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“To understand our own history, we believe that understanding the history of plague is extremely important,” said study co-author Eske Willerslev, an evolutionary geneticist with the University of Copenhagen in Denmark.

Willerslev and other researchers looked for traces of plague-causing bacteria in remains from four cemeteries near Siberia's Lake Baikal. They found remnants of plague DNA in teeth from 18 ancient hunter-gatherers.

Dating the carbon in the bones revealed that the plague triggered two outbreaks, with the first cases detected around 5,500 years ago.

The team found that the prehistoric plague developed in stages and infected several small families. It likely spread from marmots — large native rodents — when people ate their raw organs or touched infected hides during butchery. The disease also traveled between people through coughing and sneezing, the authors said.

Many of those who died were young children aged 8 to 11. Three young girls were buried side by side, two of whom were likely cousins. An aunt and nephew were found together, but her niece was in a different shared grave, according to the study published Wednesday in the journal Nature.

“People were around to bury the dead who knew who these people were when they were alive. And that’s a really human element to all of the scientific work,” said study co-author Ruairidh Macleod, who studies ancient DNA at the University of Oxford.

Kids may have been at greater risk because their immune systems weren’t as strong, researchers said.

The presence of multiple victims suggests that the prehistoric plague was capable of causing both individual cases and outbreaks, said geneticist Aida Andrades Valtueña with the Max Planck Institute for Evolutionary Anthropology. She had no role in the study.

Researchers found that this type of ancient plague evolved long before bubonic plague, which was responsible for the Black Death that struck medieval Europe. But there's evidence that earlier plagues were just as deadly. The disease decimated not only crowded cities, but also small, nomadic hunter-gatherer groups.

Knowing this can help us “understand the steps that the bacterium took to become the deadly pathogen we know today, and that can provide clues on how pathogens may emerge in the future,” Andrades Valtueña said in an email.

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