Ancient World Was a Vast Melting Pot, Study Shows
Spreading out of Africa like starlings, early humans conquered the world by embracing the strangers they encountered around the globe, not by forcing them into extinction, as many researchers believed, according to a new analysis of human genetic history.
In the textbook view, the founding fathers of modern humanity emerged suddenly from Africa about 100,000 years ago and swept into oblivion all other prehuman species--Neanderthals, for example--that they encountered.
A new and elaborate computer genealogy of 11 inherited traits compiled by Alan Templeton at Washington University in St. Louis presents a very different slant on the origins of diversity.
Templeton’s work, published today in the journal Nature, suggests that “interbreeding, not replacement,” was the rule for successive waves of primitive humans migrating out of Africa. By mingling, these ancestral human groups “strengthened the genetic ties between human populations throughout the world,” said Templeton, who studies the history and geography of genes.
In his view, the ancient world was a vast melting pot in which tribes of human ancestors scattered, rejoined and scattered again. As they did so, they gradually intermingled inherited traits across thousands of generations to mix the palette of modern humanity.
Templeton’s work is the latest riposte in a 20-year-long debate in which anthropologists, archeologists, molecular biologists and population geneticists have battled over human origins with rounds of research papers scattered like hand grenades.
Several specialists in the field enthusiastically hailed the new study of evolutionary parentage as “brilliant,” while others briskly dismissed it as “hocus-pocus” and “nonsense.”
For all their differences, both camps agree that the earliest ancestors of humankind evolved in Africa about 2 million years ago, before beginning waves of migration into Europe and Asia.
Where the scientists part company is in deciding how those ancestral groups gave rise to anatomically modern people--with small, pointed jaws; smooth foreheads; high, rounded skulls; and advanced mental abilities--emerged.
Frustrated by the ambiguous fossil record, researchers have turned to the genes that code for growth and development to flesh out this missing chapter of human evolutionary history.
To reach his conclusions, Templeton combined published data on 11 parts of the human genome. He analyzed mitochondrial DNA--genetic material that each person inherits directly from his or her mother--as well as data from genes carried on the Y chromosome, which is inherited only from fathers. He also looked at genes on other chromosomes that can be inherited from either parent.
His analysis detected considerable gene mixing and evidence of two separate waves of migration out of Africa into Asia and Europe, the first from 420,000 to 840,000 years ago and a more recent one from 80,000 to 150,000 years ago.
Other researchers agree that there was generous mixing of valuable genetic traits. But they disagree about when it might have happened.
“If those ancestral populations were all in different valleys in Africa, you would see the same thing,” said Henry Harpending, an anthropologist at the University of Utah. “This may have been gene flow between ancestral populations in Kenya, Uganda and Tanzania.”
Templeton had made a “valiant effort,” but his study “all seems too iffy to me,” Harpending said. “Going from his findings to this sweeping picture of human evolution is a jump I can’t see.”
Molecular biologist Rebecca L. Cann at the University of Hawaii said Templeton offered “strong genetic support” for considering Africa the geographic wellspring of humankind, but his broader analysis was “overambitious.”
On the other side of the debate, paleoanthropologist Milford Wolpoff at the University of Michigan said he felt “vindicated” by Templeton’s work. Wolpoff has long championed the idea that modern humans evolved more or less simultaneously around the globe by sharing their best characteristics.
Templeton’s analysis “shows that human evolution is about traits and not about kinds of people,” Wolpoff said. “Lots of things separate human populations, but more things unite them.”
Said molecular biologist Peter Underhill at Stanford University, who uses male DNA to track human migrations, “My problem with this whole field is that it is not dispassionate anymore. There is a lot of name calling . . . . That is a shame.”
