Ancient plague mystery cracked after DNA found in 4,000-year-old animal remains

Scientists have found DNA from the plague bacterium Yersinia pestis in the tooth of a domesticated sheep dating to more than 4,000 years ago, marking the first evidence that the disease infected animals as well as humans. The find, unearthed at Arkaim in the Southern Ural Mountains near the border with Kazakhstan, suggests an animal reservoir helped seed Bronze Age spread of plague across Eurasia and offers a missing piece of how the disease traveled before the medieval Black Death.

Published in the journal Cell, the study describes the painstaking work of separating tiny, damaged fragments of ancient DNA from contamination by soil, microbes and modern humans. "It was alarm bells for my team," said Taylor Hermes, a University of Arkansas archaeologist who studies ancient livestock and disease spread. "This was the first time we had recovered the genome from Yersinia pestis in a non-human sample." Hermes said the discovery reframed the plague's history, underscoring a dynamic between people, livestock and a still unidentified reservoir for the bacterium.

Led by researchers at the Max Planck Institute for Infection Biology, with senior authors Felix M. Key and Christina Warinner of Harvard University and the Max Planck Institute for Geoanthropology, the study illustrates the harsh realities of ancient-DNA work. The DNA in ancient samples is often broken into tiny fragments, sometimes only around 50 letters long, and researchers must distinguish these from soil microbes and modern contamination. Animal remains are particularly challenging to study because they are not as well preserved as human remains. The study is based on a single ancient sheep genome, so scientists caution that more samples are needed to map the full history of the Bronze Age plague.

Findings shed light on how plague likely spread through close contact between people, livestock and wildlife as Bronze Age societies kept larger herds and traveled farther with horses. Sheep could have picked up the bacteria from another animal, such as rodents or migratory birds, and then transmitted it to humans, the researchers said. The results emphasize how many deadly diseases begin in animals and jump to humans, a risk that remains today as people move into new environments and interact with wildlife and livestock.

"It had to be more than people moving," Hermes said. "Our plague sheep gave us a breakthrough. We now see it as a dynamic between people, livestock and some still unidentified 'natural reservoir' for it."

The researchers plan to study more ancient human and animal remains from the region to determine how widespread the plague was and which species may have played a role in spreading it. They also hope to identify the wild animal that originally carried the bacteria and better understand how human movement and livestock herding helped the disease travel across vast distances, insights that could help anticipate how animal-borne diseases continue to emerge.

Researchers (not pictured) found plague-causing Yersinia pestis DNA in the remains of a Bronze Age sheep.

The study, conducted by scientists at the Max Planck Institute for Infection Biology, was supported by the Max Planck Society, and includes contributions from Harvard University and the Max Planck Institute for Geoanthropology. The researchers say more samples from the region are needed to determine how widespread bronze-age plague was and which species may have carried the bacterium before it moved into human populations.

If confirmed in additional remains, the findings could reshape long-standing views of how the plague entered human communities and spread across vast distances long before the medieval period. The discovery also underscores the enduring connection between animal health and human health, a theme that remains central to contemporary efforts to monitor zoonotic disease risk as populations expand into new environments.