In the Middle Ages a huge disease called the Black Death killed about one third of people in Europe. Fleas that lived on rats carried a bacterium named Yersinia pestis and spread the illness.
But a much older version of this bacterium existed 5,000 years ago, long before fleas could move it. Scientists have wondered how the disease traveled so far without fleas.
New research gives a big clue. An international team, led in part by archaeologist Taylor Hermes, found DNA from Y. pestis inside the bone of a sheep that lived about 4,000 years ago.
The sheep came from Arkaim, a fortified town in the Southern Ural Mountains of today’s Russia, close to Kazakhstan. This shows that animals may have helped the ancient plague move across Eurasia.
The study was published in Cell and involved scientists from Harvard, Germany, Russia and South Korea.
Hermes also co‑leads a project that looks at ancient DNA from livestock. By studying bones and teeth, his team tracks how animals like sheep, goats and cattle moved from the Fertile Crescent to far‑away lands, shaping early societies.
Working with ancient DNA is tough. Samples are full of DNA from soil, microbes and even the researchers themselves. The useful bits are often tiny – sometimes only 50 DNA letters long, compared with the human genome’s 3 billion letters.
Animal bones are especially hard to preserve. Many were cooked or left in trash pits, where heat and weather break down genetic material.
While re‑examining sheep bones from Arkaim, the team noticed one bone carried Y. pestis DNA. "It was a big surprise," Hermes said. "It was the first time we saw the plague’s genome in an animal sample."
Earlier finds of the same ancient plague strain in human bones from far‑apart places raised a question: how did it travel so far? The sheep evidence suggests a mix of people, livestock, and an unknown animal that kept the bacterium alive – perhaps steppe rodents or migratory birds.
In modern times, rats were the reservoir for the medieval plague, while fleas spread it. Today, bats often act as reservoirs for viruses like Ebola.
Hermes recently received a five‑year grant from Germany’s Max Planck Society to keep digging around Arkaim. The goal is to find more human and animal remains that might hold plague DNA.
The Bronze Age saw the Sintashta culture raise larger herds and become skilled horse riders. More contact with animals and longer journeys likely exposed people to hidden disease carriers.
Even though these events happened thousands of years ago, the findings warn us today. Expanding into natural habitats can disturb ecosystems and raise the chance of new diseases jumping to humans.
Hermes says we need to respect nature’s delicate balance and protect the ecosystems we rely on.