In the Jordanian city of Jerash, specifically beneath the Roman hippodrome, an international multidisciplinary team of researchers discovered direct genetic evidence of the bacterium Yersinia pestis, the plague-causing bacterium, inside human teeth buried around 1500 years ago.

This finding settles one of the oldest historical scientific debates, as historians have discussed for centuries the causes of this devastating plague that killed millions and reshaped the Byzantine Empire.

The Justinian Plague first appeared in historical records in Pelusium (modern-day Farama) in Roman Egypt before spreading throughout the Eastern Roman or Byzantine Empire. While traces of this bacterium had previously been found thousands of kilometers away in small villages in Western Europe, no evidence had been found within the empire itself or near the epicenter of the outbreak.

According to two studies published simultaneously in the journals Genes and Pathogens, the first recorded pandemic, known as the Justinian Plague, was indeed caused by this bacterium, the same one behind the Black Death eight centuries later.

After originating in Egypt, the plague spread around the Mediterranean until 544 AD, then continued northward to Europe and the Arabian Peninsula until 549 AD, followed by recurring waves over two centuries, meaning it was not a single wave that ended quickly.

Scientists examined eight human teeth extracted from hastily constructed burial chambers beneath the former Roman horse racing track in Jerash, suggesting a rapid mass death event, especially since the transformation of a recreational site into a mass grave indicates an emergency crisis and shows how urban centers were likely overwhelmed by epidemics.

The team used targeted ancient DNA techniques to capture genetic fragments specific to the plague, successfully reading and confirming the microbe’s identity.

The key finding was that the discovered strains were very similar among the victims, consistent with a swift outbreak. Scientific dating places these infections roughly between 550 and 660 AD, right in the heart of the Byzantine plague era.

Teeth—especially dental pulp—act as precise reservoirs of blood and its contents at the time of death. After centuries, fragments of bacterial DNA may remain preserved within these protected chambers.

Researchers today use molecular probes that “capture” specific plague genome fragments amid a heap of degraded DNA, then read the sequences with highly sensitive sequencing technologies. When the same signals recur in multiple individuals from the same place and time, it strongly indicates an epidemic outbreak rather than isolated infections.

Moreover, by analyzing hundreds of ancient and modern genomes of Yersinia pestis, including those recently recovered from Jerash, researchers found that this bacterium had been widespread among human populations for thousands of years before the Justinian outbreak.

The team also discovered that later plague pandemics, from the 14th-century Black Death to cases still appearing today, did not descend from a single ancestral strain but arose independently and repeatedly from ancient animal reservoirs, spreading in multiple waves across different regions and eras.

This recurring pattern sharply contrasts with the COVID-19 pandemic, which originated from a single spillover event and evolved primarily through human-to-human transmission. Together, these groundbreaking findings reshape our understanding of how epidemics emerge and spread, helping humanity prepare for the next pandemic.