The first sequencing of genomes from human remains of the Late Upper Palaeolithic period has revealed a previously unknown 'fourth strand' of ancient European ancestry. Previously, ancient Eurasian genomes had revealed three ancestral populations contributing to contemporary Europeans in varying degrees.
     Following the 'out of Africa' expansion some 45,000 years ago, some hunter-gatherer populations migrated north-west, eventually colonising much of Europe from Spain to Hungary. Other populations settled around the eastern Mediterranean and Levant, where they would develop agriculture around 10,000 years ago. These early farmers then expanded into Europe.
     The newly discovered lineage arose from populations that split from the western hunter-gatherers shortly after leaving Africa, settling in the Caucasus region between the Black and Caspian seas, where mountains form a natural boundary separating Europe and Asia.
     The Caucasus hunter-gatherer genome shows a continued mixture with the ancestors of the early farmers in the Levant area, which ends around 25,000 years ago - just before the time of the last glacial maximum. Cut off from other major ancestral populations for as long as 15,000 years, Caucasus hunter-gatherer populations shrink, until migrations begin again as the glaciers recede.
     Around 5,000 years ago, horse-borne Steppe herders called the Yamnaya sweep into Western Europe, bringing metallurgy and animal herding skills, along with the Caucasus hunter-gatherer strand of ancestral DNA now present in almost all populations from the European continent.
     The sequencing of ancient DNA recovered from two separate burials from just south of the Caucasus Mountains in Western Georgia - one over 13,000 years old, the other almost 10,000 years old - reveals that the Yamnaya owed half their ancestry to previously unknown and genetically distinct hunter-gatherer sources.
     Dr Andrea Manica, of Cambridge University's Department of Zoology and one of the study's lead authors, says: "The question of where the Yamnaya come from has been something of a mystery up to now. 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. This Caucasus pocket is the fourth major strand of ancient European ancestry, one that we were unaware of until now."
     While the Caucasus hunter-gatherer ancestry would eventually be carried west by the Yamnaya, the researchers found it also had a significant influence further east. A similar population must have migrated into South Asia at some point, says Eppie Jones, a doctoral student and first author of the paper.
     "India is a complete mix of Asian and European genetic components. The Caucasus hunter-gatherer ancestry is the best match we've found for the European genetic component found right across modern Indian populations," Jones says. Researchers say this strand of ancestry may have flowed into the region with the bringers of Indo-Aryan languages.

Edited from Phys.org (16 November 2015)