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takyon writes:

Scientists analyzed genetic data gathered from various early European skeletons, and found that "for later migrations from the Pontic Steppe during the late Neolithic/Bronze Age" there were "approximately five to 14 migrating males for every migrating female". This third wave of "Yamnaya" migration mainly contributed to autosomal DNA:

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.

Ancient X chromosomes reveal contrasting sex bias in Neolithic and Bronze Age Eurasian migrations (DOI: 10.1073/pnas.1616392114) (DX)

Original Submission