Scientists say they have extracted ancient DNA from the skull of a man buried in the highlands of Ethiopia 4,500 years ago that supports the theory that Eurasian farmers migrated into Africa some 3,000 years ago.
A pair of USF St. Petersburg anthropologists, Drs. John and Kathryn Arthur, were among the members of the international team of scientists, whose work was published in the journal Science.
This Stone Age resettlement had previously been theorized, but the rare find allowed scientists to see what DNA looked like well before the time the migration would have taken place. A comparison with modern populations around the world allowed them to see that the migrants left their genetic mark in the furthest corners of Africa.
"This is the first ancient human genome found in Africa to have been sequenced," said Marcos Gallego Llorente, a geneticist at the University of Cambridge and member of the international team.
Previously, scientists had only been able to sequence DNA from samples found in northern and arctic regions, because the climate there allows genetic material to survive for longer. In 2011, archaeologists with the help of local people discovered a cave containing the bones of a man - dubbed Mota - who died around 2,500 BC and from whose temporal bone they managed to extract intact DNA.
"Bayira allows us to piece together the population history of southwestern Ethiopia,” Kathryn Arthur said in a news release from USFSP. The Arthurs were led to Mota Cave by Gamo elders in 2011. “His genome is important for understanding later population movements between Africa and Eurasia.”
By comparing this ancient DNA with modern samples, researchers were able to map genetic changes that have taken place in the past 4,500 years. They found that East African populations now have as much as a quarter Eurasian ancestry, while those in the far west and south of the continent still have at least 5 percent of their genome from Eurasian migrants.
“Bayira’s genetic sequence does not contain any West Eurasia genes, supporting the idea that more recent population movements are responsible for Eurasian admixture into modern African populations,” said John Arthur, who we profiled for his research of beer in ancient Ethiopia in a 2013 University Beat report. “Thus, his genome is important for understanding the out-of-Africa expansion of Homo sapiens and later population movements between Africa and Eurasia.”
"This paper is exciting because it is the first to get ancient DNA from Africa. I think the analyses are also interesting, in particular, the claim that all sub-Saharan Africans today have a substantial amount of ancestry from back-to-Africa migrations," said David Reich, a geneticist at the Harvard Medical School who wasn't involved in the study. "This is a fairly surprising claim, but the analyses seem thorough."
The researchers traced this injection of genes to an event known as the 'Eurasian backflow.' It describes a period some 3,000 years ago when people from the Near East and Anatolia streamed into the Horn of Africa, a reverse migration to that which led the first humans out of Africa about 100,000 years ago.
"It is possible that there were even more ancient migrations back to Africa," said Llorente, "but what we can say for sure is that there was a very big migration after the time Mota lived."
It was so big, in fact, that the number of migrants flooding into the Horn of Africa may have amounted to over a quarter of the population of the region at the time.
It's not clear why they moved, though one theory that's been suggested is that farmers looking for fertile land traveled up the Nile. Wheat and barley, which first emerged in the Near East, appeared as crops in East Africa around 3,000 years ago.
Paul Heggarty, a linguist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who also wasn't involved in the study, said the Eurasian backflow theory ties in with research about the spread of Semitic languages from the Near East to Ethiopia.
By analyzing the kinds of genes the Stone Age farmers carried to Africa, the scientists also found they were closely related to the same population that had brought agriculture to Europe about 7,000 years ago. Today, those ancient farmers' closest genetic relatives are found on the island of Sardinia.
This means that modern-day migrants from East Africa crossing the Mediterranean to Europe may encounter distant cousins whose ancestors took a different path than theirs thousands of years ago.
“The significance of what John and Kathryn Arthur have discovered cannot be overstated,” said V. Mark Durand, Interim Regional Vice Chancellor of Academic Affairs. “Research findings like theirs may come along just once in a lifetime.”
“Our identities are tied to our heritage,” said Kathryn Arthur. “It is important to us that everyone can understand our work because of it’s relevance to everyone’s past or personal history.”
