It is a happy accident that this year marks not only the bicentenary of Darwin's birth (12 February 1809), but also the 150th anniversary of his most famous work, The Origin of Species (1859). The celebrations look set to be spectacular. There will be a week-long festival at Cambridge in July, not to mention a commemorative cover for the Penguin Classics edition of Origins designed by Damien Hirst. Down House, the Kent retreat where Darwin mulled, dithered and doodled for 40 years, is all set to go multi-media.

The half-century lag between Darwin's birth and the publication of his mould-cracking book has always been explained by his reluctance to unleash Evolution on the world. Instead of following up the wildly popular travelogue The Voyage of the Beagle (1839) with a sustained account of how all earthly life was descended from a common ancestor, Darwin took refuge in nausea, trips to the spa, and the endless collection of additional empirical data. Whenever anyone pressed him on whether he believed Man to be descended from the same bacterial soup as Galapagos tortoises, polar bears or, indeed, monkeys, he immediately retreated into a panicky fog. It was only when Alfred Russel Wallace, a Johnny-come-lately to the field, looked set to scoop him, that Darwin was finally persuaded into print on the subject. Even then, mindful of the incendiary implications of his theory, he was careful to leave out any reference to Man, and stuck to finches instead.

That it was the possibility of having to share the intellectual credit for his Big Idea that finally jerked Darwin out of his inertia is entirely apt. For what was once said of Herbert Spencer, the slightly younger man who coined the phrase "survival of the fittest", was also true of Darwin: he believed in the evolution of everything except his own ideas. Darwin's latest biographer Keith Thomson, however, refuses to take the naturalist's account of himself as sui generis on trust, and instead sets about fleshing out the young man's intellectual debts as he read, listened and argued his way to maturity. First there was Edinburgh University in the mid 1820s, where the 16-year-old Charles was sent to join his elder brother to prepare for a career as a doctor. According to Darwin's posthumously-published Autobiography (1887), Edinburgh was an intellectual wasteland, populated by dull geology professors whose dry-as-dust approach to the subject would turn anyone off the topic. Thomson, however, makes a convincing case for it as a key centre of European thought, fizzing with the latest controversies about earth formation and, indeed, species evolution. Likewise, while Darwin dismissed his next intellectual berth, Christ's College, Cambridge, as a place of listless dissipation, Thomson identifies it as the place where the young man encountered some of the country's leading naturalists, absorbing from them the careful habits of observation and notation which would stand him in such good stead aboard the Beagle.