Archaeological sheep bones unveiled at a Bronze Age site in the Eurasian steppe. Credit: © Björn Reichhardt
A new study has uncovered ancient plague DNA in 4,000-year-old sheep bones, shedding light on how the pathogen was able to infect thousands of people across the world.
The bacterium Yersinia pestis was an early form of plague that circulated during the Late Neolithic and Bronze Age periods about 5,000 to 2,000 years ago. The pathogen is a genetically unique and prehistoric form of plague that infected humans across Eurasia before it presumably went extinct.
“We have over 200 Y. pestis genomes from ancient humans, but humans aren’t a natural host of plague,” says Ian Light-Maka, lead author of the study from the Max Planck Institute for Infection Biology in Germany.
Most human pathogens, like plague, are zoonotic in origin. This means the plague went through a process known as spillover where the pathogen jumped from animals into humans.
“One of the first steps in understanding how a disease spreads and evolves is to find out where it’s hiding, but we haven’t done that yet in the ancient DNA field,” says Light-Maka.
The team from the Max Planck Institute, Harvard University, the University of Arkansas and Seoul University investigated the bones and teeth of Bronze Age livestock at the archaeological site of Arkaim in Russia to trace the spillover.
Excavations on the Eurasian Steppe have yielded thousands of animal bones from Bronze Age livestock. Credit:
© Taylor Hermes
They conducted genetic analysis that revealed humans and sheep were infected by nearly identical Y. pestis plague strains. The results have been published in Cell.
“If we didn’t know it was from a sheep, everyone would have assumed it was just another human infection – it’s almost indistinguishable,” says Dr Christina Warinner, professor of Scientific Archaeology at Harvard University in the US.
With animal-to-human infections increasing – due to human-driven changes in climate, land use, population density and connectivity – the study demonstrates the impact animal domestication has on the spread and emergence of zoonotic diseases.
The team hypothesises that the plague was passed through an unknown wild animal to sheep and then onto humans.
The 4,000-year-old sheep tooth from which the Y. pestis genome was recovered. Credit: © Taylor Hermes
“The ancient sheep as well as human infections are likely isolated spillovers from the unknown reservoir, which remains at large,” says Dr Felix Key, a senior author from the Max Planck Institute for Infection Biology.
“Finding that reservoir would be the next step.”
The pathogen in question has a distinct plague evolution which lacked the necessary genetic machinery to pass through fleas. This suggests that the missing spillover was not from fleas.
“Arkaim was part of the Sintashta cultural complex and offered us a great place to look for plague clues,” says Dr Taylor Hermes, co-author of the study from the University of Arkansas.
“They were early pastoralist societies without the kind of grain storage that would attract rats and their fleas and prior Sintashta individuals have been found with Y. pestis infections. Could their livestock be a missing link?”
The researchers suggest that the findings fit within the historical context of the Bronze Age, as there was an increase in livestock herding which may have led to greater contact between humans and sheep.
Co-author Dr Taylor Hermes overlooks a herd of sheep in central Eurasia, where the innovation of horseback riding allowed herders to manage more livestock over larger pastures. It also likely brought these herds into closer contact with wild reservoirs of pathogens. Credit: © Emma Davolt
“The Sintashta-Petrovka culture is famous for their extensive herding over vast pastures aided by innovative horse technologies, this provided plenty of opportunity for their livestock to come into contact with wild animals infected by Y. pestis,” says Warinner.
While questions remain about how the pathogens spread so far over a short amount of time, the researchers hope this study is just the beginning.
“There will be more and more interest in analysing these collections,” says Key, “They give us insights that no human sample can.”