Recent analysis of ancient DNA from hunter-gatherers buried near Lake Baikal, Siberia, indicates a devastating plague outbreak occurred as early as 5,500 years ago. This discovery challenges the traditional belief that significant disease outbreaks emerged only with the advent of agriculture during the Neolithic Revolution.

“Before the Neolithic Revolution, we didn’t anticipate major disease outbreaks affecting entire communities,” states Ruairidh MacLeod from Oxford University. “This evidence reveals a catastrophic epidemic impacting the entire hunter-gatherer community around Lake Baikal—similar to challenges we face today.”

Plague bacteria have caused multiple pandemics, including the Plague of Justinian (541 AD), the Black Death (1346), and the third plague pandemic (1855), resulting in over 15 million deaths worldwide.

The plague bacterium can cause severe infections, including pneumonic and septicemic plague. However, the most recognized form is bubonic plague, which results from flea bites that infect the lymph nodes, leading to painful swelling known as “buboes.”

Advancements in ancient DNA sequencing techniques enable researchers to detect plague bacterium in the bones and teeth of individuals buried centuries ago. Notably, this has shown that plague has been affecting rural regions in Sweden and other areas for over 5,000 years.

These revelations have prompted theories suggesting plague could have contributed to the Neolithic decline—a stark population decrease in Europe. However, early strains of plague bacterium were found to lack a vital gene necessary for transmission through fleas.

Infected fleas possess a protein encoded by the ymt gene that clogs their intestines, enriching plague bacteria near their mouthparts. “This condition propels the flea to bite excessively, searching for new blood meals,” explains MacLeod.

Researchers hypothesize that the plague bacterium likely did not become a widespread epidemic until it acquired the ymt gene. Despite this, MacLeod’s team identified the bacterium in 18 out of 42 hunter-gatherer remains unearthed across four archaeological sites near Lake Baikal.

“We finally have compelling evidence that strains of plague from this era were indeed lethal,” asserted MacLeod.

It appears there were two separate outbreaks, with the initial one commencing roughly 5,500 years ago. Evidence shows instances of siblings buried side by side, indicating they perished concurrently, with multiple individuals interred together.

Notably, most identified plague victims were children and adolescents. This high mortality rate among young individuals aligns with historical records indicating that children were disproportionately affected by plague.

It is also evident that some individuals survived, receiving standard burial rites. “This provides poignant insight into the responses of hunter-gatherer communities,” MacLeod remarked.

According to MacLeod, hunter-gatherers had greater exposure to wild animals compared to agricultural populations, making them more susceptible to zoonotic diseases. Researchers suggest that marmots, known reservoirs of the plague, may have transmitted the disease, as evident from archaeological findings indicating these animals were hunted for food. Even today, local populations remain at risk from plague through contact with marmots or consuming undercooked marmot meat.

Once one hunter-gatherer was infected, pneumonic plague could swiftly spread via respiratory droplets.

Current genomic analyses indicate that the plague bacterium emerged between 9,800 and 5,700 years ago, with a more recent estimate appearing more plausible. Hence, the plague outbreak might be older than previously believed, but still not overly ancient.

“This study is unique in several aspects,” states Nicholas Raskovan from the Pasteur Institute in Paris. He highlights this research as the oldest known plague outbreak, occurring furthest east and affecting hunter-gatherers rather than agricultural societies.

“This evidence strongly suggests that the emergence of plague was not solely linked to agricultural practices,” Raskovan concludes. “This research indicates that non-agricultural societies had lethal infectious diseases, which is particularly fascinating and may have significantly influenced population decline during the Neolithic era in Europe.”