In the year 541, as many of 50 million people died of the plague. The plague swept through Europe, northern Africa, parts of Asia, possibly leading to the downfall of the Roman Empire. Until now, though, no one knew for sure exactly what caused that pandemic.

Ancient teeth have given scientists the material to confirm the exact bacteria strain that caused the plague by reconstructing its DNA. Finding the teeth was key: when housing developers accidentally uncovered a burial site outside of Munich, archaeologists confirmed that the graves dated to the time of the plague.

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“They found some that had multiple individuals buried together, which is often times indicative of an infectious disease,” Northern Arizona University evolutionary biologist David Wagner told National Public Radio. “And so in this particular case, we examined material from two different victims. One of those victims was buried together with another adult and a child, so it’s presumed that they all may have died of the plague at the same time.”

The dental pulp inside their teeth contained enough traces of blood to find the DNA of the plague bacteria.

After sequencing the DNA, scientists were able to track the spread of the disease. They think the bacteria started in China, jumping from rodents to humans — and that it wasn’t related to the Black Death, as was previously believed.

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“These results show that rodent species worldwide represent important reservoirs for the repeated emergence of diverse lineages of Y pestis into human populations,” the authors wrote in The Lancet Infectious Diseases.

If it weren’t for modern medicine and antibiotics, the pathogen could cause another pandemic, Northern Arizona University microbiologist Paul Keim told NPR.

Photo: Graduate student Jennifer Klunk of McMaster University examines a tooth used to decode the genome of the ancient plague. Credit: McMaster University