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Scientists have solved a mystery that puzzled them for decades: Why were so many dead children buried by hunter-gatherers in Russia 5,500 years ago? It turns out they were killed by the earliest known outbreak of the plague, revealing new insights about the disease.
Yersinia pestis, the bacteria most famous for causing the Black Death that devastated Europe during the 14th century, was already deadly to humans thousands of years before that, reports the study published Wednesday in Nature by an international team of scientists, including some Canadians.
Not only that, but it was capable of spreading through hunter-gatherer societies, and not just people who lived in the crowded settlements that came later.
“It was a complete surprise that we discovered this really, really early evidence for large-scale lethal outbreaks of plague amongst these hunter gatherer communities,” said Ruaridh Macleod, postdoctoral researcher at Oxford University and the lead author of the new study, in a news conference describing the findings.
Decades-old mystery
Andrzej Weber, professor of anthropology at the University of Alberta, and Angela Lieverse, a professor of archaeology at the University of Saskatchewan, have been studying remains from a prehistoric hunter-gatherer society near Lake Baikal in Russia for decades.
Weber is the director of the Baikal Archeology Project, which has been running for 40 years, uncovering the lives of the people who lived here seasonally millennia ago, in small communities of dozens to 100 people, fishing, hunting and gathering at the lake and the Angara River that flows out of it.
Lieverse recalled that about 20 years ago, they noticed that about two-thirds of the people buried at one of the archeological sites were children under the age of 12, “which is really, really unusual for that area, and we never really had a good explanation about why.”
Lieverse’s specialty is studying the human bones — she was able to sort the bones to identify the age and biological sex of each set of remains. She also looks for signs of disease, but she noted that only slower-moving ailments such as malnutrition, tuberculosis or cancer tend to show up in the bones — infectious diseases kill too quickly.
The team collaborated with other scientists, including Ruaridh MacLeod, an ancient DNA specialist. He was interested in figuring out family relationships among the remains, but decided to test for different pathogens as a side project.
When he found the bacteria that causes the plague, “it just set off a light bulb,” Lieverse recalled.
She said plague is endemic in the region today. It’s known to be carried by marmots, large rodents that were eaten by the hunter-gatherers. They were even sometimes buried with decorative items made of marmot teeth.
Child-killing strain
One question the researchers had was why this strain of plague was so deadly and hit children so hard. A genetic analysis showed that it contained a “superantigen” that can trigger extreme inflammatory complications such as Kawasaki syndrome (which affected some children infected with COVID-19), especially in children, whose immune systems are different from those of adults.
On the other hand, this plague strain did not have the genes that let the bubonic plague spread through fleas.
Before this study, the earliest known plague strain dated to about 5,200 years ago. But because it was missing the genes to spread efficiently through fleas and rodents, many researchers thought it would have been unlikely to cause major outbreaks.
In the Lake Baikal outbreak, many small family groups were affected, suggesting human-to-human transmission.
McLeod said in one case, three young girls between the ages of seven and nine were found to have died at the same time, and DNA testing showed they were sisters and cousins.
The outbreak, he said, was “clearly having a very tragic impact on the children in these particular communities.”
The researchers said this kind of discovery is important to understand how deadly pathogens like the plague changed and evolved over time, and what they could be capable of in the future.
Lieverse said the prehistoric remains and the DNA evidence tell a “sad and tragic story.”
“Thinking of these little sisters and cousins who all died horribly from this nasty infectious disease breaks your heart,” she said. “But in a way, we can tell their story now. And I think that’s very powerful.”
