For many years now there has been the theory that the first modern humans originated in Africa.
Now a massive scientific study of human genetic diversity reveals that the theories are true.
Researchers compared 650,000 genetic markers in nearly a thousand individuals from 51 populations around the globe—an unprecedented level of detail for a human genetic study.
"You get less and less variation the further you go from Africa," said Marcus Feldman, an evolutionary biologist at Stanford University in California and a study co-author.
Such a pattern fits the theory that the first modern humans that settled the world started in Africa less than 100,000 years ago.
As each small group of people broke away to found a new region, it took only a sample of the parent population's genetic diversity.
"If you keep sampling like that, then mathematically you must lose variation," Feldman explained.
This massive research appears in many publications including journal Science.
The new study, as well as related research in the journal Nature, offer the sharpest pictures to date for understanding variation in the human genome.
(Read "Europeans Less Genetically Diverse Than Africans" [February 20, 2008].)
Previous studies have looked only at a thousand or so genetic markers and compared them between three or four populations. The new studies examine hundreds of thousands of markers in dozens of populations.
"It's sort of like looking at Mars with the naked eye versus with a big, very powerful telescope," said Richard Myers, a geneticist and study co-author also from Stanford University.
The fine resolution, for example, shows a shared chunk of genes between the Yakut in northeastern Siberia and Native Americans, which fits the archaeological record of migration across the Bering Strait.
The study also found, for the first time, distinctions between the northern and southern Chinese populations and separated out various populations in Europe.
But perhaps even more striking, Myers said, is how similar humans are to each other. Some 90 percent of the genetic variation occurs within populations, not among them.
"That turns out to be very profound, because it's not like we've got these 51 populations that are different species," he said. "We're really, really close to each other."
In fact, there's no single genetic marker that identifies a person as French or Japanese or Papuan. Rather, patterns of thousands of these little markers within the group distinguish one population from the next.
"Those genes which we classically use like skin color and eye color and hair structure to differentiate what we commonly call races is a tiny fraction of all the variation there is," Feldman, the evolutionary biologist, noted.
Spencer Wells is a population geneticist and director of the Genographic Project, which is charting the migratory history of humans around the globe.
(The National Geographic Society, which owns National Geographic News, is a sponsor of the project.)
Wells, who was not involved in the research, said confirmation of an African origin for modern humans is "the most important story that comes out of this study."
In particular, the pattern of variation shows that the route of migration out of Africa was into the Middle East and then to the rest of Eurasia, the Americas, and Oceania,
he pointed out.
"That tends to agree with what we're seeing on the Y-chromosome side," Wells said, referring to his genetic studies of male inheritance.
Populations in the Middle East have a unique signature of African, European, and Asian characteristics, Meyers, the geneticist, added.
"It looks like a gateway. You see a lot more mixture there ... that's one of the types of findings you get by looking at this level of detail," he said.
Henry Harpending is an anthropologist at the University of Utah in Salt Lake City who studies genes to understand the pace of evolution.
"We're barely scratching the surface in what we're learning from this."
Massive Genetic Study Supports "Out of Africa" Theory
John Roach
for National Geographic News
February 21, 2008
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