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Ancient Ethiopian Genome Highlights Influence of Eurasian Backflow Into Africa

NEW YORK (GenomeWeb) – An international team of researchers has sequenced an ancient Ethiopian genome that has provided insight into the evolutionary history of modern humans.

Researchers led by Andrea Manica from the University of Cambridge sequenced DNA isolated from a 4,500-year-old man found in a cave in southern Ethiopia. By comparing this ancient Ethiopian genome to those of other ancient and contemporary human genomes, Manica and his colleagues found that the Eurasian genetic backflow into Africa some 3,000 years ago came from a population closely related to the Early Neolithic farmers that colonized Europe some 4,000 years prior, as they reported today in Science.

This backflow, they further noted, was more extensive and reached deeper into Africa than previously thought.

"Roughly speaking, the wave of West Eurasian migration back into the Horn of Africa could have been as much as 30 percent of the population that already lived there — and that, to me, is mind-blowing," Manica said in a statement.

Manica and his colleagues sequenced DNA from the petrous bone of the ancient Ethiopian man, called Mota, after a cave where he was buried, to a mean 12.5X coverage. They estimated contamination to be between 0.29 percent and 1.26 percent.

They noted that Mota lacked derived alleles found in Eurasian populations linked to skin and eye color, and was likely lactose intolerant. He did, though, have all three altitude adaptation alleles found in modern highland Ethiopian populations.

At the same time, radiocarbon dating estimated the skeleton to be about 4,500 years old. This, the researchers noted, means that Mota's remains predate both the Bantu expansion and the West Eurasian backflow some 3,000 years ago.

Using an f4 ratio estimate, the researchers gauged the West Eurasian portion of Mota's genome, failing to find any.

To search for the source of that West Eurasian backflow, the researchers modeled the Ari, a contemporary Ethiopian highland population that's the most closely related to Mota, as a mixture of Mota and ancient and contemporary West Eurasian populations. In this analysis, modern Sardinians and the early Neolithic Stuttgart individual stood out, they reported.

In previous analyses, Sardinians have been shown to be the closest modern population related to Neolithic farmers, they noted.

This, Manica and his colleagues said, suggests that the West Eurasian backflow into Africa came from the same genetic source as the Neolithic expansion in Europe from the Near East or Anatolia. They added that a previous haplotype analysis had uncovered a link between modern Ethiopians and Anatolia and noted that archaeological evidence dates the arrival of domesticated Near East crops like wheat, barley, and lentils in Africa to about 3,000 years ago.

All together, this suggested to the researchers that the direct descendants of the farmers who brought agriculture to Europe may also have had a role in bringing new forms of food production to the Horn of Africa.

Using Mota as an unmixed African reference and the Stuttgart farmer as the source of the West Eurasian backflow, the researchers traced how this gene flow influenced populations in Africa. They found that the backflow had a broader geographical impact than originally thought — for instance, it appears to spread to West and Southern Africa.

They also reported that an additional 4 percent to 7 percent of the genomes of most populations in Africa could be traced to a Eurasian source.

"Genomes from this migration seeped right across the continent, way beyond East Africa, from the Yoruba on the western coast to the Mbuti in the heart of the Congo — who show as much as 7 percent and 6 percent of their genomes respectively to be West Eurasian," first author Marcos Gallego Llorente, also from Cambridge, said.

As Mota predates modern demographic events that shuffled population genetics in Africa, the researchers argued that his genome could be used as a reference to study the out-of-Africa expansion and other demographic events.