The importance of plate tectonics to the history of life on earth is not just one of geography. Plate tectonics in part affects the pattern of global climate, which in geological terms fluctuates over a comparatively short space of time, and has undoubtedly contributed to the relatively sudden changes that have occurred in the predominant life form on earth. The juxtaposition of continents at crucial stages in the development of these animals has at certain times been important in their spread throughout the world and has produced marked differences between forms found on different land masses.


HISTORY OF LIFE
THE ORIGINS OF LIFE

The sun and the solar system were formed from a vast shapeless cloud of interstellar gas, spinning slowly in space at a rate of about once in ten million years. As it rotated it began to contract under the influence of its own gravity and, in consequence, to rotate more rapidly. The forces involved flattened the gas cloud into a disc in which material became concentrated at the centre to form the sun. Across the disc, eddies appeared that began to accrete material, forming the foundations of what later became the planets. Dust particles consisting mainly of droplets of iron and particles of silica compounds began to solidify. The droplets coalesced into lumps and collected together in the eddies under the influence of gravity. The iron, being heavier, sank to the middle and the silica remained on the outside to give the protoplanets an iron core surrounded by a stony mantle. The inner planets - Mercury, Venus, Earth and Mars - were formed in this way. The other planets aggregated from lighter material such as carbon dioxide and ammonia, which condensed from the gas as the temperature continued to fall. At this time the compaction of materials in the early sun triggered off the process of nuclear fusion and the sun began to radiate energy - a process that has continued for the last 5000 million years and will continue for 5000 million years to come.

In the days before proper scientific investigation, man believed that the earth as he knew it and all the living creatures with which he was familiar were the result of a single supernatural act of creation that had been carried out at a particular date in the relatively recent past. Fossil sea creatures found far in land, which were later to provide evidence for major changes in the distribution of land and sea, were dismissed as being the result of a punitive flood.

It is possible that the earth's first atmosphere was rich in hydrogen, methane and ammonia, similar in composition to the atmospheres of the outer planets. As time went on water vapour and carbon dioxide would have been added to these gases by outgassing from the newly formed rocks. The water at first would have remained as a vapour since the heat of the atmosphere at this time would not have allowed it to condense. On the other hand, it is equally possible that the primeval atmosphere of hydrogen, methane and ammonia was mostly driven away by the heat of the sun soon after formation and that the earth's first stable atmosphere was composed chiefly of carbon dioxide and water vapour vented from the interior through fumaroles and volcanoes. In either case the water that condensed and fell as rain when the earth became cool undoubtedly contained molecules of ammonia, methane and hydrogen dissolved in it. If this solution was subjected to high-energy influxes such as lightning bolts or ultraviolet radiation from the sun, chemical reactions would have occurred that would have synthesized complex organic molecules such as amino acids - the materials from which living things are built.
On the other hand there may be a totally different explanation for the origin of complex organic molecules. Simple organic compounds such as formaldehyde are present in interstellar dust -particles of carbon produced in stellar explosions. Molecules of these organic substances may have accumulated on the particles and have subsequently united into the long chemical chains of complex organic molecules that represent the first step in the chemistry of life. Gas emitted from stars may contain oxygen, carbon and nitrogen. If the gas contains more oxygen than carbon or nitrogen, organic molecules such as polysaccharides (simple sugars) may form. If nitrogen is the most abundant element the production of nucleic acids and chlorophyll - the energizing substance of growing plants - is more likely. Interstellar dust can, under the influence of gravitational forces, clump together, and in certain circumstances fall into orbit around a sun as a comet. If such a comet struck the earth in the early days of the planet's formation, as is more than likely, interstellar organic molecules would have reached the surface of our planet.
Whatever the case, it is certain that the hot seas on the steaming surface of the earth 4500 million years ago contained the complex organic molecules that are necessary for the building and development of living things.

Given suitable atmospheric and surface conditions it is possible for life to exist on a planet that falls within a belt around the Sun known as the ecosphere. The belt extends from just inside the orbit of Venus to just outside the orbit of Mars. Mercury, with a maximum surface temperature of 370C, is much too hot to support life, and the outer planets, becoming progressively colder through to Neptune and Pluto, where the maximum temperature is well below -200C, are much too cold.

