A team led by evolutionary geneticists Gulsah Merve Kilinc and Anders Gotherstrom, both of Stockholm University, extracted DNA from the remains of 40 human skeletons previously excavated in parts of eastern Siberia. Among those samples, DNA from Yersinia pestis, the bacterium that causes plague, was found in two ancient Siberians, the researchers report in this month’s Science Advances. One person lived about 4,400 years ago. The other dated to roughly 3,800 years ago.

It’s unclear how the plague bacterium first reached Siberia or whether it caused widespread infections and death, Gotherstrom says. But he and his colleagues found that genetic diversity in their ancient samples of human DNA declined sharply from around 4,700 to 4,400 years ago, possibly the result of population collapse.

But it’s possible that the ancient Siberians were infected with a version of Y. pestis that wasn’t virulent. If so, the bacterium wouldn’t have killed enough people to alter the genetic structure of Siberians. Genetic data from only two individuals provides too little evidence to confirm that they possessed a virulent strain of Y. pestis, Poinar says.

The genetic findings do provide a glimpse of previously unknown ancient population shifts in that region.

Ancient individuals included in the new research dated from around 16,900 years ago, shortly after the last Ice Age peaked, to 550 years ago. The researchers compared those ancient Siberians’ DNA to DNA from present-day humans in different parts of the world and to previous samples of ancient human DNA — mainly from Europe, Asia and North America. The analyses showed that despite Siberia’s harsh climate, groups near Lake Baikal and regions farther east mixed with various populations in and outside of Siberia from the Late Stone Age up to medieval times.

The two plague-carrying Siberians, in particular, came from regions that had experienced major population transformations during much of the sampled time period, the researchers say. Those events could have included migrations of plague-carrying people from outside Siberia. For instance, the 4,400-year-old skeleton was found just west of Lake Baikal, a region that witnessed the emergence of several distinct genetic groups — with roots mainly farther to the west and southwest of Lake Baikal — between about 8,980 and 560 years ago.