DNA comparison of Africa's ethnic groups quantifies genetic diversity: differences could reveal details of modern human origins.
Until now, most genetic surveys of this type have used data from just a few African groups assumed to reflect Africa's genetic diversity. But the new research shows that "no single African population is representative of the diversity of the continent," says study coauthor Sarah Tishkoff of the University of Pennsylvania in Philadelphia.
Tishkoff and her colleagues analyzed particular DNA sequences--series of the chemical letters that encode genetic information--from more than 3,000 people from 121 different populations scattered throughout Africa. Researchers divided the participants based on self-identified ethnic groups.
To reach remote groups, such as the Pygmies of Cameroon and the hunter-gatherers of Tanzania, researchers drove off-road and set up makeshift labs with equipment powered by their car battery.
"This is by far the most in-depth analysis in terms of the number of populations analyzed," comments evolutionary geneticist Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
The researchers found that the genetic sequences were highly diverse from one population to the next. "We knew that African populations were diverse in culture, art, religious ideas," says Roy King of Stanford University School of Medicine. "Now we see that genetic diversity goes along these same lines."
Because modern humans originated in Africa, there has been more time for changes to accumulate in the African DNA sequences than there has been in sequences from people in other parts of the world, Tishkoff says.
It turns out that the San bushmen of southern Africa have the most distinct, and therefore oldest, genetic sequences, the team reports. Researchers theorize that the San homeland could be the spot where modern humans began their exodus from Africa to the rest of the world. This location, near the coastal border of Namibia and Angola in southwestern Africa, fits with previously proposed sites of the out-of-Africa migration. But the scientists note the area could just be where the San settled most recently.
Tishkoff and her colleagues also found that some sequences were relatively similar across various populations, often correlating with similarities in culture and language. These patterns serve as "genetic footprints" revealing the past migration of different groups around the continent. Somewhat similar groups--such as three hunter-gatherer groups now living in different parts of Africa, bur who all speak languages that incorporate click sounds--probably shared common ancestors, the scientists say.
Genetic information from African-Americans living in three U.S. cities and an additional state was also collected and analyzed. On average, African-Americans inherited 71 percent of their DNA from western Africa, 8 percent from other locations in Africa and 13 percent from Europe, the team says. Most of the African-Americans in the study had mixed ancestry from different regions of western Africa, which made tracing ancestry to particular ethnic groups difficult, Tishkoff says.
The researchers are quick to point out that the data set is incomplete. "We analyzed 121 populations out of a possible 2,000," Tishkoffsays. Details of the analysis are sure to change as more information becomes available, but the study is a good starting point, the researchers say.
"We're hoping to provide a framework for others to build on," Tishkoff says.
Connecting ethnic groups
The largest genetic study of the people of Africa found that similar genetic sequences often correlate with similarities in language and culture. Colors represent areas occupied by similar populations.
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|Date:||May 23, 2009|
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