These individuals were members of hunter-gatherer groups that settled in the Caucasus region, where southern Russia meets Georgia, about 45,000 years ago, after our species trekked out of Africa to populate other parts of the world. At the time, Europe was populated by Neanderthals.
The Caucasus hunter-gatherers later became isolated there for millennia during the last Ice Age, the scientists said.
The thaw at the end of the Ice Age brought them into contact with other peoples, leading to the advent of a culture of horse-riding herders who swept into Western Europe around 5,000 years ago, bringing metallurgy and animal-herding skills, they added.
"Modern Europeans are a mix of ancient ancestral strands," Trinity College Dublin geneticist Daniel Bradley said. "The only way to untangle the modern weave is to sequence genomes from thousands of years ago, before the mixing took place."
Until now, only three such ancestral strands had been identified flowing from ancient populations.
The Caucasus inhabitants comprised a previously unidentified "fourth strand," said University of Cambridge geneticist Andrea Manica, noting that they contributed significantly not only to the ancestry of Europe but also to people in Central Asia and the Indian subcontinent.
Bradley called the finding "a major new piece in the human ancestry jigsaw." The Caucasus region is located at a crossroads of the Eurasian landmass, with nearby migration routes heading both west and east.
The Caucasus hunter-gatherers lived in caves and in small groups of probably no more than 20 to 30 people, University College Dublin archaeologist Ron Pinhasi said.
One of the two sets of remains came from the Kotias Klde cave near the village of Sveri in western Georgia and the other remains came from about 25 miles (40 km) away in the Satsurblia cave near the village of Kumistavi, Tengiz Meshveliani of the Georgian National Museum said.
The research was published in the journal Nature Communications.
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