Ancient Siberians were killed off by a mysterious plague, new genetic analysis has suggested.
Researchers from Stockholm University have revealed that swathes of the population in northeast Asia were culled by plague bacterium which somehow made its way to Siberia.
These "die-offs" then changed the genetic structure of the people living in the area who began trekking to what is now North America some 5,500 years ago, experts believe, indicating a far more complex ancestral history than previously thought.
A team led by evolutionary geneticists Gülşah Merve Kilinç and Anders Götherström extracted DNA from the remains of 40 human skeletons excavated in parts of eastern Siberia.
DNA from Yersinia pestis, the bacterium that causes plague, was found in two of the ancient Siberians, according to research published in Science Advances.
One of the plague victims person lived around 4,400 years ago, while the other was dated to roughly 3,800 years ago.
Prof Götherström said it remains unclear how the plague could possibly have reached Siberia or how widespread the infections were.
He and his colleagues discovered that genetic diversity in the ancient samples of human DNA declined sharply from around 4,700 to 4,400 years ago, likely the result of population collapse at the time.
This theory would fit with evidence of plague discovered in other ancient Siberian DNA last year.
However evolutionary geneticist Hendrik Poinar of Canada's McMaster University, who did not participate in the study, says it's possible the ancient people were infected with a non-virulent strain of plague.
If so, the bacterium wouldn't have killed enough victims to alter Siberians' genetic structure.
Prof Poinar said genetic data from only two individuals provides too little evidence to confirm that they possessed a virulent strain of Y. pestis.
There were a series of population shifts in the northeast of Asia from approximately the peak of the last Ice Age (16,900 years ago) to about 550 years ago, scientists now believe.
Researchers who analysed ancient Siberian genetic material and compared it to that of present day humans say it shows that despite the harsh Siberian climate, groups near Lake Baikal and other regions did breed with other populations in and outside of Siberia from the Late Stone Age up to medieval times.
The two plague-carrying Siberians analysed in the new study came from regions that had experienced major population transformations during much of the sampled time period.
This could have been partially caused by people with plague migrating in and around the area, resulting in distinct genetic groups.