Around 5,500 years ago, an outbreak of the bacterium Yersinia pestis—the cause of plague—struck hunter-gatherer communities near Lake Baikal in Siberia, marking the earliest known occurrence of the disease in human history. Genetic analysis of ancient DNA retrieved from 46 bodies buried at four local sites revealed the presence of early strains of the pathogen, particularly affecting children and adolescents.

The discovery, detailed in a study published recently in the journal Nature, challenges previous assumptions that significant plague outbreaks arose only after the advent of agriculture and densely populated settlements. Instead, the evidence indicates that plague was capable of causing lethal outbreaks in relatively small, mobile bands of prehistoric hunter-gatherers who subsisted on diverse wildlife, including elk, deer, moose, fish, seals, and notably, marmots.

Researchers identified Yersinia pestis DNA in 18 of the sampled individuals, a prevalence notably higher than that found in some medieval plague burial sites. The analysis also found that these ancient strains lack genetic features present in later pandemic versions of the pathogen, particularly those enabling efficient flea-borne transmission and the characteristic painful swellings known as buboes. Instead, the strains carried a gene variant associated with severe inflammatory responses, which may explain the disproportionate mortality among children aged 8 to 12.

“This represents a transitional stage in plague evolution—already causing severe disease but lacking the adaptations seen in later pandemics,” said evolutionary geneticist Eske Willerslev of the University of Copenhagen and the University of Cambridge, senior author of the study. Lead author Ruairidh Macleod of the University of Oxford added that the evidence suggests the plague outbreak had a unique mortality pattern, markedly impacting younger individuals.

The plague’s origin is further linked to marmots, which were a known food source and symbolic presence in the burials, as manifested by pendants made from marmot teeth. Researchers believe initial human infections likely arose from contact with infected marmots, possibly through handling or consuming undercooked meat. Following zoonotic transmission, human-to-human spread probably occurred, potentially via respiratory droplets.

This prehistoric outbreak predates the next oldest known instance of plague by several centuries; that case dates to between 5,300 and 5,000 years ago in Latvia, approximately 3,000 miles distant. The findings shed new light on the early history and geographic emergence of one of humanity’s most consequential pathogens, which has caused devastating epidemics including the 6th-century Plague of Justinian and the 14th-century Black Death in Europe.

The study underscores how plague’s history extends far deeper into prehistory than previously recognized and emphasizes the significant impact it had even on small, dispersed human populations long before the rise of sedentary civilizations.