The first thing on earth that could properly be termed 'alive' was a molecule with the unique property of reproducing itself. To do this it must have been able to break down complex molecules such as polysaccharides and use their constituent parts to build a mirror image of itself. Any characteristic of the basic molecule that helped it in this task would have enhanced its chances of survival and that characteristic would have been perpetuated in the replication process. Any feature that hindered it would have led to that molecule's extinction. Evolution had begun.
This activity continued until all the original polysacchandes present in the primeval 'soup' had been used up. The proto-organisms would have then run out of food had they not evolved the ability to synthesize their own from inorganic substances using the sun's energy. This process, known as photosynthesis, was made possible by the presence of the chlorophyll molecules.
Eventually more than one complex molecule became involved in each replicating body and there appeared the compact organic unit known as the cell. Some of the most primitive cells lacked a central nucleus, the site of the cells' reproductive machinery, and this function was instead spread throughout the cytoplasm. It was the cells with nuclei, however, that were to go on to greater things, and in the course of evolution smaller cells became incorporated into larger ones, remaining there to perform certain vital intercellular functions. Eventually complex structures arose consisting of more than one cell, each cell having its own particular role to play in keeping the whole unit alive. The organism had evolved.
The evolution of the first multi-celled organisms may have come about in one of two possible ways. Either by free-living cells of different types coming together as a single unit, or by cells failing to separate completely during subdivision and remaining together as a complex entity. Regardless of their formation, these multiple-celled organisms must as whole units have been more successful than the sum of their parts or they would not have survived.
The cells of multi-celled creatures are not identical and have quite different functions depending on the tissues or organs they constitute. In the higher forms of life, some are structural elements such as bone cells, others such as blood cells provide defence against disease and transport food, whereas others such as nerve cells form the organism's sensory and communications system. Cell differentiation in most cases occurs at the embrionic stage. To begin with an embryo's cells are all identical. The initial fertilized cell divides into two daughter cells which divide into four cells and so on until several hundreds, of identical cells have been produced. However at a particular point in the embryo's development this stage ceases and specific cells are produced that are designed to fulfil definite roles. It is unclear how this cell differentiation occurs. All cell nuclei contain the same genetic information, but only part of it is used in the production of a new cell. Some agent within the cell, most likely within the nucleus itself, must determine which piece of the genetic code is used to produce the new cell so that it can fulfil the function allotted to it.


HISTORY OF LIFE
EARLY LIVING FORMS

Two forms of symmetry exist in the invertebrate world, radial symmetry (A), in which animals are symmetrical about an axis running through them from top to bottom, and bilateral symmetry (B), in which animals are symmetrical about a plane running the length of their bodies.

Throughout the early oceans single-celled and multi-celled organisms, both plants and animals, flourished, The plants were able to absorb energy from the sun and to photosynthesize food from inorganic material. The animals unable to produce their own food directly from sunlight obtained energy by eating plants. This contrast in feeding methods is the basic difference between plants and animals, and is reflected in the structure and physiology of the two types of organism. Plants, needing only sunlight and inorganic materials, have no need to move if situated in favourable positions, and their cells are therefore stiff walled and rigid. They have flat, energy-absorbing surfaces (leaves) which orientate towards the sun, and anchoring structures (roots) through which they absorb nutrients and which also prevent them from being blown or washed away. Animals, on the other hand, need in most cases to move from one plant to another and have therefore evolved more flexible cell walls and muscular systems to make movement possible. They have developed sensory organs and nervous systems through which they evaluate their surroundings and by which they transmit messages to their muscles.
Associated with its power of movement is an animal's overall geometry. Those that are not just shapeless sedentary lumps filtering food from passing water currents have a symmetry that is either radial or bilateral.

The acorn worm (C), Balanoglossus spp., is a hemichordate, an intermediate stage between the invertebrates and the chordates - a group that includes the vertebrates. The similarity between the larvae of the acorn worm and that of starfish (A) and sea cucumbers (B), which are both echinoderms, may indicate the chordate's invertebrate ancestry.

