For many people, the plague is a reminder of rats, crowded medieval cities, and the devastating epidemics that swept across Europe during and after the Middle Ages.
New research suggests the disease has a much more deadly history. A study published in Nature Found that plague was killing people in small hunter-gatherer groups as early as 5,500 years ago, thousands of years before farming communities and cities emerged.
An international team of scientists examined ancient DNA from human remains recovered from four hunter-gatherer cemeteries near Lake Baikal in eastern Siberia. By sequencing genetic material preserved inside ancient teeth, researchers reconstructed bacterial genomes and identified previously unknown early strains of plague.
“Whether early forms of plague were mild or virulent is a matter of debate, but our findings show that these ancient strains were already highly lethal,” says senior author Eske Willerslev, a professor at the University of Copenhagen and the University of Cambridge.
Ancient DNA reveals prehistoric plague outbreak
Researchers combined genetic evidence with archaeological findings and radiocarbon dating to piece together what happened within these prehistoric communities.
“Based on plague DNA, genetic relationships between victims, archaeological analysis and radiocarbon dating, we have built a very clear, complete picture of what happened during these outbreaks,” says lead author Ruairidh MacLeod, who worked as a PhD student at the University of Cambridge and is now a research fellow at the University of Oxford.
The team detected DNA yersinia pestisThe bacteria responsible for plague were found in 18 of the 46 individuals studied. This means that about 40 percent of the remains have evidence of infection. According to the researchers, this detection rate is higher than that reported from some medieval plague burial sites.
Evidence shows that the early plague was highly lethal
Earlier research had indicated that ancient strains yersinia pestis It lacked some of the genetic characteristics that later allowed bubonic plague to spread efficiently through flea and rodent hosts. Because of this, many scientists believed that early forms of the disease were unlikely to cause large or deadly outbreaks.
The new findings point in a different direction.
In the two largest cemeteries, researchers found unusually large numbers of children and young teenagers among the dead. For decades, archaeologists had struggled to explain this pattern.
“The unusually high number of children and the short period of time was a real puzzle that we’ve been trying to solve since the 1990s. Finding out that the cause was plague is extraordinary, but it makes a lot of sense,” says archaeologist Andrzej Weber of the University of Alberta, principal investigator of the Baikal Archaeological Project.
Radiocarbon dating revealed that many of the burials took place over a relatively brief period. In some cases, it appears that siblings or parents and children died at the same time and were buried together.
Unique genetic factors may increase severity
The researchers also identified a specific superantigen in ancient plague strains. This toxin-producing genetic factor has not been found in later historical plague strains.
Superantigens can trigger powerful immune responses and are associated with severe inflammatory responses, potentially making infections more dangerous.
“This discovery transforms our understanding of early outbreaks of plague: even before efficient flea-borne transmission of the bacterium evolved, these ancient strains possessed a powerful combination of virulence factors that could make infection extremely lethal,” says senior author Martin Sikora, associate professor at the University of Copenhagen.
The results suggest that despite the lack of flea-borne transmission mechanisms associated with bubonic plague, some of the earliest known plague outbreaks may have been just as deadly as later forms of the disease, especially for children.
Clues to the origin of the plague
The study also supports the idea that plague first emerged in central or north-east Asia before spreading to Eurasia through wild rodent populations.
Archaeological evidence indicates that the hunter-gatherers in the study had close contact with marmots, large burrowing rodents that still spread plague today. Researchers believe the disease may have passed directly from infected marmots to humans, causing outbreaks in these prehistoric communities.