Scientists have found compelling new evidence that the ancestors of modern humans originated in Africa and not independently in different regions of the world. The finding may help resolve a long-standing anthropological debate and may challenge the idea that humans can be classified in discrete ethnic groups.
The origin of modern humans is a debated topic. One leading theory postulates that the ancestors of all modern humans originated in East Africa and then left the continent around 100,000 years ago. From there, they colonized the world, displacing previously established hominids such as Neanderthals in Europe. The best evidence so far for this theory is the higher genetic diversity (and thus inferred older age) of the African population compared to populations from other continents.
A minority theory proposes that modern humans evolved simultaneously in different regions of the world from populations of archaic humans and that both contributed to the current human genetic pool. The modern Homo sapiens species is, therefore, seen as the result of the sharing of genes and behaviors between archaic and modern humans (Science, 12 January 2001, p. 230).
In the new study, geneticist François Balloux and colleagues at the University of Cambridge show that geographic distance from Ethiopia (the place where the oldest human remains have been found) correlates with the genetic diversity of 51 present human populations distributed worldwide. The research gives support to the theory that, as humans left Africa, some versions of their genes became progressively lost over the migration routes. Thus, populations farther from Ethiopia are characterized by lower genetic variability.
The team, which publishes its results today in Current Biology, further observed that genetic diversity decreases very smoothly along ancient colonization routes with increasing distance from Africa. Under a multiregional origin scenario there is no way such a decline in genetic diversity with increasing distance from East Africa could be observed, says Balloux.
"We were surprised such a simple analysis gave such a clear result," Balloux says. He adds that genetic diversity decreases smoothly between neighboring populations. The lack of stark differences suggests that humans cannot be classified in discrete ethnic groups or races on a genetic basis.
"I do find it fascinating that the correlation between diversity and geographical location seems so simple and monotonic," says Svante Pääbo, Director of the Max-Planck-Institute of Evolutionary Anthropology in Leipzig, Germany. "It fits nicely with what we believe we know about the spread of modern humans."