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

Researchers have found DNA from Yersinia pestis, the plague bacterium, in the tooth of a Bronze Age sheep, providing the first direct evidence that the disease infected animals as well as people thousands of years ago. The discovery, published in the journal Cell, suggests close contact between humans and livestock helped the plague spread across parts of Eurasia during the Bronze Age, well before the Black Death.

The genetic signal came from a domesticated sheep tooth found at Arkaim, a fortified Bronze Age settlement in the Southern Ural Mountains near the Kazakhstan border, and the remains date to more than 4,000 years ago. The team described it as the first time researchers recovered the Yersinia pestis genome from a non-human sample. "It was alarm bells for my team," study co-author Taylor Hermes, a University of Arkansas archaeologist who studies ancient livestock and disease spread, said in a statement. "This was the first time we had recovered the genome from Yersinia pestis in a non-human sample."

The researchers note that pulling pathogen DNA from ancient animals is technically demanding. Ancient DNA is often degraded and mixed with soil, microbes and even modern DNA, making the signal hard to distinguish. The fragments recovered from ancient samples are typically tiny, sometimes only about 50 letters long, which amplifies the challenge of separating genuine pathogen sequences from contamination.

Bronze Age lifestyles, with larger herds and longer travel by horse, may have created networks through which plague could move. The Bronze Age, roughly 3300 to 1200 BCE, saw intensified animal husbandry and long-distance exchange that could carry diseases across vast landscapes. When the plague later resurfaced in medieval Europe during the 1300s, it would go on to kill an estimated one-third of the continent’s population.

"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 finding sheds light on how plague likely moved through Bronze Age societies that began keeping larger herds and traveling farther with horses. The Bronze Age saw more widespread use of bronze tools, large-scale animal herding and increased travel, conditions that may have facilitated disease exchange between animals and humans.

Researchers believe sheep likely picked up the bacteria from another animal, such as rodents or migratory birds, that carried it without getting sick, and then passed it to humans. They say the results highlight how many deadly diseases originate in animals and can jump to people, a risk that continues as people expand into new environments and interact with wildlife and livestock.

Still, the study has limitations. It is based on a single ancient sheep genome, which constrains what scientists can conclude about the scale of spread. Additional samples from the region are needed to gauge how common animal infection was and which species may have served as reservoirs.

The work was led by scientists at the Max Planck Institute for Infection Biology, with senior authors Felix M. Key of the Max Planck Institute for Infection Biology and Christina Warinner of Harvard University and the Max Planck Institute for Geoanthropology. The research was supported by the Max Planck Society, which has funded follow-up work in the region.

To better understand ancient zoonoses, researchers plan to study more ancient human and animal remains from the region to determine how widespread the plague was and which species played a role in its spread.

The findings offer a glimpse into how animal-borne diseases emerged and spread in the Bronze Age, a context that helps scientists interpret later pandemics and current risks as humans and livestock continue to interact more closely with wildlife and each other.

The researchers plan to examine additional remains from the region to broaden the geographic and species scope of the search and to try to identify the wild animal that originally carried Yersinia pestis, as well as to map how human movement and livestock herding may have helped the disease travel long distances.

The findings reinforce that many deadly diseases begin in animals and jump to humans, a pattern that remains highly relevant as people expand into new environments and interact more closely with wildlife and livestock. The study contributes to a growing understanding of how plague circulated in a world without modern sanitation and underscores the need for more ancient DNA research to fill gaps in the timeline of human disease.