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 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.