The oldest-known plague outbreak occurred about 5,500 years ago among hunter-gatherers in Siberia's Lake Baikal region, according to a study published Wednesday in the journal Nature. Researchers analyzing ancient DNA from bodies buried at four archaeological sites in the region identified the oldest-known strains of Yersinia pestis, the bacterium that causes plague. The findings suggest the outbreak disproportionately affected children and adolescents. The study's authors said the discovery provides new insight into the origins of one of history's deadliest pathogens and supports the theory that marmots were its original host species before it spread across Eurasia.
Groundbreaking Discovery
Eske Willerslev, an evolutionary geneticist at the University of Copenhagen and the University of Cambridge and senior author of the study, said: "The findings fundamentally change how we think about the origins and early impact of one of humanity's most consequential pathogens." Researchers said the ancient strains appear to have carried genetic traits that made them particularly lethal to young people, traits that are no longer present in modern forms of the bacterium.
Comparison with Other Evidence
The next oldest-known evidence of plague dates to between 5,300 and 5,000 years ago in Latvia, roughly 5,000 kilometers (3,106 miles) from the Siberian sites. Ruairidh Macleod, an evolutionary geneticist at the University of Oxford and lead author of the study, noted: "It's only with the development of methods for studying ancient DNA that we've discovered it's been around a lot longer than what we know from historical records. It's a zoonotic disease, a pathogen maintained primarily in rodents rather than humans, but which has repeatedly spilled over into humans with devastating effects."
Challenging Assumptions
The findings challenge longstanding assumptions that major plague outbreaks emerged only after humans adopted agriculture and established densely populated settlements. They also contradict theories suggesting early strains of the disease were relatively mild. At Lake Baikal, researchers detected Yersinia pestis in 18 of 46 individuals examined, a prevalence rate higher than that observed in some medieval plague burial sites. Macleod said the evidence of a large-scale deadly outbreak among small groups of hunter-gatherers living in a remote forested landscape came as a "complete surprise."
The plague later caused some of the deadliest pandemics in history, including the Plague of Justinian and the Black Death, which killed large portions of Europe's population.
