Massive migration from the steppe was a source for Indo-European languages in Europe - Nature Letter
We show that the populations of Western and Far Eastern Europe followed opposite trajectories between 8,000–5,000 years ago. At the beginning of the Neolithic period in Europe, ~8,000–7,000 years ago, closely related groups of early farmers appeared in Germany, Hungary and Spain, different from indigenous hunter-gatherers, whereas Russia was inhabited by a distinctive population of hunter-gatherers with high affinity to a ~24,000-year-old Siberian6. By ~6,000–5,000 years ago, farmers throughout much of Europe had more hunter-gatherer ancestry than their predecessors, but in Russia, the Yamnaya steppe herders of this time were descended not only from the preceding eastern European hunter-gatherers, but also from a population of Near Eastern ancestry. Western and Eastern Europe came into contact ~4,500 years ago, as the Late Neolithic Corded Ware people from Germany traced ~75% of their ancestry to the Yamnaya, documenting a massive migration into the heartland of Europe from its eastern periphery. This steppe ancestry persisted in all sampled central Europeans until at least ~3,000 years ago, and is ubiquitous in present-day Europeans. These results provide support for a steppe origin9 of at least some of the Indo-European languages of Europe.
a, Proposed routes of migration by early farmers into Europe ~9,000−7000 years ago.b, Resurgence of hunter-gatherer ancestry during the Middle Neolithic 7,000−5,000 years ago. c, Arrival of steppe ancestry in central Europe ... 4,500 years ago (Click to enlarge - original http://www.nature.com/nature/journal/vaop/ncurrent/images/nature14317-sf4.jpg )