We hear a lot about the ‘Out of Africa’ theory that humans originated in Africa and then migrated to other countries around the world. But a fascinating new study published in the Proceedings of the National Academy of Sciences has revealed a back-to-Africa gene flow in which the genes from west Eurasia spread into southern African populations.

The research team led by Harvard Medical School’s David Reich examined several southern African Khosian populations — broadly defined as indigenous populations that speak non-Bantu languages with click consonants – and compared the DNA to a dataset included 1,040 people from 75 populations worldwide. They uncovered evidence of two events, one which took place around 3,000 years ago in which there was gene flow from European or Middle Eastern populations to eastern African populations, and the other which traced back to about 1,300 years ago in which gene flow spread from an already mixed east African population to southern Africans.

"The most striking inference from this analysis is the presence of west Eurasian ancestry in southern Africa," said Reich and his colleagues. "Several lines of evidence suggest that the population that brought this ancestry to southern Africa was an already-admixed population from eastern Africa."

Joseph Pickrell, a co-author of the study, said that he and his colleagues had uncovered hints of ancestry from individuals resembling present-day populations in Italy in southern and eastern African populations. This apparent mixing, likely through an intermediate population in eastern African, underscored the effect of uncharacterized back-to-Africa gene flow.

The west Eurasian admixture event with the eastern African populations was the more ancient admixture event. Still, they added that the ultimate source of this west Eurasian ancestry is an "open question."

While the archaeological record from that timeframe is limited, the researchers said that architecture in Ethiopia from this time period bears a strong resemblance to that of southern Arabia and the timing broadly coincides with the introduction of Ethiosemitic languages to Africa. Pulling together the genetic, archaeological, and linguistic data, Reich and his colleagues proposed that people from west Eurasia moved into Ethiopia some 3,000 years ago, distributing west Eurasian ancestry throughout the region.

By April Holloway