At the beginning of the Cambrian period hard-shelled animals appeared for the first time in large numbers. As normally only a creature's shell becomes fossilized, the history of life is only well known from this time onwards. By the Cambrian all major groups (phyla) of animals, both radially and bilaterally symmetrical, had evolved. The animals with radial symmetry consisted of the coelentrates (jellyfish and corals) and the echinoderms (starfish and sea urchins). Those with bilateral symmetry fell into four main groups; the brachiopods - an almost extinct group of shellfish; the molluscs - bivalved shellfish, sea snails and nautilus-like cephalopods; the arthropods - represented primarily by the trilobites; and several classes of worms and worm-like creatures.
From one group of these worm-like animals, the chordates, came the first backboned animals in the Silurian - a class of primitive jawless fish and the ancestors of all vertebrates. At this time, too, the plants first came on to land. From shallow coastal waters emerged a group of plants that could survive without being totally immersed in water. They evolved stiff stems, to give them more support, and an internal plumbing system to transport water and dissolved minerals up from the ground and carry manufactured food down from the leaves.

The similarity between lobe-finned fish such as Eusthenopteron and early amphibians such as Ichthyostega gives a clear indication of amphibian ancestry. In Ichthyostega the fish's uniform spinal column has been replaced by a much heavier and stronger structure and a fully developed rib cage, capable of supporting the creature on dry land. Its limbs, although much longer than a fish's fins, are similar in form.

As a side effect of photosynthesis free oxygen was liberated into the atmosphere; the proportion of oxygen increased while that of carbon dioxide decreased, making the composition of air more congenial to animal life. The arthropods were the first animals to take advantage of the improved atmospheric conditions and both scorpions and millipedes existed among the early plants.
The succeeding Devonian period is known as the Age of Fishes. First to evolve from the primitive jawless types were placoderms such as Dinichthys the armoured fishes, which had jaws evolved from the bones of the gill arches. Before the end of the Devonian they were largely replaced by cartilaginous fish such as Cladoselache, the forerunners of the sharks and rays. Bony fish, more versatile and widely distributed, existed alongside these cartilaginous species. They formed two main groups - the ray-finned fish, which were to prove most successful, and lobe-finned fish such as Eusthenopteron. The last named is the most significant of the two from an evolutionary point of view. Living in shallow freshwater pools which periodically dried out gave them the evolutionary stimulus to survive out of water. When the pools disappeared Eusthenopteron dragged itself overland to the next area of water by means of a pair of muscular fins evolved from stabilizing organs. At these times it was able to breathe air through primitive lungs developed from outgrowths of the pharynx. Vertebrate life on land had begun, even though it was only as a temporary measure to allow the continuation of an aquatic existence. By the end of the Devonian the amphibians, able to spend most of their adult lives on land, had appeared. One of the earliest, Ichthyostega, showed the typical arrangement of five-toed limbs supported on strong girdles of bones found in land animals. It nevertheless retained fish-like features in the shape of the tail and skull.
The Carboniferous period that followed was the time of the great coal forests. It was also the Age of Amphibians; the lush swamps that characterized the lowlands of the period were ideal for their development and consequently a large number of new forms appeared. Some were small and eel-like, such as Dolichosoma, others such as Eogyrinus assumed an alligator-like form and existence. Still others, such as Diplocaulus, became broad and flattened and lived entirely in mud. The skulls of these creatures were more advanced than the fish-like structure of Ichthyostega. The nasal passages were well defined, indicating that they belonged to sophisticated air-breathing animals. These animals gave rise to both the later more highly advanced amphibians and to the reptiles.

The earliest fish were jawless (agnathans), their moutb being no more than openings to the digestive track. The jawed fish first appeared in the Devonian. The most primitive, the placoderms, were a highly diverse group of armoured species with jaws and teeth formed from bony head plates. Cartilage-skeletoned fish, the ancestors of the sharks and rays, also appeared at this time. The bony fish, the most successful group, also descended from the agnathans, can be divided into two classes, the lobe-finned fish, which had fleshy fins, and the ray-finned fish, which had fins composed of skin supported by horny fans. Most fish species present during the Age of Man belonged to the ray-finned class. The lobe-finned fish were represented by only four genera.


