Researchers delving into the genetic makeup of modern Europeans say DNA has revealed an ancient tribe that was a "fourth strand" of European ancestry.

The three generally accepted ancestral populations that gave rise to modern Europeans—indigenous hunters, Middle Eastern farmers and a migratory Bronze Age population from the east—have now been joined by another, they say.

DNA from ancient remains from the Caucasus proves that a small but important portion of the European genome comes from a distinct population of hunter-gatherers who survived the ice age by sheltering for several thousand years in the Caucasus Mountains on the present day Russian-Georgian border, a study appearing in Nature Communications explains.

Around 22,000 years ago, as the ice age receded, those Caucasus peoples began to merge with horse-riding herders of the steppes of Eastern Europe, a culture known as the Yamnaya, the researchers explain.

That culture spread into Western Europe about 5,000 years ago, joining the earlier ancestral populations of the indigenous hunter-gatherers who entered Europe 40,000 years ago, and a Middle East farming population that moved into the area 7,000 years ago.

The Yamnaya brought Bronze Age metallurgy and herding skills, as well as the Caucasus ancestral DNA now found in almost all populations across continental Europe, the researchers explain.

The mystery of origin of the Yamnaya and their DNA history has long puzzled scientists, says researcher Andrea Manica at the University of Cambridge in Britain.

"We can now answer that as we've found that their genetic make-up is a mix of Eastern European hunter-gatherers and a population from this pocket of Caucasus hunter-gatherers who weathered much of the last ice age in apparent isolation," he explains.

That Caucasus pocket represent a fourth major strand of ancient European ancestry, one that was unknown before, he says.

DNA extracted from remains found in two burials in Georgia, one more than 13,000 years old and the other nearly 10,000 years old, reveals the Yamnaya culture owed half its ancestry to that fourth strand, the previously undiscovered and genetically distinct Caucasus hunter-gatherers, the researchers say.

That Caucasus strand split from western hunter-gatherers soon after anatomically modern humans left Africa and moved into Europe, they say.

Sheltering in the Caucasus Mountains during the ice age may have cut those eastern hunter-gatherers off from the other major ancestral groups for as many as 15,000 years before the retreating ice and glaciers allowed them to migrate and merge with other groups, they explain.

"We knew that the Yamnaya had this big genetic component that we couldn't place, and we can now see it was this ancient lineage hiding in the Caucasus during the last ice age," says Manica.

Researchers also said those hunter gatherers appeared to head east as well as west when the ice age abated, and have also left an imprint on modern populations in central and south Asia, possibly marking the arrival of Indo-Aryan languages there.

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