SEVEN hundred and forty centuries ago, give or take a few, the skies darkened and the Earth caught a cold. Toba, a volcano in Sumatra, had exploded with the sort of eruptive force that convulses the planet only once every few million years. The skies stayed dark for six years, so much dust did the eruption throw into the atmosphere. It was a dismal time to be alive and, if Stanley Ambrose of the University of Illinois is right, the chances were you would be dead soon. In particular, the population of one species, known to modern science as Homo sapiens, plummeted to perhaps 2,000 individuals.

The proverbial Martian, looking at that darkened Earth, would probably have given long odds against these peculiar apes making much impact on the future. True, they had mastered the art of tool-making, but so had several of their contemporaries. True, too, their curious grunts allowed them to collaborate in surprisingly sophisticated ways. But those advantages came at a huge price, for their brains were voracious consumers of energy—a mere 2% of the body's tissue absorbing 20% of its food intake. An interesting evolutionary experiment, then, but surely a blind alley.

This survey will attempt to explain why that mythical Martian would have been wrong. It will ask how these apes not only survived but prospered, until the time came when one of them could weave together strands of evidence from fields as disparate as geology and genetics, and conclude that his ancestors had gone through a genetic bottleneck caused by a geological catastrophe.

Not all of his contemporaries agree with Dr Ambrose about Toba's effect on humanity. The eruption certainly happened, but there is less consensus about his suggestion that it helped form the basis for what are now known as humanity's racial divisions, by breaking Homo sapiens into small groups whose random physical quirks were preserved in different places. The idea is not, however, absurd. It is based on a piece of evolutionary theory called the founder effect, which shows how the isolation of small populations from larger ones can accelerate evolutionary change, because a small population's average characteristics are likely to differ from those of the larger group from which it is drawn. Like much evolutionary theory, this is just applied common sense. But only recently has such common sense been applied systematically to areas of anthropology that have traditionally ignored it and sometimes resisted it. The result, when combined with new techniques of genetic analysis, has been a revolution in the understanding of humanity's past.

And anthropology is not the only human science to have been infused with evolutionary theory. Psychology, too, is undergoing a makeover and the result is a second revolution, this time in the understanding of humanity's present. Such understanding has been of two types, which often get confused. One is the realisation that many human activities, not all of them savoury, happen for exactly the same reasons as in other species. For example, altruistic behaviour towards relatives, infidelity, rape and murder are all widespread in the animal kingdom. All have their own evolutionary logic. No one argues that they are anything other than evolutionarily driven in species other than man. Yet it would be extraordinary if they were not so driven in man, because it would mean that natural selection had somehow contrived to wipe out their genetic underpinnings, only for them to re-emerge as culturally determined phenomena.

Understanding this shared evolutionary history with other species is important; much foolishness has flowed from its denial. But what is far more intriguing is the progress made in understanding what makes humanity different from other species: friendship with non-relatives; the ability to conceive of what others are thinking, and act accordingly; the creation of an almost unimaginably diverse range of artefacts, some useful, some merely decorative; and perhaps most importantly, the use of language, which allows collaboration on a scale denied to other creatures.

There are, of course, gaps in both sets of explanations. And this field of research being a self-examination, there are also many controversies, not all driven by strictly scientific motives. But the outlines of a science of human evolution that can explain humanity's success, and also its continuing failings, are now in place. It is just a question of filling in the canvas—or the cave wall.