From NBC News:
... Cooper and his colleagues analyzed mitochondrial DNA, which resides in the cells' energy-making structures and is passed on through the maternal line, from 37 skeletal remains from Germany and two from Italy; the skeletons belonged to humans who lived in several different cultures that flourished between 7,500 and 2,500 years ago. The team looked at DNA specifically from a certain genetic group, called haplogroup h, which is found widely throughout Europe but is less common in East and Central Asia.
The researchers found that the earliest farmers in Germany were closely related to Near Eastern and Anatolian people, suggesting that the agricultural revolution did indeed bring migrations of people into Europe who replaced early hunter-gatherers [about 7,500 years ago].
But that initial influx isn't a major part of Europe's genetic heritage today.
Instead, about 5,000 to 4,000 years ago, the genetic profile changes radically, suggesting that some mysterious event led to a huge turnover in the population that made up Europe.
The Bell Beaker culture, which emerged from the Iberian Peninsula around 2800 B.C., may have played a role in this genetic turnover. The culture, which may have been responsible for erecting some of the megaliths at Stonehenge, is named for its distinctive bell-shaped ceramics and its rich grave goods. The culture also played a role in the expansion of Celtic languages along the coast.