A 2,000-year plague across Eurasia: the Bronze Age **Yersinia pestis** clue found in a sheep

Long before the Middle Ages and the Black Death, a plague circulated through Eurasia for around 2,000 years. Until now, that prehistoric outbreak had been identified only in human remains. For decades, researchers have struggled to explain how the Bronze Age plague managed to travel such huge distances - but fresh evidence points to an animal that may have helped carry it.

Following livestock from the Fertile Crescent across Eurasia

In a wide-ranging, ongoing project examining how domesticated animals moved alongside people, archaeologists analysed tiny traces of DNA preserved in the bones and teeth of Bronze Age cattle, goats, and sheep. The goal is to map how herds spread from the Fertile Crescent in the Middle East and onward across Eurasia.

Working with animal DNA of this age is notoriously difficult. Ancient genetic material is rarely complete: it is typically broken into small fragments and heavily contaminated by DNA from organisms that lived in the animal during life and entered the remains after death.

“A complex genetic soup”: contamination that can reveal pathogens

"When we test livestock DNA in ancient samples, we get a complex genetic soup of contamination," University of Arkansas archaeologist Taylor Hermes explains.

"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."

That second possibility became crucial when Hermes and colleagues encountered an unexpected pathogen while examining a 4,000-year-old domesticated sheep unearthed at Arkaim - an archaeological site in the Southern Ural Mountains of Russia.

Bronze Age plague bacteria on a tooth at Arkaim (Yersinia pestis)

In the remains of that sheep, the team detected plague DNA: Yersinia pestis. The genetic signature appeared on a single tooth, and it belonged to an ancient strain that could not infect fleas - unlike later forms of the bacterium that spread during the Middle Ages.

Because Y. pestis had not yet evolved the ability to use fleas as a vector in the Bronze Age, archaeologists have long questioned how the disease travelled so widely among people. Yet many humans clearly died from infection: their skeletons still contain genetic traces of the same plague strain, recovered from sites separated by thousands of kilometres.

This new result is the first time the Late Neolithic Bronze Age lineage of the bacterium has been detected in a non-human animal. The researchers first shared the finding in a preprint earlier this year, and the study has now been peer reviewed.

How sheep could have helped spread the disease across the Eurasian Steppe

The team argues it is plausible that domestic sheep, moving over the broad grasslands of the Eurasian Steppe, could have met a wild animal carrying the bacterium, remained unaffected, and then passed the pathogen between flocks and the people tending them. At the same time, the researchers stress that they cannot exclude the alternative direction of spread: transmission from humans to sheep.

"It had to be more than people moving. Our plague sheep gave us a breakthrough," Hermes says.

"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."

Why ancient animal pathogens are hard to detect

Finding ancient pathogen DNA in animals is especially challenging. Animals are not typically buried with the care given to humans, so their remains are often less well preserved.

In addition, many domestic animal remains recovered by archaeologists are effectively food waste - leftovers from meals - meaning the bones may have been cooked, which is an extremely effective way to degrade DNA.

"Moreover, people tend to avoid consuming visibly sick animals, and therefore faunal assemblages are likely biased toward healthy animals," biologist Ian Light-Maka, of the Max Planck Institute for Infection Biology, Hermes, and their colleagues write in their published paper.

"Even when infected animals are consumed, a single animal may infect many people, and the probability of that specific animal being found and later studied may be low."

Only the third ancient animal with Y. pestis - and the clearest yet

This is only the third occasion on which any strain of Y. pestis has been reported in ancient animals. The earlier two cases involved a medieval rat and a Neolithic dog, but in both instances the DNA was too fragmented to support robust conclusions.

Hermes says the new evidence is particularly compelling because of where the sheep was discovered. Arkaim is a human settlement associated with the Sintashta culture - a group known for producing sophisticated bronze weapons, riding horses, and contributing genetic ancestry that spread into Central Asia. Traces of the Late Neolithic Bronze Age (LNBA) plague strain have also been identified in people from this population.

At the time this infected sheep was alive, the Sintashta were only beginning to expand their livestock herds. Their horse-riding capabilities allowed them to cover larger areas quickly, which could have increased contact with wild species that harboured plague.

"Nevertheless, it is not possible with a single genome to reconstruct a complete understanding of the ecology of the LNBA lineage across the diverse set of cultures and geographies afflicted by this prehistoric plague lineage, and our results suggest its reservoir remains at large," the authors conclude.

This research is published in Cell.