First, the meaningless and oft-quoted "statistic" that we share 98.6 per cent of our genetic material with chimps. What does this mean? The spurious accuracy of the last 0.6 per cent means very little, and even less when you know that we also share a large proportion of our genes with fish, flowers and even fungi.
Second, molecular biologists do not "know how long it takes for changes to accumulate in DNA", they merely have an estimate, which means that there is some uncertainty about the date at which the last common ancestor of chimps and humans lived.
Given this uncertainty, the scenario that Gribben sketches is indeed possible, but it is also possible that the divergence took place earlier, before the origin of upright walking, leaving the chimps, gorillas and all their ancestors comfortably walking on their knuckles.
The nonsense about the "Australopithecines" being the ancestors of modern chimps and gorillas would take too long to untangle here, but the vast weight of evidence (including the molecular studies) speaks against this. In addition to the fact that the idea of a large and a small species of Australopithecine (thought by Gribben to be ancestral to the "large" gorilla and "small" chimpanzee) is now out of step with palaeontological research, Gribben also misreads the molecular evidence.
Even if the chimps did diverge after the origin of bipedal walking, the gorillas are an altogether earlier offshoot. While Gribben points out that we must avoid seeing humans as the centre of the evolutionary universe, it is he that falls straight into this trap by so casually conflating the chimps and the gorillas merely because they are "hairy apes". The point here is that we are closer cousins to the chimps that either chimps or humans are to the gorillas (who, by the way, don't live a "life in the trees".)
Dr MARK LENEY
New College, Oxford