The African part of Nicholas Wade's story

Our April selection, Before the Dawn by Nicholas Wade, was concerned solely with the small group of people who crossed the Red Sea from Africa and populated the rest of the world. At our last meeting, some of us wondered what was going on with the ones who stayed behind in Africa. Their story is suggested in the current (April 24) issue of The Economist:

A study carried out under the auspices of the Genographic Project used the mitochondrial DNA of more than 600 living Africans to show how genetic diversity has developed in Africa. The team paid particular attention to samples taken from the Khoi and San people of southern Africa. 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 Khoi and San 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. Homo sapiens may have come close to extinction. 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.