Scientists have deciphered the genome of the bacterium behind the world's first recorded pandemic that swept through the eastern Mediterranean about 1,500 years ago.
Researchers found Yersinia pestis, the microbe that causes plague, in a mass grave in the ancient city of Jerash in Jordan, near the pandemic's epicentre.
The findings, published in the journal Genes, confirm that the pathogen was behind the Justinian Plague, resolving a long-standing mystery.
The Justinian Plague, which lasted from 541AD to 750AD, killed tens of millions of people and reshaped the Byzantine Empire, but what actually caused the devastating pandemic was widely debated.
Although there was some circumstantial evidence, direct proof of the responsible microbe remained elusive.
"This discovery provides the long-sought definitive proof of Y pestis at the epicentre of the Plague of Justinian," study co-author Rays HY Jiang from the University of South Florida said.
"For centuries, we have relied on written accounts describing a devastating disease, but lacked any hard biological evidence of plague's presence. Our findings provide the missing piece of that puzzle,” Dr Jiang added.
In historical records, the plague first appears in Pelusium in present-day Egypt before spreading across the Eastern Roman Empire.
While some evidence of Y pestis as the microbe responsible for the pandemic had been found thousands of miles away in small western European villages, there was no proof from within the empire itself – until now.
"Using targeted ancient DNA techniques, we successfully recovered and sequenced genetic material from eight human teeth excavated from burial chambers beneath the former Roman hippodrome in Jerash, a city just 200 miles from ancient Pelusium,” Greg O'Corry-Crowe, another author of the study, explained.
DNA analysis revealed that the plague victims carried nearly identical strains of Y pestis, confirming that the bacterium was present within the Byzantine Empire between 550AD and 660AD.
This points to a rapid, devastating outbreak, consistent with historical descriptions of the plague causing mass deaths.
"Jerash was one of the key cities of the Eastern Roman Empire, a documented trade hub with magnificent structures,” Dr Jiang said.
“That a venue once built for entertainment and civic pride became a mass cemetery in a time of emergency shows how urban centres were very likely overwhelmed.”
A companion study on the plague, published in the journal Pathogens, shows the bacteria was circulating among human populations for millennia before the Justinian outbreak.
It also reveals that later plague pandemics, from the Black Death of the 14th century to rare isolated cases appearing today, did not rise from a single ancestral strain.
Instead, different outbreaks seem to spur independently and repeatedly from longstanding animal reservoirs.
The findings highlight that pandemics are not singular historical catastrophes but repeating events driven by human congregation, mobility and environmental change – themes that remain relevant to this day.
"We've been wrestling with plague for a few thousand years, and people still die from it today," said Dr Jiang.
"Like Covid, it continues to evolve, and containment measures evidently can't get rid of it. We have to be careful, but the threat will never go away.”
