Three young girls lie side by side in the frozen Siberian earth near Lake Baikal. Two of them were probably cousins. All three died of the plague — roughly 5,500 years ago, which is about 200 years earlier than scientists had previously believed. This single burial tells a story that no textbook quite prepares you for: the Black Death was not a medieval catastrophe that came from nowhere.

It had a very long prehistory, and it began somewhere out on the Siberian steppe. Researchers led by evolutionary geneticist Eske Willerslev of the University of Copenhagen examined remains from four ancient cemeteries near Lake Baikal in Russia. They recovered plague DNA from the teeth of 18 hunter-gatherers whose bones were carbon-dated to reveal two distinct outbreaks. The bacteria almost certainly jumped to humans from marmots — large, burrowing rodents that were hunted for food — when people ate raw organs or handled infected hides while butchering them.

From there, the disease spread between people through coughing and sneezing, which means it was already adapting to human transmission thousands of years before the Black Death wiped out roughly a third of Europe in the 14th century.