Expansive genetic diversity in Africa revealed

Study is largest DNA comparison yet among continent's populations

The largest genetic study of African populations reveals a greater genetic diversity of the continent’s cultural groups than previously known, scientists say. The study also yields insight into the origins of modern humans and the ancestry of African-Americans, researchers report online in Science April 30 and also during an April 29 press teleconference.

OUTREACH Hadza and Datog people in northern Tanzania listen as researchers explain the largest genetic study of African populations. PHOTO FROM Sarah Tishkoff

CULTURE MAP Researchers analyzed genetic information from 121 different ethnic groups, each represented by a circle on the map. The colored sections represent areas where populations were found to have similar genetic sequences, which often correlated with similarities in language, culture and lifestyle. These similarities suggested common ancestry among the groups. MAP CREDIT: Floyd A. Reed

Until now, most genetic studies have used data from just a few African groups that were 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 the genetic information of more than 3,000 people from 121 different populations of self-identified ethnic groups scattered throughout Africa.

Obtaining the samples was no mean feat. 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. 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 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. The 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.

The team found that different African populations had differences in their gene sequences. “We knew that African populations were diverse in culture, art, religious ideas. Now we see that genetic diversity goes along these same lines,” comments Roy King of Stanford School of Medicine.

At the same time, some gene sequences were relatively similar across various populations, often correlating with similarities in culture and language. These patterns serve as “genetic footprints” revealing the migration of different groups around the continent, Tishkoff says. These somewhat similar groups — such as three hunter-gatherer groups now living in different parts of Africa, but who all speak languages that incorporate click sounds — probably shared common ancestors, the scientists say.

The researchers also analyzed genetic information from African-Americans living in four different U.S. cities. On average, African-Americans inherited 71 percent of their DNA from West Africa, 8 percent from other locations in Africa and the remaining 13 percent from Europe, the team says. Most African-Americans had mixed ancestry from different regions of West Africa, making tracing ancestry to particular ethnic groups difficult without more in-depth analysis, 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,” says Tishkoff. 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,” says Tishkoff.

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