For thousands of years, a prehistoric plague moved across Eurasia with terrifying efficiency. Scientists found its traces in human remains scattered far apart long before the Black Death gave the plague its grim reputation. What puzzled experts was how it spread so without fleas, which later caused outbreaks during medieval times.
That mystery may finally have an unlikely answer. According to new research published in Cell and reported by ScienceAlert, the world’s earliest known carrier of plague appears to have been a domesticated sheep. The finding offers the first clear evidence that Bronze Age plague spread through animals moving alongside humans.
Researchers identified DNA from Yersinia pestis, the bacterium that causes plague, on a 4,000-year-old sheep tooth excavated at Arkaim, an archaeological site in what is now southern Russia. The strain belongs to the Late Neolithic Bronze Age lineage, an early form of plague that couldn’t infect fleas. Without that mechanism, scientists have long struggled to explain how the disease managed to cross continents.
The discovery emerged from a larger effort to track how livestock migrated with humans from the Fertile Crescent across Eurasia. The team examined ancient DNA from cattle, goats, and sheep, material that’s rarely intact and often contaminated.
“When we test livestock DNA in ancient samples, we get a complex genetic soup of contamination,” said Taylor Hermes, an archaeologist at the University of Arkansas and lead author of the study. “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.”
Plague Spreading Sheep?
That opportunity paid off with the Arkaim sheep. The plague DNA matched strains previously found in human remains thousands of miles apart, connecting animal movement to human infection for the first time during this period.
“It had to be more than people moving. Our plague sheep gave us a breakthrough,” Hermes said.
The researchers now see the spread as a three-way interaction between humans, livestock, and an unidentified natural reservoir, possibly rodents native to the Eurasian steppe. Sheep roaming wide grasslands could have encountered infected wildlife and passed the bacterium through herds and to people. The team also notes that transmission could have gone the other way.
Ancient animal diseases are rarely preserved. Animals weren’t buried with care, and many remains come from meals, often cooked, which destroys DNA. That makes this find especially rare.
One genome doesn’t solve the entire mystery. But one ancient sheep has narrowed the search, offering a clearer picture of how one of humanity’s earliest pandemics may have traveled with the animals people depended on most.
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