Jared Diamond begins his marvellously entertaining book The Third Chimpanzee by pointing out that if an extraterrestrial zoologist had surveyed the Earth four million years ago it may never have noticed a few hairy, chimp-like primates wandering around East Africa. Our ancestors were a minor element of a declining lineage among very diverse and impressive fauna.
But something happened over the next three million years. This minor player expanded its range geographically and ecologically. There was a massive explosion in tool use and our ancestors became extraordinarily cooperative, ultimately coming to depend utterly on a network of interactions with others.
This lineage changed its sexual, social and family behaviour in striking ways. For example, I can recognise my daughter but my male ancestors of a few million years ago probably could not. Male chimps still have little chance of recognising their daughters.
The rapidity of these changes shows that something extraordinary happened in our history. In 1975, when Alan Wilson and his collaborators first used molecular dating to estimate a divergence between the human and chimp lineages, his claims were hotly controversial because palaeoanthropologists did not think that five million years was enough time to evolve the dramatic differences between our lineage and its sisters.
Equally...
