Researchers believed almost all modern-day Europeans were descended from a mixing of early farmers and hunter-gatherers 7,500 years ago, but new research suggests Ancient North Eurasians may have also made a contribution to the gene pool.
The group is believed to have travelled across the Bering Strait into the Americas more than 15,000 years ago, Harvard Medical School reported.
"Prior to this paper, the models we had for European ancestry were two-way mixtures. We show that there are three groups," said David Reich, professor of genetics at HMS and co-senior author of the study. "This also explains the recently discovered genetic connection between Europeans and Native Americans. The same Ancient North Eurasian group contributed to both of them."
The team found ancient Near Eastern farmers and their European descendants can trace their roots to an even older lineage called the Basal Eurasians. The Harvard researchers along with colleagues from the University of Tübingen collected and sequenced DNA samples from more than 2,300 present-day people from across the globe and of nine ancient humans from Sweden, Luxembourg and Germany. The ancient bones were from eight hunter-gatherers who walked the Earth about 8,000 years ago, before the arrival of farming, and one from about 7,000 years ago.
"There was a sharp genetic transition between the hunter-gatherers and the farmers, reflecting a major movement of new people into Europe from the Near East," Reich said.
Ancient North Eurasian DNA wasn't found in the hunter-gatherers or early farmers, suggesting they came on the scene at a later time.
"Nearly all Europeans have ancestry from all three ancestral groups," said Iosif Lazaridis, a research fellow in genetics in Reich's lab and first author of the paper. "Differences between them are due to the relative proportions of ancestry. Northern Europeans have more hunter-gatherer ancestry - up to about 50 percent in Lithuanians - and Southern Europeans have more farmer ancestry."
In the past, the North Eurasians were considered to be a "ghost population," but that all changed when researchers found the remains of two Ancient North Eurasians in Siberia in January.
"This deep lineage of non-African ancestry branched off before all the other non-Africans branched off from one another," Reich said. "Before Australian Aborigines and New Guineans and South Indians and Native Americans and other indigenous hunter-gatherers split, they split from Basal Eurasians. This reconciled some contradictory pieces of information for us."
In the future, the team hopes to determine when the Ancient North Eurasians arrived in Europe.
"We are only starting to understand the complex genetic relationship of our ancestors," said co-senior author Johannes Krause, professor of archaeo- and paleogenetics at the University of Tübingen. "Only more genetic data from ancient human remains will allow us to disentangle our prehistoric past."
The findings were published in a recent edition of the journal Nature.
