The present-day Europeans trace their ancestry back to three and not just two ancestral groups. The first is an indigenous hunter-gatherers; the second is Middle Eastern farmers that migrated to Europe around 7,500 years ago; and a novel third is a more mysterious population that spanned north Eurasia and genetically connects Europeans and native Americans.

"We find a major surprise: Europeans are a mixture of three ancient populations, not two," David Reich from Harvard Medical School, one of the lead investigators of a new study, stated in a press release on Thursday.

An international consortium led by researchers from the University of Tübingen and Harvard Medical School, along with the scientists from the Centre for Cellular and Molecular Biology (CCMB), Hyderabad, have analysed ancient human genomes from a 7,000-year-old early farmer from the linearbandkeramik (LBK), a sedentary farming culture, from Stuttgart in southern Germany, a 8,000-year-old hunter-gatherer from the Loschbour rock shelter in Luxembourg and seven 8,000-year-old hunter-gatherers from Motala in Sweden.

"In order to compare the ancient humans to present-day people, the team also generated genome-wide data from about 2,400 humans from almost 200 diverse worldwide contemporary populations, including the enigmatic tribal population of Andaman and Nicobar Islands", says Kumarasamy Thangaraj, one of the authors of this study and senior principal scientist at the CCMB.

"It seems clear now that the third group linking Europeans and native Americans arrives in Central Europe after the early farmers," explains Johannes Krause from the University of Tübingen and director of the Max Planck Institute for History and Sciences in Jena, Germany.

Using the large data set of present-day and ancient human data, the researchers were able to calculate the proportion of the ancestral components in present-day Europeans. They found all Europeans have ancestry from all three ancestral groups. "Differences between them are due to the relative proportions of ancestry. Northern Europeans have more hunter-gatherer ancestry-up to about fifty percent in Lithuanians-and Southern Europeans have more farmer ancestry."

However, even the early farmers themselves had some hunter-gatherer ancestry. They were not unmixed descendants of the original near Eastern migrants that brought farming to Europe. How Europeans received their northern Eurasian ancestry remains an open question: "The northern Eurasian ancestry is proportionally the smallest component everywhere in Europe, never more than twenty per cent, but they find it in nearly every European group they have studied and also in populations from the Caucasus and near East.

A profound transformation must have taken place in west Eurasia after the Neolithic Revolution", the researchers said.

The researchers also analyzed genes with known phenotypic association and show that some of the hunter-gatherers likely had blue eyes and darker skin, whereas the early farmers had lighter skin and brownish eyes. "Both the hunter-gatherers as well as the early farmers displayed high copy numbers of amylase genes in their genomes, suggesting that both populations had already adapted to a starch-rich diet. However, none of the ancient humans was yet adapted to digest milk sugar into adulthood, which most of the European, Middle Eastern and northwest Indians have acquired about 7,000 years ago", says Thangaraj.

"This collaborative efforts by geneticists, archaeologists, anthropologists, linguists and historians around the world has helped in understanding the origin of the contemporary European populations. I am glad that CCMB is part of this endeavour", said Ch Mohan Rao, director of CCMB.

"By analysing the genomes of contemporary populations, we have previously found that the present day Indians have emerged from two ancestral populations (ASI-ancestral south Indians and ANI-ancestral north Indians); while ASI do not have any genetic affinity outside India, ANI showed up to 70 per cent genetic affinity with European." However, it would be interesting to see that which one of the three ancestral European populations is related to ANI, says Lalji Singh one of the authors of this study.

According to the researchers, archaeological evidence suggests that the transition to a farming lifestyle in central Europe occurred around 7,500 years ago, with the appearance of the LBK. It has long been debated whether that change in subsistence strategy involved the mass migration of people from the near East bringing innovative technologies and domestic animals to Europe or whether it was due to a transmission of cultural practices passed on from neighbouring populations. Recent genetic studies on ancient hunter-gatherers and early farmer remains have suggested a massive migration of people to Europe coinciding with the spread of farming. The size and distribution of the genetic components contributed to indigenous European hunter-gatherers, however, remain unclear.