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Massive Genetic Study Supports "Out of Africa" Theory

John Roach
for National Geographic News
February 21, 2008
 
A massive new study of human genetic diversity reveals surprising insights into our species' evolution and migrations—including support for the theory that the first modern humans originated in Africa—scientists said today.

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 settled the world in stepping-stone fashion after leaving 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.

The research appears in tomorrow's issue of the journal Science.

Sharpest Detail Yet

The new study, as well as related research published yesterday 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.

Africa to the Middle East

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.

Ongoing Selection?

Henry Harpending is an anthropologist at the University of Utah in Salt Lake City who studies genes to understand the pace of evolution.

(Related: "Human Evolution Speeding Up, Study Says" [December 11, 2007])

While the new study is solid science, Harpending said he'd like to see the researchers address how natural selection has affected global human diversity.

In particular, he pointed out, he'd be interested in whether increased natural selection caused by new environments triggered humans to become less diverse—not just the small size of migrating groups.

"If there's a lot of selection going on, a new gene shows up, it's favored, and pretty soon it replaces the others and it drags the neighboring part of the chromosome with it," he said.

These "selective sweeps," he added, "destroy diversity" because the selection quickly inserts new genes in entire populations.

"In the last few years, we've discovered that a lot of the genome is under selection and that may be driving these patterns," he said.

Myers noted that his research team has made all of its data publicly available, meaning that other scientists are likely to use the information to perform all sorts of additional studies such as these.

"We're barely scratching the surface in what we're learning from this."

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