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Gene study questions origin theory

London, Dec. 27: A controversy over the most widely accepted theory of the origin of modern man was reignited yesterday by an analysis of the human genetic code.

The analysis sheds doubt on the theory that early people moved out of Africa and completely replaced local populations.

Instead, the findings suggest that there was at least limited interbreeding between our African ancestors 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 Prof Henry Harpending of the University of Utah, co-author of the study. “There is instead some sign of interbreeding.”

If that conclusion is correct, it questions the “replacement theory” of human evolution, a theory Prof Harpending has advocated for more than a decade. “Hypotheses are called into question by data every day in science,” he said. “That’s the way it works.”

The journal, Proceedings of the National Academy of Sciences, published the new findings by a 20-strong team, including Prof Harpending and led by Stephen Sherry, an anthropologist, and Gabor Marth, a mathematician, of the National Centre for Biotechnology Information at the National Institutes of Health in Bethesda, Maryland.

Most anthropologists agree that human ancestors first spread out of Africa 1.8 million years ago, establishing new populations in Europe, Asia and elsewhere. The “multi-regional theory” holds that modern humans evolved from those multiple populations.

The competing “replacement theory” says the local populations, including Europe’s Neanderthals, went extinct when they were replaced between 80,000 and 30,000 years ago by another wave of human immigrants from Africa.

Scientists can analyse 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 over time so that if the human population were large at a specific point in prehistoric time, 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”, said Prof Harpending.

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, analysed genetic spelling mistakes in the human code — mutations called SNPs (single nucleotide polymorphisms) — and found a bottleneck in the human population; what looks like a sharp reduction in the number of people when ancestors of modern humans colonised Europe roughly 40,000 years ago.

Prof Harpending says one possible explanation is that there was a large population of humans who migrated from Africa, yet kept largely to themselves and mated only 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 possibility is that the prehistoric African population was large 100,000 years ago, but only a very small number — perhaps a few dozen — of those Africans migrated to other areas some 80,000 years ago, ultimately replacing local populations.

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