How did Bronze Age plague spread? A sheep might solve the mystery

(Press-News.org) In the Middle Ages, a plague killed a third of Europe’s population. Fleas carried the plague bacterium, Yersinia pestis, transmitting the Black Death from infected rats to millions of people. 

Another, earlier strain of Y. pestis emerged 5,000 years ago in the Bronze Age. It infected people throughout Eurasia for 2,000 years and then vanished. Unlike the Middle Age plague bacterium, this earlier Bronze Age strain could not be transmitted by fleas. How the plague circulated for so long across a vast area has long been a mystery. 

Now, an international team of researchers, including University of Arkansas archaeologist Taylor Hermes, has found the first evidence of a Bronze Age plague infection in a non-human host. The scientists discovered Y. pestis DNA in a 4,000-year-old domesticated sheep from Arkaim, a fortified settlement located in the Southern Ural Mountains of present-day Russia near the border of Kazakhstan. The discovery provides a missing link for how the Bronze Age plague spread so widely. 

The results, titled “Bronze Age Yersinia pestis genome from sheep sheds light on hosts and evolution of a prehistoric plague lineage,” were published in the journal Cell. The other authors are from Harvard University and top institutions in Germany, Russia and South Korea. 

A LUCKY DISCOVERY  Hermes co-directs a large, ongoing study of ancient livestock DNA. By analyzing the DNA in animal bones and teeth, Hermes and his collaborators are tracing how domesticated cattle, goats and sheep spread from the Fertile Crescent throughout Eurasia and gave rise to nomadic societies and 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.” 

The research is highly technical and time intensive. A host’s DNA must be filtered out from all the other DNA in the sample. Organisms that lived in the surrounding ground where the bones and teeth were buried leave their own DNA. The researchers themselves contaminate the samples with DNA from their saliva and skin cells. The recovered DNA fragments are often only 50 base pairs long. In comparison, a complete strand of human DNA is more than 3 billion base pairs. 

Animal remains are also rarely as well preserved as human remains, which are normally carefully buried. Heat from cooking the animals and the leftovers being left in trash piles and exposed to the elements degrade genetic material. 

While analyzing livestock samples excavated from Arkaim in the 1980s and 1990s, Hermes and his colleagues realized a sheep bone contained DNA from Y. 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.” 

THE NEXT QUESTION TO ANSWER Scientists have found numerous examples of identical strains of Bronze Age plague in humans thousands of kilometers apart. How did the disease spread so widely? 

“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 that carries the bacteria but does not get sick from it. In the Middle Ages, rats were the reservoir and fleas were the vector for Y. pestis. Today, bats are often the natural reservoir for pathogens like Ebola and the Marburg virus. 

To continue this work, Hermes has received a five-year grant from Germany’s Max Planck Society for 100,000 Euros to dig for human and animal samples in the Southern Urals near Arkaim. He will be looking for more examples of Y. pestis infections. 

The Bronze Age was the moment when people in the Sintashta culture began to maintain larger herds of livestock, while also riding horses well for the first time. The Bronze Age plague was likely a result of that closer contact with animals and frequently moving into areas where they would be exposed to the reservoir. Even though it occurred thousands of years ago, Hermes believes the ancient plague has lessons for us today. 

When we encroach on natural environments with new economic needs, there can be deadly consequences. We should appreciate the delicate inner workings of the ecosystems we might disturb and aim to preserve the balance. 

“It’s important to have a greater respect for the forces of nature,” he said. 

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