White Americans are both genetically weaker and less diverse than their black compatriots, a Cornell University-led study finds.
Analyzing the genetic makeup of 20 Americans of European ancestry and 15 African-Americans, researchers found that the former showed much less variation among 10,000 tested genes than did the latter, which was expected.
They also found that Europeans had many more possibly harmful mutations than did African, which was a surprise.
"Since we tend to think of European populations as quite large, we did not expect to see a significant difference in the distribution of neutral and deleterious variation between the two populations," said senior co-author Carlos Bustamante, an assistant professor of biological statistics and computational biology at Cornell.
It's been known for years that all non-Africans are descended from a small group, perhaps only a few dozen individuals, who left the continent between 50,000 and 100,000 years ago.
But the Cornell study, published in the journal Nature Thursday, indicates that Europeans went through a second "population bottleneck," probably about 30,000 years ago, when the ancestral population was again reduced to relatively few in number.
• Click here for the Cornell press release, and here for the full article in Nature.
The doubly diluted genetic diversity has allowed "bad" mutations to build up in the European population, something that the more genetically varied African population has had more success in weeding out.
"What we may be seeing is a 'population genetic echo' of the founding of Europe," said Bustamante.
The Cornell team hopes to study other population groups in search of similar results.
For example, Native Americans show even less genetic diversity than Europeans, having descended from a few thousand people who entered North America about 10,000 years ago.
That fact was reinforced by a larger-scale study, also published in Nature, led by scientists from the Universities of Michigan and Virginia who analyzed genetic samples of 485 individuals scattered around the globe whose DNA is recorded in a French databank.
As would be expected with the "out of Africa" theory, the researchers found Africans had the greatest amount of genetic diversity, followed in turn by Middle Easterners, then Europeans and South Asians at about equal levels, then East Asians.
Native Americans had the least genetic diversity of all, indicating that part of the world was settled last.
"Previously, we've been able to look at the genome and say, 'This part is from Africa, this is from Asia,'" explained Virginia research Andrew Singleton to Wired News. "Now we can look past that and say, 'It's from this part of Africa or Eurasia.'"
A third study, published in the journal Science on Friday, may be the most fascinating of all.
Drawing on 935 individual samples from the French databank, a Stanford University team found deep traces of long-ago population movements, all originating from a "ground zero" in Ethiopia, Kenya or Tanzania.
For example, the Pygmies of the Congo forest were found to be quite close to the Bushmen of Namibia — but both were very different from most other sub-Saharan groups.
The fierce and proud Bedouin nomads of the Middle East actually have a lot of European and South Asian blood.
The Asian-looking Hazara of Afghanistan are correct in claiming ancestry in Mongolia, but the Han, the dominant ethnic group in China, may be disappointed to discover they're actually two peoples, one north, the other south.
Native Americans have at least one closely related group in Asia — the Yakuts of eastern Siberia, who themselves are related to other hunter-gatherer Siberian tribes, some of whom build wooden teepees.
The Basques in northeastern Spain and southwestern France may be right to demand their own nation — they're not closely related to anyone else. Surprisingly, neither are the residents of Sardinia off the coast of Italy.
As with the other large-scale study, the Stanford team found the greatest diversity outside of Africa among people living in the wide crescent of land stretching from the eastern shore of the Mediterranean to northern India.
Not only was the region among the first colonized by the African migrants, they theorize, but the large number of European and East Asian genes among the population indicates that it's long been the human highway, with large numbers of migrants from both directions conquering, trading and generally reproducing along its entire length.
• Click here for the abstract of the Stanford study, and here for a comprehensive write-up of all three studies in the Washington Post.