Writing in the New York Review of Books, John Maynard Smith, one of Britain's greatest biologists, remarked ‘If one was able to re-play the whole evolution of animals, starting at the bottom of the Cambrian (and, to satisfy Laplace, moving one of the individual animals two feet to its left), there is no guarantee – indeed no likelihood – that the result would be the same. There might be no conquest of the land, no emergence of mammals, and certainly no human beings’. This review, written with characteristic flair and economy, was addressing three books on evolution, two by S. J. Gould and the third by E. Mayr. Maynard Smith was raising this issue because both the authors under review have been forthright in claiming that the emergence of human intelligence during the course of evolution has a vanishingly small probability. The logic of the argument, that because we are unique on this planet then nothing like us can occur elsewhere, is gently checked by Maynard Smith: ‘This argument seems to me so manifestly false that I fear I must have misunderstood it’. However, he, Mayr and Gould, and I imagine almost anyone else, would agree that the likelihood of ‘exactly the same cognitive creatures – with five fingers on each hand, a vermiform appendix, thirty-two teeth, and so on’ evolving again if, somehow, the Cambrian explosion could be rerun is remote in the extreme.

What, however, of the emergence of more general biological properties?