About 7,500 years ago early farmers from the near east moved into Europe and met hunter-gatherers who had lived on the continent for tens of thousand of years previously. Researchers have known, based on genetics and archeology, that these two groups combined to make up the bulk of the European gene pool.

New research from Harvard Medical School and the University of Tübingen in Germany however has found that those two groups do not tell the full genetic story. Ancient North Eursians, the same group that crossed the Bering Strait to settle the Americas also contributed part of the genetic mix.

“Prior to this paper, the models we had for European ancestry were two-way mixtures. We show that there are three groups. This also explains the recently discovered genetic connection between Europeans and Native Americans. The same Ancient North Eurasian group contributed to both of them,” said David Reich, professor of genetics at HMS and co-senior author of the study in a statement.

The researchers also found that Near Eastern farmers can trace much of their ancestry to a group called the Basal Eurasians. Ancient North Eurasian DNA wasn’t found in samples from either the hunter-gatherer groups or the Near Eastern farming immigrants which suggests that the Eurasian DNA was introduced at a time more recent than 7,500 years ago.

“Nearly all Europeans have ancestry from all three ancestral groups. 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. The Ancient North Eurasian ancestry is proportionally the smallest component everywhere in Europe, never more than 20 percent, but we find it in nearly every European group we’ve studied and also in populations from the Caucasus and Near East,” said Iosif Lazaridis, a research fellow in genetics in Reich’s lab and first author of the paper.

The research was aided considerably by the discovery, in January, of DNA from two Ancient North Eurasians found in Siberia by a different group of Archeologists.

The scarcity of DNA samples from this period makes it unlikely that even this picture of Europe’s family tree is complete. The researchers believe that it’s entirely possible that more groups will be found to have contributed to European genetics.

The research appears in the September 17 edition of the journal Nature.