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Tentative beginnings: more than 3500 million years ago

Life begins on earth between 3500 and 4000 million years ago. The astonishing proof is the fossil remains of creatures one would imagine too insubstantial to be preserved in stone.

Bacteria (single-celled creatures and still the simplest form of life today) have been discovered in African rock of that period. Many similar fossils of bacteria have been found in America and Australia, dating from closer to 3000 million years ago.

How do these creatures come to be? By this time the earth has been cooling for more than 1000 million years, while volcanoes spew out ash and lava. Meanwhile water vapour in the atmosphere is condensing to settle on the surface of the globe. Scalding land, hot seas, a thin atmosphere shot through with powerful ultraviolet light and frequent flashes of lightning - this does not seem a conducive environment for the beginning of life.

Yet a famous experiment demonstrates precisely how the first necessary ingredients emerged. In 1953 scientists imitate the earth's early atmosphere, by mixing hydrogen, carbon monoxide, ammonia, methane and water vapour in a glass flask and then subjecting it to ultraviolet light and electrical discharges.

Within a few weeks (not even the blink of an eye on a cosmic scale) complex molecules form in the mixture, including several amino acids.

Amino acids are the building blocks of proteins, and there are proteins in every cell of every living organism. So the necessary ingredients are falling into place.

But for life to occur, as opposed to a succession of inert chemical reactions, a further step is required. Out of millions of compounds, formed over millions of years, one in particular provides what is needed for the emergence of life.

The crucial ingredient is DNA, a substance capable of orchestrating the formation of proteins in such a way as to copy an existing chemical structure, while also passing on the necessary information for repeating the trick. That is an elaborate way of describing something familiar to all of us - the renewal of life in successive generations.

The discovery, also in 1953, of how DNA performs this task is one of the great moments of scientific history.

DNA enters the equation before the emergence of the first bacteria. We, like all other living creatures, descend from those bacteria; and we are still programmed by DNA. Carrying increasingly complex messages, this substance has transmitted life through thousands of millions of years from the first beginnings down to creatures alive today.

Its discovery has proved that all forms of life descend from the same origins. Those who believe in a divine Creator may argue that this was his cunning method from the start. Others have to fall back on chance.

Evolution: the theory

Chance, on which the theory of evolution depends, is exceptionally hard to believe in. But there is strong evidence, from the fossil record and from DNA, that chance has indeed brought us the amazing diversity of life.

Accidental changes in the message carried by the DNA have again and again led to altered or mutated versions of living things.

If, as occasionally happens, the mutation brings an advantage of some kind, then the mutant creature is better equipped to pass on its new version of the DNA code to future generations.

Its descendants will seem, at the first few removes, a variant of the same species. Later, after many more mutations, they may have evolved into an evidently different animal.

Our difficulty in believing this is largely a result of the short span of our own personal experience. How could there possibly have been time for so much to happen by accident?

The answer is that since mammals first evolved, there has been time for 400 million generations of mice. Even the relatively short period since the emergence of the first 'ape men' can accomodate 250,000 human generations. Such spans provide rich opportunity for change, even by chance.

From algae to fishes: 3500 - 400 million years ago

The evolution of life through the millennia can be divided, for simplicity's sake, into six stages. The first such stage, from about 3500 million to 1000 million years ago, sees the very gradual development of single-celled water creatures, such as simple algae, into slightly more complex forms.

The same evolutionary sequence can also be described in terms of rock strata, an approach which has resulted in the timescale of Geological periods. This is another way of presenting the same fossil-based information. The geological periods have scientific status, by contrast with the six stages of evolution which are adopted here purely for narrative convenience.

The second stage in the evolution of life is still confined to the oceans. Algae evolve into a wide variety of seaweeds, and there appear the first complex marine creatures - sponges, jellyfish, worms and starfish. Others of the same kind grow a solid outer coating, as a protection in the battle of life. These are the shellfish, sacrificing mobility for the safety of an enclosed existence.

More adventurous are the crustaceans, such as shrimps and lobsters, soft creatures moving with new security in a coating of armour.

