On Wednesday in the journal Nature, two teams of scientists — one based at the University of Copenhagen and one based at Harvard University — presented the largest studies to date of ancient European DNA, extracted from 170 skeletons found in countries from Spain to Russia. Both studies indicate that today’s Europeans descend from three groups who moved into Europe at different stages of history.

First hunter-gatherers 45,000 years ago.
Farmers from near east 8,000 years ago.
nomadic sheepherders from western Russia & Ukraine about 4,500 years ago.

Until about 9,000 years ago, Europe was home to a genetically distinct population of hunter-gatherers, the researchers found. Then, 9,000 to 7,000 years ago, the genetic profiles of the inhabitants in some parts of Europe abruptly changed, acquiring DNA from Near Eastern populations. . . .

From 7,000 to 5,000 years ago, however, hunter-gatherer DNA began turning up in the genes of European farmers. “There’s a breakdown of these cultural barriers, and they mix,” Dr. Reich said.

About 4,500 years ago, the final piece of Europe’s genetic puzzle fell into place. A new infusion of DNA arrived — one that is still very common in living Europeans, especially in central and northern Europe.

The closest match to this new DNA, both teams of scientists found, comes from skeletons found in Yamnaya graves in western Russia and Ukraine.

www.nytimes.com/2015/06/16/science/dna-deciphers-roots-of-modern-europeans.html?_r=2