SOUTHEASTERN SIBERIA — Ancient DNA analysis identified Yersinia pestis bacteria in human remains excavated from four late Stone Age cemeteries near Lake Baikal. Radiocarbon dating places the earliest documented plague outbreak at approximately 5,500 years ago.

The discovery predates previously documented plague evidence by roughly two centuries. Genetic testing detected plague DNA in 18 of the analyzed individuals. Researchers concluded the plague spread through at least two distinct outbreak periods separated by 400 to 600 years.

Yersinia pestis evolved from its ancestor, Yersinia pseudotuberculosis, no later than 5,700 years ago. The ancient pathogen likely originated from marmots and infected humans through contact with animal hides or consumption of raw meat. Ruairidh Macleod, a research fellow, said, "If you’re a prehistoric hunter-gatherer, you’re going to be in contact with a lot more wild species than an early farmer, and it’s the wild species that are primarily the reservoirs of the disease, not the domesticated animals."

Genetic markers indicate the ancient strain spread via person-to-person respiratory contact rather than flea vectors. Pneumonic plague affects the respiratory system and does not require the specific virulence genes necessary for bubonic plague transmission. "Pneumonic plague doesn’t require the genes that bubonic plague does. It’s spread by coughing. It’s an infection of the lungs, but it’s extremely severe, and it’s extremely deadly." Macleod said.

The pathogen possessed a superantigen protein that researchers associate with heightened mortality in children. Archaeological records show that at least two-thirds of deceased individuals from two specific burial sites were under 15 years old. Molecular biologist Nicolás Rascovan said, "It is clear evidence of an outbreak in prehistoric times that argues against agricultural lifestyles as the major driver of plague emergence."

The study was published in the scientific journal Nature. Macleod said, "The archaeologists were keen to see whether ancient DNA analysis could shed any light on what happened and it absolutely did." Evolutionary geneticist Eske Willerslev said, "To understand our own history, we believe that understanding the history of plague is extremely important."