4,000 year old sheep tooth reveals how an ancient plague spread across Eurasia

Archaeologists working in the Southern Ural Mountains have uncovered an unexpected clue about how an ancient plague once moved across Eurasia. A tiny sheep tooth found at the Bronze Age site of Arkaim has revealed the earliest known case of Yersinia pestis infection in a domesticated animal. The discovery helps explain how a prehistoric strain of plague circulated for nearly two thousand years despite lacking the flea-borne traits that later fueled the Black Death.

Arkaim was home to people linked to the Sintashta and Petrovka cultures who lived about four millennia ago. They built ringed fortifications, cared for large herds and forged metal tools and weapons. When researchers analyzed ancient livestock remains from the site, they expected to learn more about the history of domesticated animals. Instead, they uncovered signs of a pathogen that shaped human history long before medieval pandemics.

The surprise came from a tooth labeled ARK017. Radiocarbon dating placed the sheep’s life between 1935 and 1772 BCE. Scientists were screening 23 animals for traces of past infections even though ancient animal bones rarely hold pathogen DNA. Many had been discarded as food waste and exposed to heat or weather that destroys genetic material.

Despite those challenges, the sheep tooth carried fragments of Y. pestis. The team captured and sequenced enough genetic material to reconstruct most of the bacterial chromosome. The fragments showed typical signs of ancient DNA, confirming the infection came from the Bronze Age rather than modern contamination. Further testing verified that the tooth belonged to a domesticated sheep and not a wild relative.

Finding plague DNA in any animal from this era is uncommon. What made this case stand out was the version of the pathogen the sheep carried and what it revealed about disease spread during early pastoral life.

The sheep’s infection belonged to the Late Neolithic and Bronze Age lineage of Y. pestis. Scientists have studied this lineage for years and found it in human remains from across Europe and Asia. It circulated from roughly 2900 to 500 BCE but lacked genes that allow survival in fleas, which means it did not spread through flea bites.

Because of this difference, transmission in ancient communities must have followed routes unlike those of later outbreaks. When scientists compared the sheep genome with earlier human infections, it closely matched a sample called RISE386 found at a nearby settlement associated with the same cultural tradition. Both appear to have come from the same ancestral population of the pathogen.

The sheep carried ten genetic variants not found in other samples, and those changes were not caused by DNA decay. Its position on the plague family tree was well supported, and it showed signs of evolutionary pressures associated with spillover events. These are infections in hosts that do not pass the disease onward. Humans and sheep likely played that role rather than serving as long-term carriers.

Researchers believe the find points to a wider network of transmission between people, livestock and an unidentified animal reservoir that maintained the bacteria for centuries. Herding communities depended on cattle, horses and sheep, and they traveled across open grasslands where they came into contact with wild animals. That mobility, combined with close handling of livestock, created ideal chances for pathogens to jump between species.

Modern records show that sheep in some parts of Asia still become infected after contact with rodent carcasses. They do not usually pass the disease to each other but can infect people during butchering or cooking. While today’s cases cannot explain ancient ones directly, they offer a hint at how domesticated animals may have acted as occasional hosts thousands of years ago. These interactions could have helped the pathogen move across huge regions without continuous human-to-human spread.

The sheep genome also suggested it came from an infection that did not lead to further transmission. Such “dead ends” are typical for animals that are not natural reservoirs. This pattern helps explain why so many Bronze Age human infections appear genetically alike despite arising far apart. Instead of spreading steadily through people, the lineage may have repeatedly spilled over from a persistent reservoir species.

The research team also studied how this early plague lineage evolved. They found that Bronze Age strains were shaped by purifying selection, which weeds out harmful mutations. In contrast, modern plague strains show more neutral patterns, meaning mutations accumulate without strong filtering.

Several genes that came from the pathogen’s ancestor, Yersinia pseudotuberculosis, had been independently lost across multiple branches of the ancient lineage. Many of those genes influence growth and disease severity. Their disappearance suggests the microbe was adapting to new conditions as herding societies expanded and interacted with different animals and environments.

At the same time, the outer branches of the lineage, which include the sheep sample, carried a noticeable rise in mutations that change protein structure. That increase suggests the pathogen faced pressures in spillover hosts that differed from pressures in its reservoir species. Some genes with repeated changes are known to shape immune response or virulence, giving scientists new leads for understanding how plague evolved before it adapted to flea transmission.

However, one major puzzle remains. This Bronze Age lineage persisted for more than two thousand years and spread across thousands of kilometers but did not break into many distinct branches. Instead, it formed a long-lasting, tight cluster. Human travel patterns cannot fully explain that consistency. A long-term reservoir, perhaps a rodent or a migratory bird, may have repeatedly reintroduced the pathogen into new regions.

Researchers stress that ancient animal infections are difficult to detect. Animal remains decay quickly, and infections that kill animals may not be represented in archaeological sites. That makes this single sheep genome especially valuable. Its presence confirms that livestock played a part in plague ecology long before historical records described pandemics.

Scientists say this discovery should encourage more work on animal remains to build a clearer picture of prehistoric disease. Taylor Hermes, an archaeologist from the University of Arkansas, has already secured new funding to continue searching for additional infections near Arkaim. The team hopes more samples will reveal how the bacterium moved between species and across the wider steppe.

The finding also highlights the close ties between human and animal health in past societies. As communities expanded herds, traveled longer routes and spent more time in new environments, they created new openings for disease. Hermes says that pattern still holds true today as people enter habitats that once supported stable ecological relationships.

“It is important to have a greater respect for the forces of nature,” he said.

This work offers new insight into how pathogens spread among people and animals long before written records. By showing that early plague circulated through a mix of humans, livestock and a still unknown reservoir, the study may help researchers understand how emerging diseases move through mobile communities. It also underscores how changes in land use, animal management and travel can create new risks.

These lessons can guide modern public health planning, especially in regions where people live closely with animals or move across large distances for work.

The research also expands paleogenomic methods that can help identify disease pathways in other ancient contexts.

Research findings are available online in the journal Cell.

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