Genetic analysis reveals present-day Europeans descended from at least 3, not 2, groups of ancient humans.
The setting: Europe, approximately 7,500 years ago.
Agriculture was sweeping in from the Near East, bringing early farmers into contact with hunter-gatherers who had already been residing in Europe for tens of thousands of years.
Genetic and archaeological research in the last 10 years has shown that almost all present-day Europeans descended from a mixing of these two ancient populations. But in actual fact, that is not the full story.
Researchers from Harvard Medical School and the University of Tübingen in Germany have now documented a genetic contribution from a third ancestor: Ancient North Eurasians. This group appears to have contributed DNA to present-day Europeans, along with the people who traveled across the Bering Strait to the Americas over 15,000 years ago.
“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,” Reich added. “The same Ancient North Eurasian group contributed to both of them.”
Another discovery the research team made was that the ancient Near Eastern farmers and their European descendents can trace a large amount of their ancestry to a previously unknown, even older lineage called the Basal Eurasians.
Their research is published September 18th in Nature.
Peering into the past
To probe the continuous mystery of European’s heritage and their relationships to the rest of the world, the international research team– including co-senior author Johannes Krause, professor or archaeo- and paleogenetics at the University of Tübingen and co-director of the new Max Planck Institute for History and the Sciences in Jena, Germany– collected and sequences the DNA of over 2,300 present-day people from around the globe and of nine ancient humans from Sweden, Luxembourg and Germany.
The ancient bones came from eight hunter-gatherers who lived approximately 8,000 years ago, before the advent of farming, and one farmer from approximately 7,000 years ago.
The researchers also included in their study genetic sequences previously gathered from ancient humans of the same time period, including early farmers such as Ötzi “the Iceman”.
“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.” Said Reich.
It was found that Ancient North Eurasian DNA was not present in either the hunter-gatherers or the early farmers, implying the Ancient North Eurasians arrived in the area later.
“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.”
Lazaridis added, “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. A profound transformation must have taken place in West Eurasia after farming arrived.”
When the research was carried out, Ancient North Eurasians were considered a “ghost population”– an ancient group known only through the traces left in the DNA of present-day people. Then, in January, a separate group of archaeologists discovered the physical remains of two Ancient North Eurasians in Siberia. Now, said Reich, “We can study how they’re related to other populations.”
Room for more
The team was only able to progress so far in its analysis because of the limited number of ancient DNA samples. Reich thinks there could easily be more than three ancient groups who contributed to the genetic profile of today’s European people.
Reich and his colleagues found that the three-way model does not tell the whole story for particular regions of Europe. Mediterranean groups such as the Maltese, as well as Ashkenazi Jews, had more Near east ancestry than anticipated, while far northeastern Europeans such as the Finns and the Saami, as well as some northern Russians, had more East Asia ancestry in the mix.
The most startling aspect of the project for Reich, however, was the discovery of the Basal Eurasians.
“This deep lineage of non-African ancestry branched off before all the other non-Africans branched off from one another,” he 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.”
Next, the team wishes to work out when the Ancient North Eurasians arrived in Europe and to find ancient DNA from the Basal Eurasians.
“We are only starting to understand the complex genetic relationship of our ancestors,” said co-author Krause. “Only more genetic data from ancient humans remains will allow un to disentangle our prehistoric past.”
“There are important open questions about how the present-day people of the world got to where they are,” said Reich, who is a Howard Hughes Medical Investigator. “The traditional way geneticists study this is by analyzing present-day people, but this is very hard because present-day people reflect many layers of mixture and migration.
“Ancient DNA sequencing is a powerful technology that allows you to go back to the places and periods where important demographic events occurred,” he said. “It’s a great new opportunity to learn about human history.”
Contributing Source: Harvard Medical School
Header Image Source: Wikimedia
