During the Middle Ages, a devastating plague wiped out roughly one third of Europe's population. The disease spread through fleas that carried the bacterium Yersinia pestis . These fleas passed the infection from rats to humans, fueling the catastrophe known as the Black Death. But the history of plague goes back even further. An earlier form of Y. pestis appeared about 5,000 years ago during the Bronze Age. This ancient strain infected people across Eurasia for nearly two millennia before disappearing. Unlike the medieval plague, however, this earlier version could not be transmitted by fleas. For years, scientists have struggled to understand how the disease managed to spread across such a vast region without that transmission pathway. Ancient Sheep Provides a Critical Clue Researchers have now uncovered an important piece of the puzzle. An international team that includes University of Arkansas archaeologist Taylor Hermes identified the first evidence of Bronze Age plague in a nonhuman host.…