If you have overdosed on Darwin this anniversary year, the great man himself is partly to blame: he was inconsiderate enough to publish On the Origin of Species when he was exactly 50. The resulting coincidence of sesquicentennial with bicentennial was bound to excite the anniversary-tuned antennae of journalists and publishers. Anniversaries are arbitrary, of course, dependent on the accident of our having ten fingers. If we had evolved with eight instead, we would have to suffer centenaries after only 64 (decimal) years, and style gurus would prate about the changing fashions of octaves instead of decades.
Incidentally, it is not far-fetched that we might have evolved a different number of fingers. The pentadactyl limb (five digits on each) has become a shibboleth of vertebrate zoology, and even animals such as horses (which walk on their middle fingers and toes) or cows (two digits per limb) have lost the extra digits from a five-fingered ancestor. But the lungfish-like group of Devonian fishes from which all land vertebrates are descended included species with seven (Ichthyostega) or eight (Acanthostega) digits per limb. If we were descended from Acanthostega, instead of from an unsung five-fingered cousin of the same fish, who knows what feats of virtuosity pianists might now perform with 16 fingers? And would computers have been invented earlier, because hexadecimal arithmetic translates more readily than decimal into binary?
Historical accidents of this sort are rife, contrasting with the illusion of good design to provide some of our most convincing evidence that evolution happened. Sometimes the legacy of history goes beyond arbitrary accidents, and spills over into downright poor design. The vertebrate retina is installed backwards, facing away from the light, which perforce has to pass through a carpet of nerves on their way to the "blind spot" where they dive through the retina, bound for the brain. In spite of this we see tolerably well, because natural selection is good at cleaning up after its bodges. But an engineer who produced such a travesty of design would be fired instantly. The retina is a legacy of remote history.
For whatever reason lost in some Devonian swamp, our ancestors evolved with ten fingers. And that is why Darwin was exactly half a century old when he published the book that set us on the path to understanding the whole of life - its diversity, complexity, beauty, compelling illusion of design, and every detail such as why we have the eyes, fingers and toes that we do.
But why did Darwin wait until he was 50 before publishing his great idea (the best idea anyone has ever had, according to the distinguished philosopher Daniel Dennett)? The idea of natural selection came to Darwin more than two decades earlier, in 1838. He wrote out a pencil sketch of it in 1842, then a fuller version in 1844, which he asked his wife Emma to publish if he should die. Then nothing: the mysterious gap. If you were a young man of 30, in possession of the best idea anyone had ever had, would you sit on it until you were 50?
When Darwin eventually did write On the Origin of Species, he was jolted into it by another travelling naturalist, Alfred Wallace, who had the same brilliant idea in 1858. Again in an accident of history, Wallace, who was recovering from a malarial fever on the Indonesian island of Ternate, chose to send his manuscript to - of all people - Charles Darwin.
A potential priority dispute was averted by the gentlemanly behaviour of both protagonists and the smooth diplomacy of Charles Lyell and Joseph Hooker. These elder statesmen of science arranged for Wallace's and Darwin's papers to be read, in their absence, at the Linnean Society in London in 1858, where the great idea fell completely flat and was ignored by all. We remember Darwin more than Wallace because he wrote the book, published in 1859, which fell anything but flat and revolutionised our world for ever.
Hypotheses to explain Darwin's delay range from "He didn't want to upset his pious wife" to "He wanted to get all his ducks in a row before the shooting began", and there may be truth in both. But I remain mystified by the larger question of why humanity as a whole waited until the 19th century. On the face of it, any one of Newton's achievements - optics, gravity, the laws of motion, the differential and integral calculus - seem more difficult, yet Newton's annus mirabilis pre-dated Darwin by nearly two centuries. The proof of Pythagoras's Theorem, and others of Euclid's elegant compendium, pre-date Darwin by more than two millennia.
Once again, Darwin's achievement doesn't seem all that difficult. Why did it elude Aristotle? And everybody else - great philosophers, mathematicians, anatomists, thinkers and achievers of all kinds down the centuries? Why did this simple but staggeringly powerful idea have to wait until the middle of the 19th century before bursting into our consciousness through the medium of two Victorian naturalists? And why, even today, do so many people have difficulty grasping it?
So, what is this best of all ideas, the idea of evolution by natural selection? It is really the principle of the sieve, multiplied a billionfold and applied cumulatively over billions of years. Every generation is a gene sieve (Darwin didn't put it this way, because he didn't know about genes). The genes that fall through the sieve are the minority that drop through from the current generation to the next. In order to do so, the individuals possessing them have to reproduce. And in order to reproduce, they have to survive. Surviving is difficult. There are predators waiting to pounce, diseases waiting to strike. Incompetence takes its toll in missed footfalls or unheeded signals of danger.
Reproduction, too, is an obstacle course. The individual has to find a mate, woo her with alluring feathers or smells, fight off rivals with talons or antlers, feed the young and protect them from marauders. In any generation, only a minority of individuals will become long-term ancestors. The vast majority of animals that ever lived have no surviving descendants. And the genes that jostle and jockey for position in every generation are the genes that, without a single, solitary exception, have passed through the bodies of an ancestral elite, the tiny minority who managed to become ancestors.
The genes that exist are the genes that made it through a million sieves in cumulative cascade. And what was it that made them do so well? They co-operated, through the intricate processes of embryology, with other successful genes to build an unbroken succession of elite individuals, equipped by them to become ancestors. That is why the qualities of the elite are the qualities inherited by every animal and plant: because existence is tough, and competition sorted out the ancestors from the failures
An arms race
The exact equipment for survival varies from species to species, for there are many ways to survive: streamlined wings, in the case of swallows; powerful flukes in whales and spades in moles; bewildering camouflage and mimicry in insects; shimmering tail feathers in birds of paradise. All these are the outward and visible levers that propel the genes that made them through the sieves of the generations. And, to complicate matters, the survival techniques of each species open windows of opportunity for others to exploit, as Darwin recognised.
Wherever you see elaborate and complicated machinery in a living body, it is usually the end product of an arms race, run in evolutionary time, each side accumulating improved equipment to outdo the other - an arms race between predators and prey, between parasites and hosts, even between males and females of the same species.
The modern theory of evolution by natural selection can be expressed mathematically, in a calculus of changing gene frequencies. Darwin was no mathematician and he knew nothing of genetics, but he had the essence of this great and simple idea, and he expressed it with the luminous clarity of one of the greatest minds ever to emerge from the process of evolution that he discovered.
Richard Dawkins FRS was the first Charles Simonyi Professor for the Public Understanding of Science at Oxford. His latest book is “The Greatest Show on Earth: the Evidence for Evolution" (Bantam Press, £20)