Researchers have discovered ancient plague DNA in a 4,000-year-old sheep tooth, shedding light on how the disease spread across Eurasia long before the Black Death.
Long before the Black Death ravaged medieval Europe, a more ancient and enigmatic form of plague traversed vast regions of Eurasia. For decades, scientists have grappled with understanding how this ancient disease, which emerged during the Bronze Age, managed to endure for nearly 2,000 years and spread over considerable distances without the flea-borne transmission that characterized later outbreaks.
Now, researchers have made a groundbreaking discovery that may finally explain this mystery: the detection of plague DNA in the remains of a domesticated sheep that lived over 4,000 years ago.
In a study published in the journal Cell, scientists report the first known evidence that the ancient plague bacterium, Yersinia pestis, infected animals—not just humans—during the Bronze Age. This finding provides a crucial missing link in understanding how the disease circulated so widely among early societies.
The breakthrough originated from an unexpected source: a tooth from a Bronze Age sheep unearthed in what is now southern Russia. Within this ancient tooth, researchers identified genetic material from Yersinia pestis.
“It was alarm bells for my team,” said Taylor Hermes, an archaeologist at the University of Arkansas and co-author of the study. “This was the first time we had recovered the genome of Yersinia pestis from a non-human sample.”
Prior to this discovery, all confirmed Bronze Age plague genomes had been extracted from human remains, leading researchers to believe that the disease primarily spread through human-to-human contact. However, this theory never fully accounted for the remarkable geographic reach of the plague.
“This sheep changed the picture entirely,” Hermes noted.
The Bronze Age, which spanned approximately from 3300 to 1200 B.C., was marked by significant social and technological advancements. Communities began to maintain larger herds of domesticated animals, utilize horses for long-distance travel, and expand trade routes across Eurasia.
According to scientists, these developments created ideal conditions for diseases to jump between species.
“Our plague sheep gave us a breakthrough,” Hermes explained. “It had to be more than just people moving. We now see it as a dynamic between humans, livestock, and some still unidentified natural reservoir.”
Researchers believe the sheep likely contracted the bacteria from another animal—possibly rodents or migratory birds—that carried the pathogen without exhibiting symptoms. The disease could then have been transmitted to humans through close interactions during herding, slaughtering, or daily care.
This model of animal-human transmission helps clarify how the plague persisted for centuries, even before fleas became its primary vector in later outbreaks.
Extracting usable DNA from ancient animal remains poses significant challenges. Unlike human burials, which were often protected by ritual practices, animal remains were typically exposed to harsh environmental conditions.
“When we test livestock DNA, we get a complex genetic soup of contamination,” Hermes explained. “Soil, microbes, modern DNA—everything mixes together.”
The DNA fragments recovered from ancient animals are often minuscule, sometimes as short as 50 genetic “letters,” compared to the more than three billion letters in a modern human genome. Isolating meaningful pathogen DNA from this background noise requires meticulous work and advanced technology.
This makes the recent discovery particularly remarkable.
“This was a lucky find,” Hermes said. “But it also shows that livestock remains can preserve crucial information about ancient diseases.”
When the plague re-emerged in the Middle Ages as the Black Death, it spread rapidly through flea-infested rats and resulted in the deaths of an estimated one-third of Europe’s population. However, the plague during the Bronze Age exhibited different characteristics.
Earlier versions of Yersinia pestis lacked the genetic mutations that facilitate flea transmission. Without animals acting as intermediaries, scientists struggled to explain how the disease traveled so effectively across Eurasia.
The sheep genome fills that gap.
“This discovery forces us to rethink how ancient diseases moved,” Hermes stated. “Livestock weren’t just passive companions to human migration—they were active participants in disease ecology.”
Beyond unraveling an ancient mystery, the findings hold modern significance. Many of today’s most dangerous diseases—from influenza to COVID-19—originated as animal-borne infections before jumping to humans.
The Bronze Age plague may represent one of the earliest documented examples of zoonotic disease spread facilitated by human behavior.
“As people moved into new environments and interacted more closely with animals, they created pathways for pathogens,” Hermes said. “That pattern hasn’t changed.”
Researchers caution that their conclusions are based on a single ancient sheep genome, which limits the broader applicability of the findings. More samples are needed to confirm how widespread animal infection was and which species played key roles.
The study was led by scientists at the Max Planck Institute for Infection Biology, with senior authors Felix M. Key and Christina Warinner, who is affiliated with both Harvard University and the Max Planck Institute for Geoanthropology.
The research was funded by the Max Planck Society, which has also supported follow-up fieldwork in the region.
Moving forward, the team plans to analyze additional ancient animal and human remains from across Eurasia. Their objectives include identifying the original wild reservoir of the bacteria and mapping how plague spread alongside Bronze Age trade, herding, and migration routes.
“Understanding how ancient diseases emerged and spread helps us anticipate future risks,” Hermes concluded. “It reminds us that human health has always been deeply connected to the animals we live with.”
Thousands of years later, a single sheep tooth has reopened a chapter of human history and revealed how closely our fate has always been tied to the unseen world of microbes, according to Source Name.
