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Thursday, March 7, 2002
Humans emerged out of Africa again and again
By ROWAN HOOPER
Everyone knows that humans came out of Africa, but until recently nobody knew that they came in at least two major waves of migration, about 600,000 and 95,000 years ago. The finding comes from a major analysis of newly derived human genetic trees, published today in Nature.
More than that, the analysis, by Alan Templeton, of Washington University in St Louis, Mo., suggests that the migrations were not bloodthirsty "replacement events," when a new population wipes out an existing one and results in its complete genetic extinction. Instead, the early migrant humans interbred with the populations they encountered, almost as if they followed a principle of "make love, not war."
In his efforts to reconstruct the movement and history of the early human migrants, Templeton gathered evidence from many different populations and many different genes. This was a thoroughly modern study of human origins and history -- relying on DNA rather than the shape of old bones.
"In this case, there is no digging for fossils, no three-dimensional reconstructions of skulls and such, just the requirement to collect 10-ml samples of blood," says Rebecca Cann, a geneticist from the department of genetics and molecular biology at the University of Hawaii, in a commentary on Templeton's paper.
Africa has played a dominant role in shaping the modern gene pool through successive population expansions, Templeton says. Humans came out of Africa in not one, but several waves.
And there is another important message from his analysis: The finding that migrant populations spread and interbred with resident populations means that genetic interchange between populations has occurred everywhere, throughout history. The DNA evidence suggests that early human wanderers met the people they encountered with open arms, rather than raised spears. More scientific evidence piles up against the cultural idea of racial differences.
"Humans expanded again and again out of Africa," Templeton writes, "but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world."
Templeton published a paper in 1998 in American Anthropologist that proposed humans were one race, instead of a species with subdivisions, or races. His study showed that, among people now categorized by race, everyone shares about 85 percent of the same genes. The 15 percent of variation is not enough difference to separate people biologically.
But if we share so many genes, then why do we look different?
Cann describes how what we see as racial markers -- differences in skin color and so on -- came about. "Oscillations in climate are assumed to have resulted in an increased isolation of different groups, which would have promoted the 'fixation' of local adaptations in morphology, physiology and behavior," she writes. "Geographically distinct populations in Europe, Africa, South Asia, China and Australia could have had separate evolutionary trajectories if there were not enough migrants in each generation to spread any new mutations to populations in other regions."
Templeton reached his conclusions on human migration by analyzing genetic trees for maternally inherited mitochondrial DNA, paternally inherited Y-chromosomal DNA and eight other DNA regions, including two on the X chromosome. He used a computer program called GEODIS to determine genetic relationships among and within populations based on an examination of specific haplotypes, clusters of genes that are inherited as a unit.
Templeton's study is based on 10 DNA regions, while most other genetic analyses focus on just one, mitochondrial DNA, for instance. It also differs from most approaches because it uses a statistical analysis that assumes no prior model of human evolution. Most other ways of calculating genetic results have a probable model built into the math and look to see if the data are compatible with it.
GEODIS analyses pin down an older expansion out of Africa between 420,000 and 840,000 years ago and a more recent one between 80,000 and 150,000 years ago. GEODIS data also show conclusively, Templeton says, that the most recent out-of-Africa expansion event was not a replacement event.
While no one would deny the role that traditional archeology has had in increasing our knowledge of human history, studies like Templeton's -- using DNA rather than bones -- have the potential to tell us even more.
The results explain the close genetic ties between populations around the world -- the difficult job now is getting those same people around the world to accept that.
E-mail Rowan Hooper at firstname.lastname@example.org