HISTORY OF LIFE
THE AGE OF REPTILES

The reptiles were the first completely land-living vertebrate animals on earth. The amphibians from which they had evolved were reasonably well adapted to life on land, but always had to return to the water to breed, and the immature stages always, of necessity, had to lead a completely aquatic tadpole existence. This meant, in effect, that amphibian colonization of the land was confined to swampy areas near coasts, lakes and the banks of rivers.
The reptilian development that extended this range was the development of the hard-shelled egg, which, by means of impermeable membranes, enabled the embryo animal to develop in its own private armoured pool away from water. In addition the reptiles also had tough skins that resisted desiccation to a much greater extent than those of the amphibians.

 



The earliest reptiles, known as the "stem reptiles", evolved from the amphibians in the Carboniferous and developed into a variety of forms that filled all the major environmental realms - air, land and water. The ichthyosaurs, plesiosaurs and mosasaurs were aquatic, the pterosaurs were aerial and the dinosaurs and mammal-like reptiles were terrestrial The dinosaurs (the terrible lizards) are classified in two groups according to the structure of their hips. Oddly, the birds are descended from the lizard-hipped group, not as one might expect from those with bird-like hips. As often occurs in nature, where lack of sophistication implies adaptability, the crocodile shape, one of the earliest reptilian forms to evolve, ultimately proved most successful.

Although the first reptiles appeared among the coal forests of the Carboniferous period, it was not until the Permian and Triassic that the reptiles really came into their own. As large parts of the earth became very much drier at this time there was a distinct advantage in being able to live away from water; precisely the conditions needed to encourage the reptiles to expand and diversify.

Two main groups of terrestrial reptiles evolved from reptile-like amphibians, such as Seymouria - the archosaurs and the mammal-like reptiles. Early archosaurs such as Ornithosuchus were bipedal, but many later members of the group adopted a four-footed stance. Mammal-like reptiles such as Thrinaxodon, although appearing early in the evolutionary history of the reptiles, survived to become the ancestors of the mammals.

The first really successful group were the mammal-like reptiles. These had a tooth pattern that was a great improvement over the simple fish-trap teeth of the amphibians. They had long killing teeth at the front of their mouths and shorter shearing teeth at the back; an adaptation to a carnivorous mode of life that anticipated the specialized tooth patterns of the mammals. The limbs moved to a mammal-like position beneath the body so that it was now supported at the top of the legs rather than slung between them, as was the case with the amphibians and the earliest reptiles. At the end of the Triassic period the mammal-like reptiles died out leaving, as their descendants, the true mammals that were to realize their full potential in another 100 million years. The dominant reptile niche was meanwhile taken by a group known as the archosaurs, to which the later dinosaurs belonged.
The archosaurs (the ruling reptiles) first appeared in Permian and Triassic times as semi-aquatic creatures rather like crocodiles in form with powerful hind legs and strong tails - characteristics that lasted throughout the history of the whole group. When during the Permian these semi-aquatic reptiles became readapted to life on land, they found that their long tails gave sufficient balance to permit them to walk on their hind legs - the basic bipedal stance of the dinosaur had evolved.
By the beginning of the Jurassic period, the dinosaur had become the dominant form of vertebrate life and had evolved into a wide variety of forms that had spread across the earth's entire habitable land area. There were large herbivores such as Diplodocus, lightly built, fast-moving carnivorous forms such as Deinonychus and much slower moving meat-eating scavengers such as Allosaurus. It is a mistake to believe that all dinosaurs were massive beasts. Compsog-nathus, which preyed mainly on small reptiles and eggs, was no larger than the domesticated chicken. The lightly built herbivorous dinosaurs, of which Camptosaurus was a typical representative, were a particularly important group and were the ancestors of several major forms, most notably the armoured varieties of the later Cretaceous. Dinosaur armour took a number of shapes and sizes depending on its function; vertical plates as in Stegosaurus, flat bony tubercules as on Ankylosaurus and head shields as in Triceratops.
None of these previously mentioned archosaur groups was ultimately to prove to be the most successful. As tends to be the case with any group of animals, the most primitive and un-specialized members are the most enduring and in the case of the
archosaurs the longest-living member was the early crocodile form, which outlasted the more spectacular dinosaurs by more than a hundred million years, surviving with modifications until well after the Age of Man.
In time the archosaurs even dominated the skies. The earliest fliers were the pterosaurs, creatures that glided through the air on wings of skin stretched between the elongated fourth finger of their forelimbs and their back legs and tail. There were small creatures such as Pterodactylus and Rhamphorhynchus, which may have fed on insects, larger forms such as Pteranodon and Nyctosaurus, which were fisheaters, and huge creatures such as Quetzalcoatlus that were almost certainly scavengers.
In the Jurassic another flying niche was taken by the descendants of one particular group of small, flesh-eating dinosaurs, similar to Compsognathus, which had feathers instead of scales covering at least parts of their bodies (almost certainly their limbs and tails). These were the ancestors of the true birds which dominated the skies in the later Cenozoic era.
During the period of dinosaur evolution, other reptile groups developed forms that were able to exploit the great food resources that existed in the sea. To return to the realm of their remote ancestors they had to re-evolve many of the physical features that had been lost during their adaptation to a land-based existence.
The group that contained the snakes and lizards of the Age of Man gave rise, at this time, to Tylosaurus, a massive-jawed, sea-living predator which propelled itself by sinuous movements of its long body and flattened tail and used its paddle-like limbs for steering. Plesiosaurus, a slow-moving creature with a long snake-like neck which it used to lash out at and capture fish, its main food, has no surviving ancestors. The evolutionary pinnacle of the aquatic reptiles was perhaps Ichthyosaurus, a reptile that looked more like a fish. It had a streamlined body, fins and a fish's tail.
Despite all these marvellous adaptations the large reptiles died out entirely at the end of the Cretaceous period. The reason is still unclear, but their extinction coincided with drastic changes in the marine invertebrate fauna and with climatic changes that resulted in a change of vegetation types over most of the earth.


