MITOCHONDRIAL DNA is a remarkable thing. Itself the remnant of a strange evolutionary event (the merger of an ancient bacterium with the cell ancestral to all plant and animal life), it also carries the imprint of more recent evolution. In many species, humans included, it passes only from mother to child. No paternal genes get mixed into it. That makes it easy to see when particular genetic mutations happened, and thus to construct a human family tree.

The branches of that tree are now well studied. Humans started in Africa, spread to Asia around 60,000 years ago, thence to Australia 50,000 years ago, Europe 35,000 years ago and America 15,000 years ago. What have not been so well examined, though, are the tree's African roots. The genetic diversity of Africans probably exceeds that of the rest of the world put together. But the way that diversity evolved is unclear.

A study carried out under the auspices of the Genographic Project, based in Washington, DC, and just published in the American Journal of Human Genetics, goes some way towards correcting this oversight. The study's researchers, led by Doron Behar of the Rambam Medical Centre in Haifa and Spencer Wells of America's National Geographic Society, have used the mitochondrial DNA of more than 600 living Africans to show how genetic diversity has developed in Africa. In doing so, they have shed light on how modern man spread around his home continent long before he took the first, tentative steps into a bigger, wider world.

By the drought divided

The team paid particular attention to samples taken from the Khoi and San people of southern Africa. These people, known colloquially as bushmen, traditionally make their livings by hunting and gathering. Indeed, their way of life is thought by many anthropologists to resemble quite closely that of pre-agricultural people throughout the world.

Comparing Khoi and San DNA with that of other Africans shows that the first big split in Homo sapiens happened shortly after the species emerged, 200,000 years ago. Most people now alive are on one side of that split. Most bushmen are on the other. The consortium's analysis of which DNA “matrilines” are found where suggests that for much of its history the species was divided into two isolated populations, one in eastern Africa and one in the south of the continent, that were defined by this split. However, few other matrilineal splits from the first 100,000 years of the species's history have survived to the present day.

This suggests the early human population was tiny (so the opportunities for new matrilines to evolve in the first place were limited) and reinforces the idea that Homo sapiens may have come close to extinction (eliminating some matrilines that did previously exist). Indeed, there may, at one point, have been as few as 2,000 people left to carry humanity forward.

This shrinkage coincides with a period of prolonged drought in eastern Africa, and was probably caused by it. The end of the drought, however, was followed by the appearance of many new matrilines that survive to the present day. The researchers estimate that by 60,000-70,000 years ago, the period when the exodus that populated the rest of the world happened, as many as 40 such groups were flourishing in Africa—though that migration involved only two of these groups.

The African matrilines, however, seem to have remained isolated from each other for tens of millennia after the exodus. It was not until 40,000 years ago that they began to re-establish conjugal relations, possibly as a result of the technological revolution of the Late Stone Age, which yielded new and more finely crafted tools. Only the bushmen seem to have missed out on this panmictic party. They were left alone until a few hundred years ago, when their homelands were invaded from the north by other Africans and from the south by Europeans. Panmixis thus came full circle. And that particular party was certainly not a happy one.