The first genetic analysis of ancient human remains in Africa suggests a massive migration from the north around the time of the Egyptian empire, once only suspected.
Africa is considered the cradle of human genetic diversity, the place from which humans spread across the globe.
But there appeared to have been a reverse migration in which farmers from the Near East and the ancient kingdoms of what is now Turkey and Iraq flooded back into the Horn of Africa, thousands of years after early humans left the continent. They brought crops new to Africa, such as wheat and barley.
What drew them back remains a mystery.
But evidence of their return, reported in the journal Science, comes from a DNA analysis of an adult male buried face down 4,500 years ago in a cave in the Ethiopian highlands.
Lead researcher Andrea Manica, a human geneticist at the University of Cambridge in England and the study's lead author, said the mass migration had an enormous influence over the entire continent, beginning with the Horn.
"It looks like the population almost as big as a quarter of what was already present in East Africa actually arrived and settled down in the area. And that's a huge amount," Manica said.
At least 5 percent of the African genome, Manica said, is traceable to the Eurasian migration.
Far beyond East Africa, it appears these ancient farmers mingled with native populations, from the Yoruba on the western coast to the Mbutu in the Congo. There, an estimated 6 to 7 percent of the indigenous populations' DNA was imprinted by Eurasian migration.
This is the first time geneticists have been able to sequence DNA from African fossils. The genetic material breaks down in the hot, humid climate. Until now, DNA from human fossils could be extracted for analysis only from ancient remains found in the cooler regions of Europe, Asia and North America. But the cave where the skeleton was found was cool and dry enough to preserve its genetic material.
For researchers looking for the origins of human populations, this represents a breakthrough.
"Every time we do this type of analysis, you need to have an African reference," Manica said. "Until now, what we had to do was to take the genome, the information that we have from modern Africans, and use that as our effectively representative" sample.
Manica added that the DNA of this one ancient individual has provided a snapshot of the population mixing believed to have taken place thousands of years ago in continental Africa.