HISTORY OF LIFE
THE AGE OF MAMMALS

Although the mammals evolved from the mammal-like reptiles during the Triassic period they remained a small insignificant part of the vertebrate fauna for the next 100 million years, while the reptiles held sway over land, sea and air.
The characteristic physical features of mammals - the simplicity of the jaw, the evolution of the ear from the jaw bones, specialized dentition and the position of the limbs beneath the body, rather than out to the side - had all been evolved in the mammal-like reptiles. The feature that in particular separates the mammal-like reptiles from the mammals is the articulation of the jaw. A reptile's jaw is a complicated structure consisting of several bones. In a mammal, however, the lower jaw is constructed from a single bone. The additional bones present in a reptile are incorporated in the mammal's ear. This development took place independently at least four times towards the end of the reign of the mammal-like reptiles.

All mammals are descended from small mammalian insectivorous creatures such as Megazostrodon. At the end of the Mesozoic, major environmental changes resulted in the extinction of the dinosaurs and the mammals evolved to take their place as the dominant vertebrate group. The early Tertiary mammals were mainly forest-dwellers. Carnivores of the period, such as Andrewsarchus, were primitive and had teeth similar to those of reptiles. When conditions became drier during the late Tertiary, the forests receded and mammals similar to those found during the Age of Man appeared.

Several physiological features indicative of the mammals also occurred in the mammal-like reptiles. The palate which enables mammals to breathe and eat at the same time, essential for the constant supply of oxygen needed to support the warm-blooded condition, first appeared in the mammal-like reptiles. Pits in the skull and jawbones, indicating the presence of whiskers suggests that some mammal-like reptiles were at least partly hairy and again provides evidence of warm-bloodedness. The development of their teeth suggests that the young were toothless and hence must have been suckled by their mothers.
Throughout the Age of Reptiles mammals were confined to small, mouse-like creatures living on insects, seeds and also probably reptile eggs. Although in Jurassic times several distinct orders existed, distinguished by different kinds of teeth, few of these creatures outlived the dinosaurs. The ultimately most successful survivors were the placentals, those mammals that nourished their developing young within their bodies until they were at an advanced stage of development. The placentals first appeared during the Cretaceous along with a side branch, the marsupials - mammals that nurtured their developing young in an external pouch. Between them the placentals and marsupials accounted for almost the entire mammal fauna of the world during the Age of Man, although a primitive egg-laying group, the monotremes, was represented by a mere handful of species which included the platypus.
The Age of Mammals dawned at the beginning of the Tertiary with the passing of the large reptiles. Within the first ten million years or so all niches formerly occupied by reptiles had been filled by mammals and all the mammalian orders that were around at the Age of Man had been established.
On land the herbivores were represented by hoofed ungulates such as horses and the pigs, possessing plant-cropping and grinding teeth, by large elephants with pick-like tusks and by small rodents and lagomorphs such as the rats and rabbits with gnawing front teeth and crushing back teeth. These herbivorous animals were preyed upon by fierce members of the carnivore family. Insects and other invertebrates were eaten by primitive insectivores such as shrews, with strong teeth able to tear through the tough outer coverings of beetles and millipedes, and edentates such as anteaters and armadillos with no teeth at all. Primates, the monkeys and apes, evolved in the forests and lived on a wide variety of foods, as is reflected by the versatile nature of their dentition. All these mammals were placentals, but in some isolated continents such as Australia and South America many of the niches were occupied by marsupials instead.