At the very end of this stage, evolution finds a new and more fruitful use for rigid material - as a skeleton inside the body, rather than armour plating outside.

The dating of these overlapping developments is necessarily vague. But this second stage lasts from about 1000 million to 400 million years ago. The earliest known creature with a skeleton is a form of fish, evolving a little more than 500 million years ago.

The first land creatures: 400 - 300 million years ago

The third evolutionary stage sees life extends its reach from the waters on to land. Plants are the first to make the move, about 400 million years ago.

They are followed on to the land, perhaps 50 million years later, by fishes - using strong front flippers to heave themselves about, rather as a seal does.

Such creatures can only pay brief visits to this new environment, because they have no easy means of extracting oxygen from air. This disability is solved when they evolve, from about 340 million years ago, into amphibians - the family represented today by frogs, toads and newts. These, having lungs, can live indefinitely out of water.

But there is still a restriction on their movement. They must return to their original element to breed. Their eggs, like those of the fishes from which they descend, are soft and must remain moist.

It is in this same period, lasting from about 400 million to 300 million years ago, that there finally emerges the first group of creatures capable of living entirely out of water. They are the insects, which also colonize by flight the third main region of the environment. With their segmented bodies and hard outer shells, the insects descend from marine creatures akin to the crustaceans.

The insects evolve a variety of methods for taking in oxygen from the air, and they develop a much lighter form of outer shell which makes flight possible. Even so, that shell still limits the size to which any insect can develop. Like crustaceans, insects can only grow by moulting (shedding the old shell, then growing a new one). A very large insect would be so weak and vulnerable after moulting that it would stand little chance of survival.

Reptiles: 300 - 65 million years ago

The fourth evolutionary stage, from about 300 million to 65 million years ago, belongs to animals which have solved both the main problems encountered on land. They are the reptiles.

Unlike the amphibians, they lay eggs with dry shells; no longer needing to live partly in water, they can inhabit any part of a continent. And unlike the insects, they have skeletons rather than outer shells; they can grow larger than any of their predecessors or contemporaries. With these advantages there evolve, among the reptiles, the first creatures to dominate the planet.

The dinosaurs and their related species evolve about 225 million years ago. They range in size from a couple of pounds up to more than 70 tons. In their various forms they make land, sea and air their domain. Some are vegetarian, others carnivore.

The term 'extinct' may now seem to label them failures, but the dinosaurs thrive for an astonishing 160 million years. Creatures classed as hominid, the family of man, have so far achieved about 5 million.

The dinosaurs die out in a very short space of time, shown in the fossil record to be about 65 million years ago. It is not known what causes their extinction, though many theories have been put forward. The most probable one is the striking of earth by a massive asteroid,about 6 miles (10 km) in diameter, with its place of impact identified as a crater in the Yucatan peninsula in Mexico. The dust from such an event would shroud the sun for years, reducing the earth's temperature.

Certainly a cooling of the climate, however brought about, is the most likely cause of the dinosaurs' sudden demise.

Most if not all dinosaurs are cold-blooded (meaning, more accurately, that their blood adjusts to the surrounding temperature). Like any reptile today, they need to warm up in the morning sun before exerting themselves. A steady drop in average temperature would make it increasingly hard for such large reptiles to recover sufficient heat.

But there already exist, in the time of the dinosaurs, two groups of small warm-blooded animals - birds and mammals - which are well placed to thrive in a dinosaur-free environment.

Birds and mammals: from 65 million years ago

The fifth stage of evolution, from 65 million to about 5 million years ago, sees the development of a range of birds and mammals similar to those we know today.

The first members of both groups have coexisted with the dinosaurs.For many millions of years there were creatures with some characteristics of both dinosaurs and birds. The earliest known example of these hybrid forms, providing a clear link between the two species, was the famous Archaeopteryx, discovered in Germany in 1861. Structurally a dynosaur, but with feathers, the species lived about 150 million years ago. Another dramatic demonstration of the same evolutionary link was the discovery in 2001, in China, of a perfectly preserved fossil skeleton a small two-legged dromaeosaur. In its life, 130 million years ago, this small dinosaur was covered in downy fluff and primitive feathers - a device not for flight but for retaining warmth. By about 125 million years ago the first primitive birds begin to appear in the fossil record.

