- The Washington Times - Friday, December 27, 2002

The dispute over the most widely accepted theory of the origin of modern man, that early people moved out of Africa and completely replaced local populations, was reignited yesterday by an analysis of the human genetic code.
The findings in the latest issue of the scholarly journal Proceedings of the National Academy of Sciences suggested that at least limited interbreeding occurred between early African men and the residents of the regions where they settled.
"The new data seem to suggest that early human pioneers moving out of Africa starting 80,000 years ago did not completely replace local populations in the rest of the world," said Professor Henry Harpending of the University of Utah, co-author of the study. "There is instead some sign of interbreeding."
The Proceedings published the findings by a 20-member team including Mr. Harpending and led by Stephen Sherry, an anthropologist, and Gabor Marth, a mathematician, of the National Center for Biotechnology Information at the National Institutes of Health in Bethesda.
Most anthropologists agree that human ancestors first spread out of Africa about 1.8 million years ago, establishing new populations in Europe, Asia and elsewhere. The "multiregional theory" holds that modern humans evolved from those multiple populations.
The competing "replacement theory" says that the local populations, including Europe's Neanderthals, were replaced roughly between 80,000 and 30,000 years ago by another wave of human immigrants from Africa.
Scientists can analyze ancient genetic mutations in modern people to learn about how humans evolved and the size of the population over time. Mutations occur at a relatively steady rate. If the prehistoric human population was large, more mutations would occur, resulting in greater diversity in genetic mutations found today. A small population of human ancestors would result in fewer mutations, so modern humans would display less genetic diversity.
In this way, a person's genetic material "contains the whole history of the population from which you descended," Mr. Harpending said.
Earlier studies of genetic material found in the power packs of cells known as mitochondrial DNA supported the idea that a small group of perhaps 5,000 to 20,000 primitive humans migrated from East Africa, spread around the world, and rapidly expanded in population as they replaced other human populations elsewhere in Africa 80,000 years ago, and in Asia 50,000 years ago and Europe about 35,000 years ago.
The new study, however, analyzed genetic mutations called SNPs (single nucleotide polymorphisms) and found a bottleneck in the human population: A sharp reduction seems to have occurred in the number of people when ancestors of modern humans colonized Europe roughly 40,000 years ago.
Mr. Harpending says one explanation is that a large population of humans migrated from Africa, yet kept largely to themselves and mated to a limited extent with local populations in Europe and elsewhere.
Because interbreeding was still uncommon, only a few of the prehistoric European genes were incorporated into the modern human genetic blueprint, giving a false impression that the prehistoric human population collapsed or shrank in size.
Another explanation is that the prehistoric African population was large 100,000 years ago, but only a small number perhaps a few dozen of those Africans migrated to other areas about 80,000 years ago, ultimately replacing local populations.
That would explain why the human genetic blueprint could give a false impression that the human population collapsed in size even if it did not. But Mr. Harpending called it unlikely that such a small number of migrants from Africa could replace other populations worldwide.

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