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animals inherit physical traits from their parents, and these can be enhanced. To create a flock of sheep with the thickest-possible woolly coats, for instance, breeders select sheep with the thick coats, mate them together, and repeat the process, generation after generation.
    But, whereas humans select for traits they desire in an animal or plant, nature appears to select for traits that maximise an organism’s chance of survival in its environment. Such natural selection might not be as fast as the artificial selection of human breeders but it is just as effective.
    After Darwin had spent eighteen months of intense concentration on the problem, a light went on in his mind. He suddenly saw the elusive mechanism of natural selection. And it was breathtakingly simple.
    One of the striking things about the natural world is how profligate organisms are. Invariably, animals give birth to large litters of young. Plants produce vast quantities of seeds. But there is simply not enough food in the world to support so many young. Inevitably, therefore, most creatures starve to death. Crucially, Darwin realised, the only ones who survive to reproduce are those with traits that best enable them to make a living in their environment. 1 And these traits are inherited by the next generation. So, as time goes by, the prevalence of beneficial traits in a population increases at the expense of traits that do not confer survivability.
    This was it: the missing piece of the jigsaw. Evolution by
natural selection
. ‘How extremely stupid not to have thought of that,’ said Darwin’s friend and champion, Thomas Huxley. But of course Darwin had to see past the dizzying complexity of the natural world to the mechanism ticking at its heart and quietly generating its complexity. And that was no mean feat.
    Richard Dawkins has called evolution by natural selection the greatest idea in the history of science. And it certainly has phenomenal explanatory power. Modern biology is literally the story of evolution by natural selection. ‘Nothing in biology makes sense except in the light of evolution,’ wrote Theodosius Dobzhansky in 1937.
    According to his biographers, Darwin made no effort to publicise his idea, realising full well that it flew in the face of the Church’s teaching that God created all living creatures in theirfinal form. Only in 1858 – after twenty years of sitting on his explosive idea – was he galvanised into action. A letter arrived from a man called Alfred Russel Wallace, who, while observing nature in Indonesia and Malaysia, had hit on the exact same unifying idea of evolution by natural selection. 2 Stunned, Darwin locked himself in his study and began writing furiously.
    Darwin’s epochal work, published in 1859, is universally known as
The Origin of Species
, though it says essentially nothing about the ultimate origin of life, which to this day remains a deep mystery. More apposite is the book’s full, though considerably more long-winded, title:
On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life
. 3
    According to Darwin, all life on Earth today has evolved over aeons of time from a common ancestral organism by the process of natural selection. The idea conflicted not only with the biblical account of creation as a one-off event but with the Church’s claim that humans were, uniquely, forged in the image of God. According to Darwin, humans were neither at the pinnacle of creation nor special in any other way. They were just another animal.
    Just as, in the sixteenth century, the Polish astronomer Nicolaus Copernicus showed that the Earth was not at the centre of things and occupied no special place in the cosmos, Darwin showed that humans were not at the centre of things and occupied no special place in the living world. 4
    Darwin was courageous to present a theory that flew in the face of entrenched religious orthodoxy. But he was also very honest