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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.
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 370°C, 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 -200°C, 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 differenti¬ation 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.
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.
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 organiza¬tion. 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) |
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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.
INTRODUCTION BY DESMOND MORRIS 9
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
The World after Man
TEMPERATE WOODLANDS AND GRASSLANDS 36
The Rabbucks : The Predators : Creatures of the Undergrowth
:
The Tree Dwellers : Nocturnal Animals : The Wetlands
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
The Sand Dwellers : Large Desert Animals : The North American Deserts
The Grass-eaters : Giants of the Plains : The Meat-eaters
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
The Destiny of Life
Glossary : The Tree of Life : Index : Acknowledgements
