New studies of ancient DNA are shifting
scientists' ideas of how groups of people migrated across the globe and
interacted with one another thousands of years ago. By comparing nine ancient
genomes to those of modern humans, Howard Hughes Medical Institute (HHMI)
scientists have shown that previously unrecognized groups contributed to the
genetic mix now present in most modern-day Europeans.
"There are at least three major, highly
differentiated populations that have contributed substantial amounts of
ancestry to almost everybody that has European ancestry today," says David
Reich, an HHMI investigator at Harvard Medical School. Those include
hunter-gatherers from western Europe, the early farmers who brought agriculture
to Europe from the Near East, and a newly identified group of ancient north
Eurasians who arrived in Europe sometime after the introduction of agriculture.
That means there were major movements of people into Europe later than
previously thought. The team, led by Reich and Johannes Krause at the
University of Tübingen in Germany, reported their findings in the September 18,
2014, issue of the journal Nature.
In the last five years, genetic evidence has
demonstrated that migrants from the Near East brought agriculture with them to
Europe when they arrived about 8,500 years ago. But the genomes of present-day
Europeans show signs that they come from more than just the indigenous
hunter-gatherers and these early farmers.
Two years ago, Reich's group uncovered
genetic evidence that most present-day Europeans are a mixture of groups
related to southern Europeans, Near Easterners, and a third group most closely
related to Native Americans. "That was a crazy observation, but it's very
strong statistically," Reich says. "We argued that this is because of
the contribution of an ancient north Eurasian population some of whose members
contributed to the peopling of the Americas more than 15,000 years ago, and
others of which later migrated to Europe."
To clarify that early history, Reich's team,
including more than 100 collaborators worldwide, collected genetic data from
nine ancient skeletons and 203 present-day populations living all over the
world. Collaborators isolated human DNA and sequenced the complete genomes from
the bones of a 7,000-year old skeleton found in Germany and eight skeletons of
hunter-gatherers who lived in Luxembourg and Sweden about 8,000 years ago. They
compared those genomes to those of the 2,345 people in their contemporary
populations.
That required developing new computational
methods for genetic analysis. "Figuring out how these populations are
related is extremely hard," Reich says. "There's a lot that happened
in Europe in the last 8,000 years, and this history acts like a veil, making it
difficult to discern what happened at the beginning of this period. We had to
find statistics that were able to tell us what happened deep in the past
without getting confused by 8,000 years of intervening history, when massive
and important events occurred."
"What we find is unambiguous evidence
that people in Europe today have all three of these ancestries: early European
farmers who brought agriculture to Europe, the indigenous hunter-gatherers who
were in Europe prior to 8,000 years ago, and these ancient north
Eurasians," Reich says. Further analyses showed that describing
present-day Europeans as a mixture of the three populations is a good fit for
most, although not all, populations.
When the study began, the ancient north
Eurasian population was a "ghost population" – identified based on
genetic patterns without any ancient DNA. But in 2013, another group analyzed
DNA from two skeletons found in Siberia, one from 24,000 years ago and one from
17,000 years ago, and found that it shared genetic similarities with Europeans
and North Americans. The ghost, Reich says, had been found.
Although DNA from ancient north Eurasians is
present in nearly all modern Europeans, Reich's team did not find it in their
ancient hunter-gatherers or the ancient farmers. That means the north Eurasian
line of ancestry was introduced into Europe after agriculture had been
established, a scenario most archaeologists had thought unlikely.
"We have this amazing observation that
only two ancestries are represented among the first farmers, from about 7,000
to 5,000 years ago. And then suddenly everybody today has ancient north
Eurasian ancestry," Reich says. "So there must have been a later
movement of this ancestry into Europe."
Anthropologists have long thought that
densely settled populations would be resistant to the arrival of new groups.
"But this is hard evidence that exactly such a major migration
occurred," Reich says. "It's very important because it's a major
contributor to Europeans today." The time of the ancient north Eurasians'
arrival remains to be determined, but Reich says their later-than-expected
movement into Europe might help explain the complex mix of languages that
exists there today.
The team's data also reveals that the first
farmers to reach Europe from the Near East had ancestors from a previously
unidentified lineage, which Reich's group named the Basal Eurasians. Basal
Eurasians were the first people to separate from the larger group of
non-Africans, before other non-African groups diversified. Reich says that
attempts to identify the first group to split from the non-Africans had always
been puzzling: genetic evidence indicates that this is likely to be Europeans
or Near Easterners, even though some archaeological evidence has indicated that
people were in New Guinea and Australia before they were Europe.
The new analysis shows that the Near Easterners who came into
Europe 8,000 years ago brought with them a strand of ancestry that had
separated before the ancestors of Australian aborigines separated from the
indigenous people of Europe. "That population must have been hanging out
somewhere in the Near East for a very long time," Reich says. Now he would
like to know how that population fits into the archaeological history of the
region. Ancient DNA from Basal Europeans, if found, might lead to new
revelations about early human history.