For centuries, historians and biologists have studied an early form of plague that differed from the Black Death that devastated medieval Europe. A Bronze Age strain of Yersinia pestis spread across Eurasia for nearly 2,000 years, but it lacked the genetic adaptations needed for flea-borne transmission. Among the many questions scholars still have, how it managed to travel such long distances has remained a mystery.
Now, a recent study in Cell provides new evidence, involving researchers who recovered Y. pestis DNA from a 4,000-year-old domesticated sheep, identifying the first non-human host of the Bronze Age plague. This suggests the disease moved through a network involving people, livestock, and an unknown natural reservoir, rather than spreading only through human contact.
The work comes from an international team led by University of Arkansas anthropologist Taylor Hermes, whose research focuses on ancient livestock genomics.
A Plague Without Fleas
The Black Death spread rapidly across medieval Europe as fleas carrying the disease transmitted it from rodents to humans. This Bronze Age strain of the plague was different. Genetic studies show it could not be transmitted by fleas, making the long-distance spread of the disease difficult to explain.
However, ancient DNA has revealed identical Bronze Age plague strains in human remains found thousands of kilometers apart. This pattern points to a broader ecological mechanism that could move with people across open landscapes.
The new clue came from Arkaim, a fortified Bronze Age settlement in the southern Ural Mountains, near the present-day border between Russia and Kazakhstan. While analyzing livestock remains excavated decades ago, Hermes and colleagues detected Y. pestis DNA in a sheep bone.
“It was alarm bells for my team,” Hermes said. “This was the first time we had recovered the genome from Yersinia pestis in a non-human sample.”
Finding Pathogens in a Genetic Haystack
Livestock bones and teeth often contain a complex mix of genetic material from soil microbes, modern human handlers, and the animals themselves. Therefore, most of this DNA is degraded into short fragments.
“When we test livestock DNA in ancient samples, we get a complex genetic soup of contamination,” Hermes said. “This is a large barrier to getting a strong signal for the animal, but it also gives us an opportunity to look for pathogens that infected herds and their handlers.”
Unlike human burials, people often cooked, discarded, or left animal remains exposed to the elements, accelerating DNA decay. Filtering host DNA from environmental contamination requires careful sequencing and comparison with reference genomes.
In this case, the effort paid off. The team reconstructed a Bronze Age Y. pestis genome from the sheep sample, confirming that domesticated animals could carry the pathogen.
Livestock as a Missing Link
The discovery changes theories of how the Bronze Age plague may have spread. Instead of relying on fleas, the disease could have circulated through close contact among people, their animals, or other sources.
“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, which could be rodents on the grasslands of the Eurasian steppe or migratory birds.”
This model fits what archaeologists know about the Bronze Age. Cultures such as the Sintashta, associated with Arkaim, kept large herds, traveled widely, and rode horses. Livestock moved with people across large territories, creating repeated opportunities for pathogens to spread between species and regions.
A Modern Context for Disease
Hermes has received a five-year grant from Germany’s Max Planck Society to excavate more human and animal remains in the southern Urals. The research aims to determine how common animal infections were and whether other species carried the same plague lineage.
The work also has modern relevance. As human societies continue to expand into new environments and increase contact with animals, ancient patterns can once again become relevant.
“When we encroach on natural environments with new economic needs, there can be deadly consequences,” Hermes said. “We should appreciate the delicate inner workings of the ecosystems we might disturb and aim to preserve the balance.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration and a Bachelor of Science in Business Administration, as well as a certification in Data Analytics. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.