A mysterious form of plague that spread across Eurasia thousands of years before the Black Death has finally revealed a crucial clue.

Scientists analyzing ancient DNA discovered the bacterium Yersinia pestis in a 4,000-year-old domesticated sheep from a Bronze Age settlement in the Ural Mountains—the first time the pathogen has ever been found in a non-human host from that era. Because this early strain couldn’t spread through fleas like the medieval plague, researchers have long puzzled over how it traveled so widely.