From the New York Times:
About 50,000 years ago, humans from Africa first set foot in Europe. They hunted woolly mammoths and other big game — sometimes to extinction. Eventually, they began grazing livestock and raising crops.
They chopped down forests and drained swamps, turning villages into towns, then cities and capitals of empires. But even as they altered the Continent, Europeans changed, too.
Their skin and hair grew lighter. They gained genetic traits particular to the regions in which they lived: Northern Europeans, for example, grew taller than Southern Europeans.
... Recently David Reich, a geneticist at Harvard Medical School, and his colleagues analyzed the genomes of nine ancient Europeans. Eight belonged to hunter-gatherers who lived about 8,000 years ago, seven in what is now Sweden and one in Luxembourg. The ninth came from a farmer who lived 7,000 years ago in present-day Germany.
The scientists compared these genomes with those of living Europeans. As they reported last month in Nature, the study revealed something scientists never knew: Europeans today have genes from three very different populations.
The oldest of these populations were the first Europeans, who appear to have lived as hunter-gatherers. The second were farmers who expanded into Europe about 8,500 years ago from the Near East.
But most living Europeans also carry genes from a third population, which appears to have arrived more recently. Dr. Reich and his colleagues found the closest match in DNA taken from a 24,000-year-old individual in Siberia, suggesting that the third wave of immigrants hailed from north Eurasia. The ancient Europeans that the scientists studied did not share this North Eurasian DNA. They concluded that this third wave must have moved into Europe after 7,000 years ago.