The first primitive mammals arrive on the evolutionary scene slightly earlier than birds, about 170 million years ago (more than 100 million years before the extinction of the dinosaurs).

Birds and mammals alike have descended from reptiles, but both have discovered the advantage of maintaining an even body temperature (or becoming 'warm-blooded') by means of the insulation of feathers and fur. Birds, like dinosaurs, rely on a dry shell to protect the developing embryo. But the mammals have found an even more effective way of protecting the next generation - keeping the foetus safe, until ready for independent life, within the mother's body.

When the dinosaurs die out, mammals are no larger than mice and are probably nocturnal insect-eaters - not rivals for any dinosaur food supply, and hardly noticeable as a meal. In a world suddenly free of dinosaurs and rich in opportunities, these small creatures evolve in an astonishing variety of ways. They return to the plentiful food supply in life's original environment (as whales, seals or otters); they thrive even in the air (as bats); and above all they respond with ruthless success to the wide range of possibilities on land. Burrowing below the ground, leaping from tree to tree, grazing the plains or racing for the kill, mammals are soon as much in evidence around the globe as the dinosaurs have previously been.

Primates: from 45 million years ago

Of most direct relevance to our own story are the mammals in the trees, known as the primates. By about 45 million years ago they have evolved to a form somewhat resembling modern lemurs. By 22 million years ago there are primates of a kind that we would recognize today as monkeys. Running and leaping agilely among the branches of trees, they develop excellent eyesight.

They also acquire another advantage which is characteristic of all the later or 'higher' primates - flexible hands and feet, with an opposable digit (like a thumb) making it possible to hold something in one hand rather than two.

One group of primates, ancestors of the apes, adopts a different style of life in the trees, swinging from the branches rather than running along them. This affects the posture of the body, making the spine a vertical rather than a horizontal support.

These particular primates are also heavier than others - perhaps the original reason for their different behaviour in the trees. Eventually their weight makes life in the trees less convenient. Several species begin to spend much of their time on the ground.

They still move more easily in a four-footed manner, but when there is any advantage they will walk on two feet. Their normal position, when at rest, is now the upright posture of sitting.

Their evolution has also included a marked increase in brain size. Social relationships, a limited use of tools and an ability to learn are familiar aspects of the increased brain power of the apes.

The four species of modern ape - the orang utan and the gibbon in Asia, the gorilla and the chimpanzee in Africa - are thought to have descended from a common ancestor something less than 15 million years ago.

By another line of descent from that common ancestor is a creature also characterised by an upright stance, good eyesight, fingers and thumbs, and an enlarged brain. In these arbitrary divisions of evolution the sixth stage (so far) concerns ourselves.

The missing link: 5 million years ago?

Exactly where and when the genealogy of mankind branches off from that of the apes has been a matter of detailed controversy ever since the Darwinian 'missing link' was first sought. We still do not know even the approximate date, though some scientists now believe that it was not much more than 5 million years ago.

The evidence of DNA demonstrates that the place was almost certainly Africa. Comparing human DNA with that of the apes, it has been shown that the African pair of apes are more nearly related to us than the two from Asia; and of the African primates, the chimpanzee is a considerably closer cousin than the gorilla. We share 98% of our genes with chimpanzees.

From a period about 4 million years ago there begin to be fossils of creatures as close in kind to human beings as to apes. It is a measure of their intermediate status that they have been classed as hominids (hominidae man-like) but have been given the species name Australopithecus ('southern ape').

Australopithecus is a confusing name with its hint of Australia ('southern land'), but these ape-men are southerners in a different sense. Their remains have been found in the southern hemisphere, and specifically in southern and eastern Africa.

Several varieties of Australopithecus seem to have coexisted in Africa over a span of more than 3 million years. What they have in common with each other, and with us, is a preference for walking upright on two feet. They may also have used sticks and stones to fight, for they have less dangerous teeth than their ape ancestors.

With them and their immediate predecessors, something more than 4 million years ago, our own era can at last be said to be dawning. And we are already 99.95% of the way through the story.