Ancient DNA Shows Plague Killed Entire Families 3,500 Years Before Black Death

While plague is frequently associated with rats and the densely populated cities of medieval Europe, new research indicates the disease posed a deadly threat to humans nearly 5,500 years ago. Published in Nature, the study reveals that small, mobile hunter-gatherer groups in East Siberia were succumbing to the bacteria long before agriculture and urbanization created the crowded conditions typically linked to major outbreaks.

An international team analyzed ancient DNA extracted from human remains found at four burial sites near Lake Baikal in East Siberia. By sequencing genetic material preserved in teeth, researchers reconstructed ancient bacterial genomes and identified previously unknown early forms of Yersinia pestis, the bacterium responsible for plague.

“Whether the earliest forms of plague were mild or virulent has been a matter of debate, but our findings demonstrate that these ancient strains were already highly lethal,” said Eske Willerslev, a senior author and professor at the University of Copenhagen and the University of Cambridge.

To reconstruct the events within these prehistoric communities, the scientists combined genetic data with archaeological analysis and radiocarbon dating. Lead author Ruairidh Macleod, who conducted the work as a PhD student at Cambridge and is now a Research Fellow at the University of Oxford, noted that the integration of these methods provided a clear picture of the outbreaks.

The team detected Yersinia pestis DNA in 18 of the 46 individuals tested, representing a detection rate of nearly 40 percent. This figure exceeds rates reported at some medieval plague burial sites, underscoring the severity of the infection in these early populations.

Previous studies suggested that ancient strains of the bacterium lacked specific genetic features required for efficient transmission through fleas and rodent hosts, leading many scientists to assume the earliest versions caused only mild outbreaks. However, the new findings challenge this assumption.

At the two largest cemeteries, researchers observed an unusually high proportion of children and young teenagers among the deceased. For decades, archaeologists had struggled to explain the concentration of young burials at these sites over such a short timeframe.

“The unusually high number of children and the short timespan was a real puzzle that we’ve been trying to solve since the 1990s,” said Andrzej Weber, an archaeologist at the University of Alberta and principal investigator of the Baikal Archaeology Project. “Finding out that plague was the cause is extraordinary, but it makes so much sense.”

Radiocarbon dating confirmed that many of these deaths occurred in close proximity in time. The graves indicate that siblings and even parents with children died together during the same outbreaks, suggesting the disease moved rapidly through family units.

The ancient strains also contained a distinctive superantigen, a toxin-producing genetic factor absent in later historic plague strains. Superantigens can trigger an intense immune response, leading to severe inflammation and significantly increasing the danger of infection.

“This finding changes our understanding of the earliest plague outbreaks: Even before the bacterium evolved efficient flea-borne transmission, these ancient strains appear to have carried a potent combination of virulence factors that could make infection highly lethal,” said Martin Sikora, associate professor at the University of Copenhagen and another senior author of the study.

The evidence suggests that some of the earliest known plague outbreaks were as deadly as later historical epidemics, particularly affecting children. This lethality appears to have occurred even before the bacterium developed the efficient flea-based transmission associated with bubonic plague.

The study also supports the hypothesis that plague may have originated in Central or Northeast Asia before spreading across Eurasia via wild rodent populations. Archaeological evidence indicates that the hunter-gatherers around Lake Baikal had close contact with marmots, large burrowing rodents that still carry plague today.

Researchers suspect that some of these prehistoric outbreaks began when the disease passed directly from infected marmots to humans. If confirmed, this would mean plague was capable of causing fatal outbreaks in small communities thousands of years before the rise of cities and the flea-driven epidemics that later made the disease infamous.