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Evidence of ancient migration from Eurasia to Africa

Investigators of human population genetics often assume that migrations in human history have been in the direction away from Africa — from Africa to the Middle East, then on to Europe and East Asia, and then on to the Americas and Oceania. Now scientists find evidence that a number of groups in southern Africa may have some distant ancestors most closely related to Europeans and Middle Easterners, suggesting ancient migrations from Eurasia to Africa. The findings are detailed in the Proceedings of the National Academy of Sciences.

The hunter-gatherer and the pastoralist (shepherd and cattle-herder) populations of southern Africa are among the world’s most culturally, linguistically, and genetically diverse human populations. While hunter-gatherer populations have apparently lived in southern Africa for tens of thousands of years, past research suggested that within approximately the last 2,000 years, both pastoralists and agriculturalists arrived in the area, leading to the varied populations in the region today.

The majority of southern Africa is descended from Bantu-speaking agriculturalists. Population geneticist Joseph Pickrell at the New York Genome Center and his colleagues focused on southern Africa’s Khoisan (also spelled Khoe–San), hunter-gatherers and pastoralists populations who speak non-Bantu languages with click consonants. Historically, the Khoe were pastoralists while the San were hunter-gatherers.

The scientists found there were at least two major interbreeding events in the history of the Khoisan. One involved populations related to Niger-Congo-speaking African populations. Intriguingly, the other introduced ancestry most closely related to west Eurasian — European or Middle Eastern — populations. The researchers dated this latter event to about 900 to 1,800 years ago.

Interestingly, the scientists detected a similar signal of west Eurasian ancestry throughout eastern Africa. Specifically, they found evidence for two interbreeding events in the history of Kenyan, Tanzanian, and Ethiopian populations. The earlier of these events involved populations related to west Eurasians and happened about 2,700 to 3,300 years ago.

The west Eurasian gene variants the research team detected are similar in both eastern Africa and in the Khoisan. The investigators suggest west Eurasian ancestry first entered eastern Africa and then southern Africa.

Past research revealed the Khoisan diverged genetically from other African groups. This study had also found the Nama, a pastoralist Khoe group, shared a small but very distinct genetic component with east Africans — for instance, the cattle-herding Maasai — suggesting this east African component was introduced by east African groups that brought pastoralist practices to southern Africa. “I think the things we identify are probably the explanation for some of the observations in that paper,” Pickrell said.

These findings contradict assumptions that migrations in human history have been in the direction away from Africa. “I’d never really realized I’d made this assumption until we saw this clear signal of migration in the other direction,” Pickrell says. “Of course in retrospect it’s a bit of a silly assumption to make — people have probably been moving all over the world in every possible direction for tens of thousands of years.”

One uncertainty that remains is whether such findings regarding interbreeding are really the results of large movements of people over a short period of time, or smaller movements of people spread out over a long period of time. “We’ve interpreted our results as the former, and there are a few reasons that we think this is correct in this case, but it’s really quite difficult to establish this definitively,” Pickrell says.

In the future, the scientists want to investigate whether these intermingling events led to any recognizable genetic changes in southern and eastern Africa, such as lactose tolerance. Lactose tolerance was a genetic innovation that helped people sustainably derive nourishment from livestock without killing it.

In addition, future research could aim to unearth the group that moved into eastern Africa about 3,000 years ago. “There are only hints of this population in archaeology and recorded history, but presumably this was a really important time in the history of the region,” Pickrell says. “Really the only definitive way to answer the questions we raise in this study is to combine genetics and archaeology by doing DNA analysis of archaeological samples. Hopefully there will be some samples in Africa with intact DNA!”

Categories: Genetics
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