An ancestor of the bacteria responsible for plague has been found in the tooth of a sheep that lived nearly 4,000 years ago in a Bronze Age human settlement, scientists report in a new preprint study.
Millennia later, the apparent descendants of this pathogen would unleash vicious pandemics that claimed millions of human lives, including the 6th-century Justinian plague and the 14th-century Black Death.
In tracing the backstories of diseases like plague, this new research highlights the importance of looking not just at ancient human remains, but also the animals around them, the authors say.
Most human pathogens have zoonotic origins, and many likely arose in prehistoric pastoral settlements, where crowds of humans and livestock created many novel spillover opportunities.
The bacteria behind the plague, Yersinia pestis, has been intensively studied using ancient DNA, with almost 200 genomes reconstructed from traces found in human remains.
Yet we know much less about ancient plague in other species, with just one partial genome recovered from a medieval rat.
All modern strains of plague bacteria can be traced back to a common ancestor in Eurasia during the Late Neolithic and early Bronze Age, roughly 3,800 years ago, as previous research suggests.
Pastoralism was fairly new back then, as humans were only a few millennia into the shift from foraging to producing food in year-round settlements.
Those settlements were increasingly abuzz with domesticated mammals, whose population density and proximity to people raised the risk of trouble.
"In particular, the domestication of sheep, goats, pigs, and cattle and their cohabitation with people have been hypothesized as drivers for the emergence of deadly human pathogens causing infectious diseases as varied as tuberculosis, salmonellosis, measles, and plague," the researchers write.
One site that fits the profile of a springboard for early plague is Arkaim, a fortified Bronze Age settlement in the Southern Ural Mountains.
Back then, an earlier form of Y. pestis caused periodic outbreaks among humans in Eurasia, but without key genetic features of flea transmission, suggesting this plague spread without fleas.
Known as the Late Neolithic Bronze Age (LNBA) lineage, this form has been identified from dozens of human archaeological remains across Eurasia, but not from any other species.
It's now presumed extinct, but research suggests LNBA plague endured for two millennia, from about 2900 to 500 BCE, at a time of "heightened pastoralist mobility and interaction throughout the Eurasian steppes," the authors write.
The advent of horse riding, the authors propose, led to a pastoralism boom in Centra Asia's Sintashta culture 4,000 years ago.
Large, dense livestock herds were more likely to contract LNBA plague from natural reservoirs like wild rodents or birds, the authors suggest, and to enable a leap to people – even without fleas.
There is scant evidence of cultivated crops at Sintashta settlements, the authors point out, suggesting they lacked the kind of grain stores that drew flea-ridden rats into humans' midst in later plague pandemics.
Unable to transmit efficiently via fleas, LNBA plague may have spread to humans via sheep and other livestock.
"It was remarkable to discover a domesticated sheep from the Bronze Age that was infected with LNBA plague. This gave us an important clue for how plague could transmit within pastoralist communities without fleas as vectors," says University of Arkansas anthropologist Taylor Hermes.
This is the first time LNBA plague has been found in a nonhuman animal, and the researchers were able to recover the pathogen's genome – a rare feat, they note, since livestock remains tend to be jumbled, dispersed, and degraded.
These insights could help demystify the evolutionary history of plague bacteria, which remain a public health threat in some parts of the world.
"The identification of a Bronze Age Y. pestis genome from a domesticated sheep offers a novel perspective on the hidden evolution and host range of a prehistoric pathogen," they write, "and sets a precedent for the exploration of ancient diseases beyond humans."
The study, which has not yet been peer-reviewed, is available as a preprint on bioRxiv.