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Published: April 30, 2009
A new genetic survey of people in Africa, the largest of its kind,
suggests, however, that the region in southwest Africa seems,
on the present evidence, to be the origin of modern humans. The
authors have also identified some 14 ancestral populations.
The new data goes far toward equalizing the genetic picture of the world, given that most genetic information has come from European and Asian populations. But because it comes from Africa, the continent on which the human lineage evolved, it also sheds light on the origins of human life.
“I think this is an enormously impressive piece of work,” said Alison Brooks, a specialist on African anthropology at George Washington University.
The origin of a species is generally taken to be the place where its individuals show the greatest genetic diversity. For humans, when the new African data is combined with DNA information from the rest of the world, this spot lies on the coast of southwest Africa near the Kalahari Desert, the research team, led by Sarah A. Tishkoff of the University of Pennsylvania, said in this week’s issue of Science.
Dr. Brooks, who spent many years in the area, said that it had some trees but that it also had deep sand and was not particularly garden-like. The area is a homeland of the Bushmen or San people, whose language is distinguished by its many click sounds.
But the San in the past might not have been restricted to where they are now, she said. The San are thought to have once occupied a much larger area, one that probably stretched from southern Africa up the east coast to as far as present-day Ethiopia.
Since the geneticists’ calculations refer to people, not geography, the San — and therefore the site of greatest human diversity — might have been located elsewhere in the past.
Christopher Ehret, an expert on African languages at the University of California, Los Angeles, and a member of Dr. Tishkoff’s team, has detected traces of words borrowed from click languages in East African languages. This suggests that proto-Khoisan, the inferred ancestral language of all click-speakers, may have originated in East Africa, Dr. Brooks said.
The language of the first modern humans may have undergone a very early branching, Dr. Ehret said, with the Khoisan click languages on one branch and the other three language groups of Africa — Nilo-Saharan, Niger-Kordofanian and Afroasiatic — on the other branch. Clicks are difficult to pronounce fluently and with a single exception no click languages are known outside Africa.
Another finding of the Tishkoff-Ehret team is that African languages tend to be highly correlated with the genetics of their speakers, a finding that helps indicate cases of language replacement. The various Pygmy groups in Africa, the team has found, show distant genetic relationships to the San and other click-speakers, suggesting the pygmies, too, once spoke Khoisan languages but have now adopted those of their neighbors.
Another instance of a mismatch between language and genetics concerns the Luo, an ethnic group in Kenya to which President Obama’s father belonged. The Luo speak a Nilo-Saharan language and are thought of as a people of Sudanese origin, but genetically they have a heavy mixture of Bantu speakers’ genes, Dr. Tishkoff said.
Dr. Tishkoff’s team has also calculated the exit point from which a small human group — maybe a single tribal band of 150 people — left Africa some 50,000 years ago and populated the rest of the world. The region is near the midpoint of the African coast of the Red Sea.
Dr. Tishkoff and her colleagues found that the 14 ancestral
African populations they detected are now highly mixed, with the
exception of the Bantu speakers
