For thousands of years At least 10,000 years ago, a population of Mesolithic hunter-gatherers with the same genetic ancestry lived in what is now Denmark. Then, about 5,900 years ago, they were completely replaced by Neolithic farmers with roots in Anatolia (modern-day Turkey). Just a millennium later, these farmers were in turn completely replaced by herders known as Yamnaya, whose origins can be traced back to the southern Russian steppe. A study sequencing the genomes of 100 people who lived in Denmark between 10,500 and 3,200 years ago revealed these dramatic population changes.

The general pattern of hunter-gatherers becoming farmers, who in turn became herders, is known throughout Europe. In this case, however, the researchers were able to gain an unusually accurate picture of population dynamics. “We had access to many skeletons from the same small area and were able to date them very precisely because they were all radiocarbon dated,” says Morten Allentoft, an evolutionary biologist at Curtin University. “This allowed us to understand very precisely how these population fluctuations occur over time.”

In both cases, when new groups arrived in Denmark, the researchers found that the existing population was completely uprooted. The Neolithic farmers in Denmark had some ancestry from the Mesolithic period, but it turns out they had acquired this over generations as they mixed with hunter-gatherers on their way through Central Europe. The herders who replaced the farmers in Denmark also had some Neolithic ancestry, but they had brought it with them from the steppe. “What I found most surprising was how complete and rapid these upheavals were,” says Anne Birgitte Nielsen, a paleoecologist at Lund University.

Genome analyses also helped to settle an age-old debate about a number of Danish Mesolithic cultures. The successive hunter-gatherer cultures of Maglemose, Kongemose and Ertebølle were present in Denmark between about 10,500 and 5,900 years ago, and archaeologists have distinguished them based on their preferred flint blade style. The question was whether each culture represented a new population that introduced new flint-cutting styles, or whether an existing population adopted a number of new styles. “We have shown very clearly that it is the same gene pool throughout this long period of change in Mesolithic cultures,” says Allentoft. “It is not new people who came with new technologies.”