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May 11, 2009
Landmark Study Looks at Genetics of Africans, African Americans
Researchers have analyzed genetic variation in people across Africa, helping to tease apart the complex evolutionary history of Africans and African Americans. The results will also help uncover genes that contribute to disease in these populations.
Humans originated in Africa about 200,000 years ago and spread across the globe within the past 100,000 years. Africa still has over 2,000 population groups, defined by ethnicity, shared culture and language. However, African populations have not been well represented in human genetic studies.
To understand genetic variation among African populations, a team of African, American and European researchers spent years collected samples from people living in remote regions of Africa. The effort was led by Dr. Sarah Tishkoff of the University of Pennsylvania. It was funded by NIH's National Institute of General Medical Sciences (NIGMS), National Human Genome Research Institute (NHGRI) and National Heart, Lung and Blood Institute (NHLBI), among other sources.
The researchers analyzed the DNA of over 2,500 people from 121 geographically diverse African populations. They also studied 4 populations in the United States—from Chicago, Baltimore, Pittsburgh and North Carolina—and hundreds of people in 60 other areas across the world. They used over 1,300 genetic markers to reveal variations across these genomes.
The researchers reported their results in the online edition of Science on April 30, 2009. They were able to cluster African ancestral populations into 14 genetic groups. These correlated well with groupings by ethnicity, culture and language.
Most populations had high levels of mixed ancestry, reflecting historic migrations across the continent. The data suggest shared ancestry among some geographically diverse hunter-gatherer populations, such as Khoesan-speakers—who use languages containing click-consonants—and Pygmies.
The analysis places the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola, corresponding to the current San homeland. Modern populations may not reflect those of the past, but this result is consistent with those of other studies. The ancestry of modern African Americans appears to be predominantly from Niger-Kordofanian (about 71%), European (about 13%) and other African (about 8%) populations. However, the mix varies considerably among individuals.
"Our goal," Tishkoff says, "has been to do research that will benefit Africans, both by learning more about their population history and by setting the stage for future genetic studies, including studies of genetic and environmental risk factors for disease and drug response."
The researchers say that additional sampling will remain important for the future, particularly in regions that were underrepresented in this study, such as North and Central Africa.
—by Harrison Wein, Ph.D.