Mammals are grouped into three major categories - monotremes, marsupials and placentals. Monotremes (A) such as the platypus lay eggs. The females have no teats and when the young are hatched they lap their mother's milk from the surface of her abdomen (B). Marsupials (C) give birth to immature young, which in most cases are kept and fed in an abdominal pouch (D). Placentals (E), as their name implies, have a placenta, through which the embryo is fed until it is comparatively well developed. After the young are born they are fed by their mother through external teats (F).

In the air the reptilian pterosaur niche was occupied by the bats, with wings developed on extended forelimbs and fingers. As they flew mainly during the evening and at night they did not compete directly with the birds, which were much better adapted for this sort of life.
The watery niches of the plesiosaurs, pliosaurs and ichthyosaurs were taken, respectively, by the seals, whales and dolphins, which evolved streamlined fish-like bodies and paddle-shaped limbs.
The early part of the Tertiary period was characterized by an increase in the number of different species of mammals. It was as if nature were trying out all sorts of new forms to see which fitted the vacant niches best. Later the mammals settled down to a much smaller number of well-adapted species. This is analogous to the situation following the demise of man and the extinctions caused directly by him. There again a vast number of new species evolved, almost immediately, which were later whittled down to a smaller number of successful forms.
The early Tertiary land mammals were forest-dwellers. However, about half-way through the period the global climate changed and the grasslands, providing more open habitats, expanded in both the temperate and tropical latitudes. The grasses that comprised the vegetation represented a rich untapped food source, but before it could be exploited new, ridged grinding teeth had to evolve to cope with its high silica content. The open vistas made concealment difficult and compelled the herbivorous mammals occupying them to seek refuge in flight and, to remain effective, the predatory carnivores also had to become faster.
The primates, which evolved in the trees of the forests, also ventured on to the grassy plains. One particular group adopted an upright stance - a natural development of their mode of locomotion in the trees and enabled them to see over the top of tall grasses and have warning of approaching predators. Another legacy of their arboreal life was a keen co-ordination between hand and eye. This dexterity enabled them to use sticks and stones as tools to exploit the available food more effectively. A primate trait that particularly helped in food gathering was their high degree of social organization. Hunting in packs they could surround animals that would otherwise be too alert and swift to be captured. The increase in intelligence that enabled them to co-ordinate these attributes and to regulate their complex social structure paved the way for man's evolution in the future.

In the course of evolution from reptile to mammal the jaw has become more sophisticated; the jaw hinge has moved farther forward to permit more accurate chewing, the teeth have become specialized and have taken on different forms to suit particular purposes such as biting, chewing and stabbing. The reptilian jaw hinge has become incorporated within the middle ear and the mammals have developed a palate so that they can breathe and eat at the same time. A. Early mammal-like reptile - (Dimetrodon) B. Late mammal-like reptile (Oligokyphus) C. Mammal (Canis)

HISTORY OF LIFE
THE AGE OF MAN

The first men were plant gatherers and animal hunters and differed little in life style from other herbivorous and the carnivorous animals around them. They had sufficient intellect to devise tools and weapons and a social organization that made hunting and food gathering more efficient. Neither of these things, however, made any serious impact on the environment.
The first great change in their life style came when, instead of hunting and gathering animals and plants they brought them together and looked after them in a single location. This eliminated the element of danger present in hunting and reduced the probability of starvation, as there was no longer the possibility of returning empty-handed from a foraging expedition. It was the beginning of agriculture.

