Thousands of horsemen may have swept into Bronze Age Europe, transforming the local population

Call it an ancient thousand man march. Early Bronze Age men from the vast grasslands of the Eurasian steppe swept into Europe on horseback about 5000 years ago—and may have left most women behind. This mostly male migration may have persisted for several generations, sending men into the arms of European women who interbred with them, and leaving a lasting impact on the genomes of living Europeans.

“It looks like males migrating in war, with horses and wagons,” says lead author and population geneticist Mattias Jakobsson of Uppsala University in Sweden.

Europeans are the descendants of at least three major migrations of prehistoric people. First, a group of hunter-gatherers arrived in Europe about 37,000 years ago. Then, farmers began migrating from Anatolia (a region including present-day Turkey) into Europe 9000 years ago, but they initially didn’t intermingle much with the local hunter-gatherers because they brought their own families with them. Finally, 5000 to 4800 years ago, nomadic herders known as the Yamnaya swept into Europe. They were an early Bronze Age culture that came from the grasslands, or steppes, of modern-day Russia and Ukraine, bringing with them metallurgy and animal herding skills and, possibly, Proto-Indo-European, the mysterious ancestral tongue from which all of today’s 400 Indo-European languages spring. They immediately interbred with local Europeans, who were descendants of both the farmers and hunter-gatherers. Within a few hundred years, the Yamnaya contributed to at least half of central Europeans’ genetic ancestry.

To find out why this migration of Yamnaya had such a big impact on European ancestry, researchers turned to genetic data from earlier studies of archaeological samples. They analyzed differences in DNA inherited by 20 ancient Europeans who lived just after the migration of Anatolian farmers (6000 to 4500 years ago) and 16 who lived just after the influx of Yamnaya (3000 to 1000 years ago). The team zeroed in on differences in the ratio of DNA inherited on their X chromosomes compared with the 22 chromosomes that do not determine sex, the so-called autosomes. This ratio can reveal the proportion of men and women in an ancestral population, because women carry two X chromosomes, whereas men have only one.

Europeans who were alive from before the Yamnaya migration inherited equal amounts of DNA from Anatolian farmers on their X chromosome and their autosomes, the team reports today in the Proceedings of the National Academy of Sciences . This means roughly equal numbers of men and women took part in the migration of Anatolian farmers into Europe.

But when the researchers looked at the DNA later Europeans inherited from the Yamnaya, they found that Bronze Age Europeans had far less Yamnaya DNA on their X than on their other chromosomes. Using a statistical method developed by graduate student Amy Goldberg in the lab of population geneticist Noah Rosenberg at Stanford University in Palo Alto, California, the team calculated that there were perhaps 10 men for every woman in the migration of Yamnaya men to Europe (with a range of five to 14 migrating men for every woman). That ratio is “extreme”—even more lopsided than the mostly male wave of Spanish conquistadores who came by ship to the Americas in the late 1500s, Goldberg says.

Such a skewed ratio raises red flags for some researchers, who warn it is notoriously difficult to estimate the ratio of men to women accurately in ancient populations. But if confirmed, one explanation is that the Yamnaya men were warriors who swept into Europe on horses or drove horse-drawn wagons; horses had been recently domesticated in the steppe and the wheel was a recent invention. They may have been “more focused on warfare, with faster dispersal because of technological inventions” says population geneticist Rasmus Nielsen of the University of California, Berkeley, who is not part of the study.

But warfare isn’t the only explanation. The Yamnaya men could have been more attractive mates than European farmers because they had horses and new technologies, such as copper hammers that gave them an advantage, Goldberg says.

The finding that Yamnaya men migrated for many generations also suggests that all was not right back home in the steppe. “It would imply a continuing strongly negative push factor within the steppes, such as chronic epidemics or diseases,” says archaeologist David Anthony of Hartwick College in Oneonta, New York, who was not an author of the new study. Or, he says it could be the beginning of cultures that sent out bands of men to establish new politically aligned colonies in distant lands, as in later groups of Romans or Vikings.