For over 20 years, I’ve been telling the same story to students whenever I teach European history. At some point in the 14th century, the bacterium Yersinia pestis somehow moved out of the rodent population in western China and became wildly infectious and lethal to humans. This bacterium caused the Black Death, a plague pandemic that moved from Asia to Europe in just a few decades, wiping out one-third to one-half of all human life wherever it touched. Although the plague pandemic definitely happened, the story I’ve been teaching about when, where, and the history of the bacterium has apparently been incomplete, at best.

In December, the historian Monica Green published a landmark article, The Four Black Deaths, in the American Historical Review, that rewrites our narrative of this brutal and transformative pandemic. In it, she identifies a “big bang” that created four distinct genetic lineages that spread separately throughout the world and finds concrete evidence that the plague was already spreading from China to central Asia in the 1200s. This discovery pushes the origins of the Black Death back by over a hundred years, meaning that the first wave of the plague was not a decades-long explosion of horror, but a disease that crept across the continents for over a hundred years until it reached a crisis point.

As the world reels beneath the strains of its own global pandemic, the importance of understanding how humans interact with nature both today and throughout the relatively short history of our species becomes more critical. Green tells me that diseases like the plague and arguably SARS-CoV-2 (before it transferred into humans in late 2019 causing Covid-19) are not human diseases, because the organism doesn’t rely on human hosts for reproduction (unlike human-adapted malaria or tuberculosis). They are zoonotic, or animal diseases, but humans are still the carriers and transporters of the bacteria from one site to the other, turning an endemic animal disease into a deadly human one.

The Black Death, as Monica Green tells me, is “one of the few things that people learn about the European Middle Ages.” For scholars, the fast 14th-century story contained what Green calls a “black hole.” When she began her career in the 1980s, we didn’t really know “when it happened, how it happened, [or] where it came from!” Now we have a much clearer picture.

“The Black Death and other pre-modern plague outbreaks were something everyone learned about in school, or joked about in a Monty Python-esque way. It wasn't something that most of the general public would have considered particularly relevant to modernity or to their own lives,” says Lisa Fagin Davis, executive director of the Medieval Academy of America. But now, “with the onset of the Covid-19 pandemic, suddenly medieval plagues became relevant to everyone everywhere.”

The project that culminated in Green’s article unfolded over many years. She says that the first step required paleogenetic analysis of known victims of the plague, including a critical study 2011. Paleogenetics is the study of preserved organic material—really any part of the body or the microbiome, down to the DNA—of long dead organisms. This means that if you can find a body, or preferably a lot of bodies, that you’re sure died in the Black Death, you can often access the DNA of the specific disease that killed them and compare it to both modern and other pre-modern strains.