Large-scale DNA analysis reveals diversity across the continent.
A wide-ranging DNA analysis of Africans has revealed a detailed picture of the continent's rich genetic diversity, as well as traces of the evolutionary history and migrations of various groups. The work could help scientists target the most diverse populations for further study and improve their chances of spotting genetic variants associated with disease.
Carried out by an international team of scientists over nearly a decade, the study suggests that Africans originated from 14 ancestral groups that mixed freely with each other to create the distinct populations that exist today. The team also found that African Americans tend to show ancestry primarily from western Africa, as might be expected from the history of the slave trade. The researchers based their analysis on DNA samples from about 100 African Americans and more than 2,400 Africans across the continent.
"The scope of the data is really extraordinary," says Molly Przeworski, a population geneticist at the University of Chicago, Illinois, who was not involved in the work. "We just didn't know as much as we should about African population genetics, and this is a major step in that direction."
Genes and languages
Modern humans first evolved in Africa about 200,000 years ago, before migrating to other parts of the world. Today, Africa has more than 2,000 groups with different ethnicities and languages. But genetic studies of Africans have been limited to small numbers of populations or have not covered large parts of the genome. Although geneticists knew that Africans show more genetic diversity within groups than non-Africans do, the details of genome-wide variation in many populations remained unclear.
“We just didn't know as much as we should about African population genetics. Molly Przeworski , University of Chicago”
A team led by geneticist Sarah Tishkoff of the University of Pennsylvania in Philadelphia has now published research that includes DNA samples from 2,432 Africans from 113 populations, including groups in Nigeria, Cameroon, Tanzania, Kenya and the Sudan plus non-African samples from Yemen. They looked for differences at 1,327 sites in the genome and combined the results with existing genetic data from 8 African and 59 non-African groups. The team then ran statistical analyses to cluster the individuals by genetic similarity and determine their ancestry.
The results confirm that Africans have the highest within-population diversity worldwide, and suggest that they originated from 14 ancestral groups. Most African populations seem to show genetic traces from multiple ancestral groups, supporting previous archaeological and linguistic evidence for migrations across the continent that would have led to mixing.
The analysis also suggests that hunter-gatherers from different regions and cultures, including pygmies in central Africa and click-language groups in southern Africa, may have descended from one ancestral population. The genetic clusters generally aligned with ethnicity and language, although the team found exceptions in cases where groups had lost, or possibly replaced, their languages. The research appears in Science1.
Roadmap of diversity
While the overall results are not surprising, the study gives a fine-scaled view of genetic variation across a large number of African populations, says Noah Rosenberg, a geneticist at the University of Michigan, Ann Arbor, who plans to collaborate with Tishkoff. "They show just how much diversity in Africa actually exists," he says.
The team also analysed DNA from four African American populations and found an average of 69-74% ancestry from a largely western African language family called Niger-Kordofanian. Most individuals probably have mixed ancestry from multiple West African groups, the analysis suggests. Because the differences between these groups are subtle, the team says that without even more genetic data it may be difficult for African Americans to pinpoint their particular tribes of origin.
David Reich, a geneticist at Harvard Medical School in Boston, Massachusetts, says the research could help scientists determine which African groups should be studied further in order to capture the continent's deep diversity. "This study will provide a roadmap," says Reich, who has collaborated with Tishkoff. By targeting the most divergent groups in Africa, he says, scientists might find more genetic variants linked to disease.
Tishkoff S. et al. Science doi:10.1126/science.1172257 (2009).