Should the European repopulate Africa

Who were the first Europeans?

Within a few centuries, people with a portion of Yamnaya DNA had spread to the British Isles. There and elsewhere, almost none of the European farmers who already lived there survived the onslaught from the east. In what is now Germany, "70 percent to perhaps even 100 percent of the local population have been replaced," says Reich. "Something very dramatic happened 4,500 years ago."

Until then, the Neolithic farming societies in Europe had been successful for millennia. They had populated entire stretches of land from Bulgaria to Ireland, often in complex village communities with hundreds and sometimes thousands of people. The archaeologist Volker Heyd of the Finnish University of Helsinki estimates that around 3000 BC Up to seven million people lived in Europe. In the British Isles, the Neolithic people were just building Stonehenge.

(Archaeologists are amazed: artificial islands older than Stonehenge)

For many archaeologists, the idea that his bunch of nomads could swap out such a well-established civilization in a matter of centuries was just implausible. "How the hell were these decentralized groups of shepherds supposed to overturn a firmly entrenched Neolithic society, even if they had horses and could fight well?" Asks Kristian Kristiansen, an archaeologist at the University of Gothenburg in Sweden.

Evidence of this can be found in the teeth of 101 people who lived in the steppes at the time when the Yamnaja moved west. In seven of the samples, geneticists found not only human DNA but also DNA from an early form of Yersinia pestis - the plague bacterium that wiped out roughly half of the European population in the 14th century.

Unlike the Black Death, which was transmitted by fleas, this earlier version of the bacterium was transmitted from person to person. The steppe nomads had apparently lived with the disease for centuries and possibly developed immunity to it - much like the Europeans who colonized America and brought smallpox with them. And just as smallpox and other diseases decimated Native Americans, so may the yamnaya plague brought with them to the Neolithic settlements. That could explain both the surprising collapse of society and the rapid spread of Yamnaya DNA from Russia to the British Isles.

(The black death is rampant in Madagascar)

"Plague epidemics paved the way for the expansion of the Yamnaja," says Morten Allentoft. The evolutionary biologist at the Natural History Museum in Copenhagen helped identify the ancient plague DNA.

However, this theory raises an important question: evidence of the plague was only recently documented in Neolithic skeletons, and no one has yet found mass graves containing epidemic victims. If the European peasants were actually wiped out by the plague, they left little trace.

Whether they brought the plague or not - the Yamnaja brought domesticated horses and, with their wagons, a mobile way of life to Stone Age Europe. With their innovative metal weapons and tools, they may even have helped usher in the Bronze Age in Europe.