December 23, 2002 — A new analysis of human genetic history deals a blow to the theory that early people moved out of Africa and completely replaced local populations elsewhere in the world. The findings suggest there was at least limited interbreeding between our African ancestors and the residents of areas where they settled.
“The new data seem to suggest that early human pioneers moving out of Africa starting 80,000 years did not completely replace local populations in the rest of the world,” says Henry Harpending, a University of Utah anthropology professor and co-author of the new study. “There is instead some sign of interbreeding.”
If that conclusion is correct, it contradicts the “replacement theory” of human evolution – a theory Harpending has advocated for more than a decade.
“Hypotheses are called into question by data every day in science. That’s the way it works,” he says.
The journal Proceedings of the National Academy of Sciences is publishing the new findings in its online edition the week of Dec. 23, 2002. The study’s 20 co-authors include three from the University of Utah: Harpending; Alan Rogers, also a professor of anthropology; and Stephen Wooding, a postdoctoral researcher in human genetics.
The study was led by anthropologist Stephen Sherry and mathematician Gabor Marth of the National Center for Biotechnology Information at the National Institutes of Health in Bethesda, Md. Sherry is a former student of Harpending’s when both were at Pennsylvania State University. Other co-authors of the new study are from the Washington University School of Medicine in St. Louis, The Johns Hopkins University School of Medicine in Baltimore and the University of California, San Francisco.
Most anthropologists agree human ancestors first spread out of Africa roughly 1.8 million years ago, establishing new populations in Europe, Asia and elsewhere. The “multiregional theory” holds modern humans evolved from those multiple populations. The competing “replacement theory” says that the local populations, including Europe’s Neanderthals, went extinct when they 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 human population over time. Mutations occur at a relatively steady rate over time. 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 in modern people. A small population of human ancestors would result in fewer mutations, so modern humans would display less genetic diversity.
So a person’s genetic material “contains the whole history of the population from which you descended,” Harpending says.
Earlier studies of genetic material known as mitochondrial DNA and microsatellites supported the notion that a small group of perhaps 5,000 to 20,000 primitive humans migrated from East Africa, spread around the world, a 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 mutations called SNPs (single nucleotide polymorphisms) in DNA from the nucleus of human cells studied for the Human Genome Project, the effort to map the entire human genetic blueprint. The analysis indicates there was a bottleneck in the human population – what looks like a sharp reduction in the number of people – when ancestors of modern humans colonized Europe roughly 40,000 years ago.
Researchers are not sure what this means because it conflicts with studies of other kinds of human genetic information, which support the idea that a rapidly expanding African population spread globally and replaced local populations elsewhere.
“If Africans moved out of Africa and then populated the whole world, we would see that in the genetic evidence as an expansion in population size,” yet the new study indicated the population shrank instead, Rogers says.
The evidence five years ago indicated migrating Africans did not interbreed with local populations, while the new study indicates they did, Rogers notes, adding that the conflicting genetic data mean “the question is still open.”
Harpending says one possible explanation for the new data is that there was a large population of humans who migrated from Africa, yet they kept largely to themselves and mated only to a limited extent with local populations in Europe and elsewhere. Because interbreeding still was 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, Harpending says.
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. 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 Harpending believes it is unlikely that such a small number of migrants from Africa could spread globally and ultimately replace other populations.