An ancient ancestor of the pathogen that would later cause the Black Death and other major pandemics has been identified in a Bronze Age domestic sheep in Russia 鈥� making it one of the oldest pathogens ever found in an animal.
Its DNA closely matches that of plague bacteria found in European human skeletons from the same period, providing the first evidence that the disease could have spread between humans and their own livestock well before the pathogen evolved to jump from rodents to people via fleas.
For thousands of years, Yersinia pestis bacteria have caused various forms of plague in people, affecting the lymph nodes, blood or lungs. The bubonic plague of the Middle Ages spread among people and rats through flea bites, but earlier variants of Y. pestis 鈥� like the now-extinct Late Neolithic and Bronze Age (LNBA) lineage 鈥� lacked the ymt gene that makes flea-based transmission possible.
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快猫短视频s have found LNBA bacteria DNA in dozens of Neolithic and Bronze Age human skeletons scattered across Eurasia, suggesting widespread infectious disease at a time when human populations were dropping.
It is possible for the pneumonic form of the disease to spread through the air, but researchers still don鈥檛 know which kind of plague the ancient bacteria caused, nor how animals might have been involved in its transmission, says at the Pasteur Institute in Paris, who wasn鈥檛 involved in the new study. 鈥淭his is a piece of the puzzle that is still missing to understand plague in the Late Neolithic and Bronze Age.鈥�
To investigate further, at Harvard University and her colleagues sequenced DNA from the skeletons of 12 sheep and 11 cows in Arkaim, Russia, a Middle-Late Bronze Age site where people herded sheep, cattle and horses.
They found Y. pestis DNA in a domestic sheep tooth, which they carbon dated to between 1935 BC and 1772 BC. Then, the researchers compared this DNA sequence to 189 plague pathogen genomes collected from ancient humans, a medieval rat and modern people and animals. They determined that the Arkaim sheep had a strain that closely resembled those found in humans living at around the same time in Europe.
Warinner and her colleagues declined to speak to 快猫短视频 about the study, but, in a paper posted online, they suggest that people could have acquired the bacteria by eating or working with infected livestock. Indeed, modern people in western China sometimes catch plague by in their fields.
However, Rascovan says it might have been the other way around. Bronze Age sheep might have grazed near human cremation or burial sites, picking up Y. pestis from infected humans, he says. The people might have caught it by hunting and eating large wild rodents, such as marmots. 鈥淭here are many different possibilities of the scenarios that explain how this human-animal transmission was happening,鈥� he says.
It is also possible that humans and their animals 鈥� who frequently lived under the same roof for a warmer living environment 鈥� passed the pathogen back and forth to each other in the air, says at the University of Copenhagen.
鈥淚t鈥檚 clearly the same pathogen that must have been spreading between humans and domesticated animals,鈥� he says. 鈥淚 don鈥檛 really see how you could argue for the direction of it. I think both ways would be perfectly possible.鈥�
In any case, neither humans nor domesticated animals were likely to have been the primary reservoirs of the pathogen. Y. pestis has historically been found to linger long term in rodent populations, unlike in humans, where its presence is short-lived. The new study shows that the strain infecting both humans and the Arkaim sheep had certain mutations that are typical for bacteria in non-reservoir hosts.
bioRxiv
