In 1346, Tartar leader Khan Janibeg laid siege to a Genoese city in Crimea called Kaffa in hopes of removing the Italians from this central foothold. What happened next has become part-legend, part-historical record: As the Tartars waited outside Kaffa’s walls, the soldiers began to fall one by one to a terrible disease, the plague.

Out of frustration, the Tartar leaders catapulted the disease-ridden bodies over the walls of the city, where the residents threw the bodies aside and fled in ships back to Italy. But the plague followed them to Sicily and spread throughout Europe, killing 30 to 50 percent of the population by 1351.

New research has shown, however, that a different strain of the plague existed in England as far back as the dawn of the Bronze Age, some 4,000 years ago, showing how humans have lived alongside the disease for millennia while suffering only periodic pandemics. The study found that the ancient strains lacked the gene needed for flea-to-human transmissions. (During the Black Death, rat fleas transmitted the disease to humans on an industrial scale.)

Finding 4,000-Year-Old Traces of Plague

Researchers at the Francis Crick Institute in London took skeletal samples from two funeral sites in England – a mass burial in Somerset and a ring cairn monument in Cumbria. Out of 34 people, they found evidence of the plague bacterium, Yersinia pestis, in the remains of two children aged 10 to 12 and one woman aged 35 to 45. According to radiocarbon dating, all three had lived about 4,000 years ago.

Using a “clean room,” the researchers drilled into the teeth of the three, into their dental pulp, aka the tooth’s innermost layer containing blood vessels. In the fossilized tissue, they found DNA remnants of Y. pestis.

Read More: Scientists Reveal the Black Death’s Origin Story

Still, they didn’t conclude that the mass burial was the result of an outbreak as the remains there showed signs of physical trauma. Some of those people died with the plague in their bodies, but given the fragility of bacterial DNA, researchers couldn’t say exactly how many.

How Did Bronze Age Humans Respond to Plague?

The new study contains the oldest evidence of plague in England, although past research found evidence of it in mainland Eurasia between 5,000 and 2,500 years ago. This “LNBA” plague lineage likely flowed out of Central and Western Europe around 4,800 years ago, the study says, as people spread to the West.

“We see that this [Y. pestis] lineage, including genomes from this study, loses genes over time, a pattern that has emerged with later epidemics caused by the same pathogen,” says Pooja Swali, a researcher at Crick, in a press release.

As the plague DNA changed, the researchers say, so did ours, in an “evolutionary arms race with the pathogens themselves,” according to Pontus Skoglund, group leader of the Ancient Genomics Laboratory at Crick, in a press release. “Future research will do more to understand how our genomes responded to such diseases in the past.”