The evolution of man can be traced from an ape-like ancestor through hominids such as Australopithecus and early members of his own genus such as Homo erectus. Cro-magnon man, an early form of the species Homo sapiens itself, appeared in Europe towards the end of the Pleistocene ice ages. Man's skull developed from a massive structure containing a small brain into one of lighter bones encasing a large cerebral cavity. Man's large brain gave him the power of conscious thought and separated him from the rest of the animal kingdom.

At first the areas under cultivation were small and relatively insignificant. However, the improvements to early man's way of life were so dramatic that his populations increased markedly and more and more land had to be cleared of its natural vegetation to make room for crops and grazing animals.
As man's ingenuity and tool-making ability grew, he invented industrial processes that could produce tools with greater speed and less trouble than before. This inevitably involved heat, and forests were cut down to supply wood and mountainsides were dug away to reach coal to provide fuel. Within a few thousand years the landscape of the earth was changed out of all recognition.
Man's knowledge grew, most significantly in the field of medical science. Accidents and diseases that help to keep natural populations in check were overcome or reduced in their effects by man's endeavours. Genetic defects that, in the wild, would have proved fatal and would have been eliminated by natural selection were perpetuated because their possessors were allowed to live and reproduce. World population increased exponentially and hardly a region of the earth's surface remained untouched by man.
The ultimate effect was that, whereas other animals change and adapt through the slow process of evolution to fit into their environment, man was able to change his environment to suit his current needs, reaping a short-term advantage in the process. Living outside evolution each stage in his rapid cultural development was passed on to the next generation, not through his genes but by learning. Although he avoided the unpleasant effects of natural selection, he also did without its long-term benefits and in short called a halt to evolution as it applied to himself. The result was a world overburdened by a population of beings unable to survive without their own conscious intervention, a world given over to the essential needs of man, a world poisoned by his waste.
Ultimately the earth could no longer supply the raw materials needed for man's agriculture, industry or medicine, and as shortage of supply caused the collapse of one structure after another, his whole complex and interlocking social and technological edifice crumbled. Man, no longer able to adapt, rushed uncontrollably to his inevitable extinction.
With the dominant life form gone the animal world entered a period of evolutionary chaos that lasted tens of thousands of years. However, man's extinction provided the impetus for the formation of many new species of animals and his disappearance was of fundamental importance in shaping the world that has emerged 50 million years later.


CONTENTS

INTRODUCTION BY DESMOND MORRIS 9

AUTHOR'S INTRODUCTION 10

EVOLUTION 11

Cell Genetics : Natural Selection : Animal Behaviour : Form and Development :
Food Chains

HISTORY OF LIFE 22

The Origins of Life : Early Living Forms : The Age of Reptiles :
The Age of Mammals : The Age of Man

LIFE AFTER MAN 33

The World after Man

TEMPERATE WOODLANDS AND GRASSLANDS 36

The Rabbucks : The Predators : Creatures of the Undergrowth :
The Tree Dwellers : Nocturnal Animals : The Wetlands

CONIFEROUS FORESTS 50

The Browsing Mammals : The Hunters and the Hunted : Tree Life

TUNDRA AND THE POLAR REGIONS 58

The Migrants : The Meaching and its Enemies : The Polar Ocean :
The Southern Ocean : The Mountains

DESERTS : THE ARID LANDS 70

The Sand Dwellers : Large Desert Animals : The North American Deserts

TROPICAL GRASSLANDS 78

The Grass-eaters : Giants of the Plains : The Meat-eaters

TROPICAL FORESTS 86

The Tree-top Canopy ; Living in the Trees : The Forest Floor :
Living with Water : Australian Forests : The Australian Forest Undergrowth

ISLANDS AND ISLAND CONTINENTS 100

South American Forests : South American Grasslands : The Island of Lemuria :
The Islands of Batavia : The Islands of Pacaus

FUTURE 113

The Destiny of Life

APPENDIX 117

Glossary : The Tree of Life : Index : Acknowledgements

 


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