The largest genetic analysis of African populations to date suggests that modern humans originated in southern Africa about 60,000 years ago, not eastern Africa as is now commonly thought, researchers said Monday.
A team from Stanford University found that the Bushmen hunter-gatherers of the Kalahari Desert who speak one of the Khoisan languages characterized by the presence of clicking consonants have the greatest genetic diversity in their DNA of any people in Africa — and, indeed, the world.
High genetic diversity is generally accepted as a mark of old age of a population, and Africans in general have the greatest genetic diversity of all peoples, indicating that the human race originated on that continent. The new data, reported online today in the Proceedings of the National Academy of Sciences, go even further, showing that the greatest diversity exists in the hunter-gatherer population and that they must therefore be the most direct descendants of our oldest ancestors.
The new conclusion was possible because this was the first time scientists were able to compare data from many of the Bushmen tribes, said geneticist Brenna M. Henn of Stanford University, lead author of the paper.
But not everyone agrees with the authors' reasoning. Though the ancestors of the Khoisan-speaking peoples may have been the first modern humans, that doesn't necessarily mean that modern humans originated in southern Africa, said anthropologist Henry C. Harpending of the University of Utah.
"These Khoisan were all over southern Africa and east Africa," Harpending said. Southern Africa "is just the only place they are left. They were wiped out everyplace else. I can't imagine why that means humans originated in southern Africa."
Paleoanthropologists generally argue that modern humans originated in eastern Africa because that is where the earliest bones are found. Geneticists, however, say that is simply the area that provides the best preservation of fossils.
Recently, two other genetic studies have suggested a southern African origin, one looking just at a type of DNA called mitochondrial DNA and one examining just 800 regions in the human genome that vary between people, called genetic markers. Both have been criticized as inadequate.
The new study, headed by Henn and geneticist Marcus W. Feldman of Stanford, looked at 16 different population groups and compared more than 500,000 genetic markers in each sample. "This is the strongest genetic evidence to date," Henn said.
But the researchers themselves cautioned that the Bushmen are experiencing rapid evolutionary change, and Harpending noted that there appears to be a fair amount of foreign DNA in their genomes, implying a great deal of genetic mixing has been taking place.
"I have the feeling we are generating a lot of data we don't know what to make of," Harpending said.