During the Middle Ages, a devastating plague wiped out roughly one third of Europe's population. The disease spread through fleas that carried the bacterium Yersinia pestis. These fleas passed the infection from rats to humans, fueling the catastrophe known as the Black Death.
But the history of plague goes back even further. An earlier form of Y. pestis appeared about 5,000 years ago during the Bronze Age. This ancient strain infected people across Eurasia for nearly two millennia before disappearing. Unlike the medieval plague, however, this earlier version could not be transmitted by fleas. For years, scientists have struggled to understand how the disease managed to spread across such a vast region without that transmission pathway.
Ancient Sheep Provides a Critical Clue
Researchers have now uncovered an important piece of the puzzle. An international team that includes University of Arkansas archaeologist Taylor Hermes identified the first evidence of Bronze Age plague in a nonhuman host. The scientists detected Y. pestis DNA in the remains of a domesticated sheep that lived about 4,000 years ago.
The animal came from Arkaim, a fortified settlement in the Southern Ural Mountains of present day Russia near the border with Kazakhstan. The finding suggests that livestock may have played a role in the spread of plague during the Bronze Age, helping explain how the disease traveled so widely across Eurasia.
The research was published in Cell under the title "Bronze Age Yersinia pestis genome from sheep sheds light on hosts and evolution of a prehistoric plague lineage." The international collaboration includes researchers from Harvard University and leading institutions in Germany, Russia and South Korea.
Searching Ancient DNA for Clues
Hermes co leads a major research project that studies ancient livestock DNA. By examining genetic material preserved in bones and teeth, his team is tracing how domesticated animals such as cattle, goats and sheep spread from the Fertile Crescent across Eurasia. These movements helped shape the rise of nomadic cultures and early empires.
"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."
Working with ancient DNA is challenging and time consuming. Scientists must separate the DNA of the animal from many other sources found in the sample. Microorganisms living in the soil where bones were buried leave behind their own genetic traces. Researchers can also accidentally introduce DNA from their own skin cells or saliva.
The fragments recovered from ancient remains are extremely small. Many pieces measure only about 50 base pairs. For comparison, the full human genome contains more than 3 billion base pairs.
Animal remains also tend to be less well preserved than human remains, which are usually carefully buried. Animals were often cooked and eaten, and their bones discarded in waste piles where exposure to heat and weather gradually breaks down genetic material.
The Moment of Discovery
While studying livestock remains excavated from Arkaim in the 1980s and 1990s, Hermes and his colleagues noticed something unexpected. One sheep bone contained DNA belonging to Yersinia pestis.
"It was alarm bells for my team. This was the first time we had recovered the genome from Yersinia pestis in a non-human sample," Hermes said. "We were extra excited because Arkaim is linked to the Sintashta culture, which is known for early horse riding, impressive bronze weaponry and substantial geneflow into Central Asia."
How Did Bronze Age Plague Spread?
Researchers have previously found identical Bronze Age plague strains in human remains located thousands of kilometers apart. The question has been how the disease managed to travel such long distances.
"It had to be more than people moving. 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," Hermes said.
A natural reservoir is an animal species that carries a pathogen without becoming sick. In the Middle Ages, rats served as the reservoir for Y. pestis, while fleas acted as the vector that spread the bacterium. Today, bats often fill this role for viruses such as Ebola and the Marburg virus.
Lessons From an Ancient Epidemic
Hermes recently received a five year grant from Germany's Max Planck Society worth 100,000 Euros to continue excavations in the Southern Urals near Arkaim. His team will search for additional human and animal remains that may contain traces of Y. pestis.
The Bronze Age was a period when the Sintashta culture began managing larger livestock herds while also becoming skilled horse riders. Increased interaction with animals and expanding travel across the steppe may have exposed people to disease reservoirs in the environment.
Although these events happened thousands of years ago, Hermes believes the findings carry an important message today. Expanding economic activities into natural environments can disrupt ecosystems and increase the risk of disease spillover.
"We should appreciate the delicate inner workings of the ecosystems we might disturb and aim to preserve the balance," Hermes said.
"It's important to have a greater respect for the forces of nature," he said.
