The oldest known plague outbreak struck hunter-gatherers 5,500 years ago

The oldest-known traces of plague, around 5,500 years old, have been discovered in hunter-gatherer burials in Siberia.

Found at one of four ancient burial sites, the discovery predates the previous oldest signs of plague by several hundred years. It also indicates that hunter-gatherers were at risk from outbreaks of plague many centuries before the invention of farming and settled villages, researchers report June 17 in Nature.

“We weren’t expecting this result at all,” says archaeologist Ruairidh Macleod of the University of Oxford. “There was an expectation that these big outbreaks don’t really happen among prehistoric hunter-gatherers [but only] with people living in high-density settlements.”

Macleod and colleagues had come across unusually large numbers of children’s graves at hunter-gatherer burial sites near Lake Baikal, but initially it wasn’t clear why. The team gathered and analyzed DNA from the remains, in the hope that family ties between the interred individuals could help explain the mystery.

Their analysis of 46 people from four burial sites determined instead that at least 18 of them had been infected with Yersinia pestis — the bacterium that causes plague —when they died. They’d also been buried in mass graves alongside others, indicating they had been buried in a hurry; and the burial sites had been used only once, which suggested they were killed by a fatal outbreak of the disease.

Until now, the oldest plague traces were from a single grave in Latvia and from a mass grave at a settled Neolithic farming site in Sweden, both roughly 5,000 years ago. Those finds fit the hypothesis that plague became a virulent danger to human communities only after they began farming and living in close quarters, resulting in large numbers of rats and fleas that spread the disease.

The evidence from Lake Baikal indicates otherwise.

The most likely source was marmots, large burrowing rodents that lived alongside the hunter-gatherers and are a natural reservoir of Y. pestis. Macleod notes it’s possible the plague may have first infected some other animal that the people interacted with, such as a bird.

The Siberia strain had genes that made it deadly and virulent, the researchers found — a discovery that had not been possible with earlier finds. The analysis also suggests plague had diverged from a less lethal relative at least 5,700 years ago, probably in Central Asia. That’s the oldest, most original form of plague found so far — older than the strain from Latvia, which emerged later as the disease spread.

Nicolás Rascovan, a molecular biologist at Institut Pasteur in Paris who led the research into the ancient Y. pestis infection in Sweden, says the Lake Baikal find is “clear evidence of an outbreak in prehistoric times that argues against agricultural lifestyles as a major driver of plague emergence.”

He cautions that it can be difficult to determine exactly which species or strain of bacteria had caused the ancient outbreak. “There are still several thousands of years of Y. pestis evolution and spread” that have been overlooked, he says. “I believe there are still many surprises to come in the history of the plague.”