Une étude de « Nature » ▻http://www.nature.com/ncomms/journal/v4/n4/full/ncomms2656.html revue par le National Geographic : les origines des « européens modernes » remontreraient à moins longtemps qu’on ne le pensait, ▻http://news.nationalgeographic.com/news/2013/13/130423-european-genetic-history-dna-archaeology-science
Europeans as a people are younger than we thought, a new study suggests .
DNA recovered from ancient skeletons reveals that the genetic makeup of modern Europe was established around 4,500 B.C. in the mid-Neolithic—or 6,500 years ago—and not by the first farmers who arrived in the area around 7,500 years ago or by earlier hunter-gatherer groups.
“The genetics show that something around that point caused the genetic signatures of previous populations to disappear,” said Alan Cooper, director of the Australian Centre for Ancient DNA at the University of Adelaide, where the research was performed.
“However, we don’t know what happened or why, and [the mid-Neolithic] has not been previously identified as [a time] of major change,” he said.
Furthermore, the origins of the mid-Neolithic populations that did form the basis of modern Europe are also unknown.
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The study shows that “relatively recent migrations seem to have had a significant genetic impact on the population of Central Europe,” said study co-author Spencer Wells, who leads National Geographic’s Genographic Project.
Genetic Signature
In the study, Cooper and his colleagues extracted mitochondrial DNA—which children inherit only from their mothers—from the teeth and bones of 39 skeletons found in central Germany. The skeletons ranged in age from about 7,500 to 2,500 years old.
The team focused on a group of closely related mitochondrial lineages—mutations in mitochondrial DNA that are similar to one another—known as haplogroup H, which is carried by up to 45 percent of modern Europeans.
Cooper and his colleagues focused on haplogroup H because previous studies have indicated the mutations might have been present in Europeans’ genetic makeup for several thousand years.
It’s unclear how this haplogroup became dominant in Europe. Some scientists have proposed that it spread across the continent following a population boom after the end of the last ice age about 12,000 years ago.
But the new data paint a different picture of the genetic foundation of modern Europe: Rather than a single or a few migration events, Europe was occupied several times, in waves, by different groups, from different directions and at different times.
The first modern humans to reach Europe arrived from Africa 35,000 to 40,000 years ago. By about 30,000 years ago, they were widespread throughout the area while their close cousins, the Neanderthals, disappeared. Hardly any of these early hunter-gatherers carried the H haplogroup in their DNA.
About 7,500 years ago during the early Neolithic period, another wave of humans expanded into Europe, this time from the Middle East. They carried in their genes a variant of the H haplogroup, and in their minds knowledge of how to grow and raise crops. ’
Archeologists call these first Central European farmers the linear pottery culture (LBK)—so named because their pottery often had linear decorations.
The genetic evidence shows that the appearance of the LBK farmers and their unique H haplogroups coincided with a dramatic reduction of the U haplogroup—the dominant haplogroup among the hunter-gatherers living in Europe at that time.
Farmers Move In
The findings settle a longstanding debate among archaeologists, said Wells, who is also a National Geographic explorer-in-residence.
Archaeology alone can’t determine whether cultural movements—such as a new style of pottery or, in this case, farming—were accompanied by the movements of people, Wells said in an email.
“In this study we show that changes in the European archaeological record are accompanied by genetic changes, suggesting that cultural shifts were accompanied by the migration of people and their DNA.”
The LBK group and its descendants were very successful and spread quickly across Europe. “They became the first pan-European culture, if you like,” Cooper said.
Given their success, it would be natural to assume that members of the LBK culture were significant genetic ancestors of many modern Europeans.
But the team’s genetic analysis revealed a surprise: About 6,500 years ago in the mid-Neolithic, the LBK culture was itself displaced. Their haplogroup H types suddenly became very rare, and they were subsequently replaced by populations bearing a different set of haplogroup H variations.
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“All we know is that the descendants of the LBK farmers disappeared from Central Europe about 4,500 [B.C.], clearing the way for the rise of populations from elsewhere, with their own unique H signatures.”
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One thing that is clear from the genetic data is that nearly half of modern Europeans can trace their origins back to this mysterious group.
“About [4,500 B.C.], you start seeing a diversity and composition of genetic signatures that are beginning to look like modern [Central] Europe,” Cooper said. "This composition is then modified by subsequent cultures moving in, but it’s the first point at which you see something like the modern European genetic makeup in place.
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