June 17 (UPI) — Scientists have found the oldest known signs of a plague outbreak in DNA evidence from hunter-gatherer graves in Siberia, a study published Wednesday in the journal Nature said.

The researchers said the study involves early plague strains from outbreaks starting about 5,500 years ago among hunter-gatherers living near Lake Baikal in southeast Siberia. The graves studied contained skeletons from multiple generations, the study said.

Testing on the skeletons’ teeth showed DNA from the bacterium that causes plague — Yersinia pestis — in nearly 40%.

“These findings show that plague outbreaks happened earlierthan previously thought and were indeed lethal,” the study said. “We contend that the occurrence of outbreaks … challenges the notion that higher population densities and lifestyle changes during the Neolithic agricultural transition were prerequisites for plaque epidemics.”

The infections appear to have resulted in “acute mortality” especially among children ages 8 to 11, the study said. Many of the graves contained the remains of children. Ruairidh Macleod, the lead author and a postdoctoral fellow at the University of Oxford, told NBC News that one grave contained a group of related children, all 4 to 9 years old.

“We see three very young girls, all buried at the same time, having presumably died about the same time. We detect lots of plague DNA in all three of these individuals,” he said. “It’s clearly having a very tragic impact on the children in particular in these communities.”

A previous study documented plague in a single hunter-gatherer who died about 5,000 years ago in what is now Latvia, but it did not find evidence of human-to-human transmission or an outbreak as the study released Wednesday did, NBC News said.

Plague outbreaks, including pandemics, have affected much of human history, with the most notable case being the Black Death that killed up to half of Europe’s population in the 14th century.

Scientists have commonly thought that the first agricultural revolution of the Neolithic period, when early humans became more settled to grow crops and raise animals, was the point when plague and other epidemics began to be an issue. This study, showing plague in a hunter-gatherer community with signs of human-to-human transmission, challenges that thought.

“You have this kind of notion that the hunter-gatherer time was this kind of clear time where there were no diseases, no pathogens,” evolutionary geneticist Eske Willersev, who took part in the study, said in NBC News. “Now we can see, well, it wasn’t that easy to be a hunter-gatherer either, right? You got hit by plague again and again, and it’s probably very common.”