Bacteria can change history.

In the 14th century, a microbe called Yersinia pestis caused an epidemic of plague known as the Black Death that killed off a third or more of the population of Europe. The long-term shortage of workers that followed helped bring about the end of feudalism.

Historians and microbiologists alike have searched for decades for the origins of plague. Until now, the first clear evidence of Yersinia pestis infection was the Plague of Justinian in the 6th century, which severely weakened the Byzantine Empire.

But in a new study, published on Thursday in the journal Cell, researchers report that the bacterium was infecting people as long as 5,000 years ago.

Exactly what those early outbreaks were like is impossible to know. But the authors of the new study suggest that plague epidemics in the Bronze Age may have opened the doors to waves of migrants in regions decimated by disease.