Did the Black Death Rampage Across the World a Century Earlier Than Previously Thought?

Scholar Monica Green combined the science of genetics with the study of old texts to reach a new hypothesis about the plague

Black Death cemetery London
The genetic lineage of the plague that hit London in 1348 gave Green a data point to track the disease back to its origin. MOLA / Getty Images

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 in 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.

This has paid off in numerous ways. First, as scientists mapped the genome, they first put to rest long lingering doubts about the role Y. pestis played in the Black Death (there was widespread but unsubstantiated speculation that other diseases were at fault). Scientists mapped the genome of the bacterium and began building a dataset that revealed how it had evolved over time. Green was in London in 2012 just as findings on the London plague cemetery came out confirming without a doubt both the identity of the bacterium and the specific genetic lineage of the plague that hit London in June 1348. “The Black Death cemetery in London is special because it was created to accommodate bodies from the Black Death,” she says, “and then when [the plague wave] passed, they closed the cemetery. We have the paperwork!”

Green established herself as the foremost expert in medieval women’s healthcare with her work on a medical treatise known as The Trotula. Her careful analysis of manuscript traditions revealed that some of the text was attributable to a southern Italian woman, Trota. Other sections, though, revealed male doctors’ attempts to take over the market for women’s health. It’s a remarkable text that prepared Green for her Black Death project not only by immersing her in the history of medicine, but methodologically as well. Her discipline of philology, the study of the development of texts over time, requires comparing manuscripts to each other, building a stemma, or genealogy of texts, from a parent or original manuscript. She tells me that this is precisely the same skill one needs to read phylogenetic trees of mutating bacteria in order to trace the history of the disease.

Still, placing the Black Death in 13th-century Asia required more than genetic data. Green needed a vector, and she hoped for textual evidence of an outbreak. She is careful to add that, when trying to find a disease in a historical moment, the “absence of evidence is not evidence of absence.” Her first step was to focus on a cute little rodent from the Mongolian steppe: the marmot.

Mongols hunted marmots for meat and leather (which was both lightweight and waterproof), and they brought their rodent preferences with them as the soon-to-be conquerors of Asia moved into the Tian Shan mountains around 1216 and conquered a people called the Qara Khitai (themselves refugees from Northern China). There, the Mongols would have encountered marmots who carried the strain of plague that would become the Black Death. Here, the “big bang” theory of bacterial mutation provides key evidence allowing us a new starting point for the Black Death. (To support this theory, her December article contains a 16-page appendix just on marmots!)

The phylogenetic findings were enough for Green to speculate about a 13th-century origin for the plague, but when it came to the mechanism of spread, all she had was conjecture—until she found a description of an outbreak at the end of the Mongol siege of Baghdad in 1258. Green is quick to note that she has relied on experts in many different languages to do this work, unsurprisingly since it traverses from China to the rock of Gibraltar, and from near the Arctic Circle to sub-Saharan Africa.

No one is expert in all the languages. What Green brought was a synthetic view that drew a narrative out of cutting-edge science and humanistic scholarship and the ability to recognize the significance of what she found when she opened a new translation of the Akhbār-i Moghūlān, or Mongol News. This source was published for the first time in 2009 by the Iranian historian Iraj Afshar, but only translated into English in 2018 as The Mongols in Iran, by George Lane. The medieval Iranian source is something of a jumble, perhaps the surviving notes for a more organized text that didn’t survive. Still, the report on the Mongol siege, Green realized, held the key piece of evidence she’d been looking for. As she cites in her article, Mongol News describes pestilence so terrible that the “people of Baghdad could no longer cope with ablutions and burial of the dead, so bodies were thrown into the Tigris River.” But even more importantly for Green, Mongol News notes the presence of grain wagons, pounded millet, from the lands of the Qara Khitai.

Suddenly, the pieces fit together. “I’ve already got my eye on the Tian Shan mountains, where the marmots are,” she says, and of course marmot-Mongol interaction could cause plague there, but didn’t explain long-distance transmission. “The scenario I’m putting together in my head is some sort of spillover event. Marmots don’t hang around people. They’re wild animals that will not willingly interact with humans. So the biological scenario I had to come up with is whatever is in the marmots had to be transferred to another kind of rodent.”

With the grain supply from Tian Shan linked to plague outbreak in Baghdad, it’s easy to conjecture a bacterium moving from marmots to other rodents, those rodents riding along in grain, and the plague vector revealed. “That was my eureka moment,” she says.

She had put the correct strain of the bacteria at the right place at the right time so that one infected rodent in a grain wagon train revealed the means of distribution of plague.

“Throughout her career, Dr. Green has combined humanism and science in ways that have brought a more clear understanding of the origins and spread of plague,” says Davis, from the Medieval Academy. “Her collaborations with historians, geneticists, paleobiologists, archaeologists and others untangle the genetic complexities of plague strains.”

That kind of interdisciplinary work would have been significant to scholars at any moment, but right now takes on particular relevance. “[Green] has worked to undermine imprecise and simplistic plague narratives and to explain to a ready public the importance of understanding historic plagues in context,” adds Davis “[Her] voice has been critical as we try to make sense of our own modern-day plague.”

Green also sees the relevance, especially as her study of plague variants and pandemic came out just as new variants of the Covid-19 pathogen were manifesting around the world. She tells me that her work didn’t change because of Covid, but the urgency did. “Plague,” Green says, “is our best ‘model organism’ for studying the history of pandemics because the history of it is now so rich, with the documentary and archaeological record being supplemented by the genetic record. All the work the virologists were doing in sequencing and tracking SARS-CoV-2's spread and genetic evolution was exactly the same kind of work that could be done for tracking Yersinia pestis's evolution and movements in the past.”

She wants her fellow scholars to focus on human agency both in history—those Mongols and their wagon trains—and now. The history of the Black Death tells “a powerful story of our involvement in creating this pandemic: this wasn't Mother Nature just getting angry with us, let alone fate. It was human activity.”

The world is only now—thanks to Green and many others (see her long bibliography of scholars from a wide variety of disciplines, time periods, and parts of the world)—really getting a handle on the true history of the Black Death. Next, she tells me, she has an article coming out with Nahyan Fancy, a medieval Islamist, on further textual evidence of plague outbreaks to supplement the Mongol News. Many of these 13th-century sources were previously known, but if you start with the assumption that the plague couldn’t be present until the 14th century, you’d never find them.

She imagines scholars may find plague in other places, once they start looking. In the meantime, the stakes for understanding how diseases move remains crucial as we wrestle with our own pandemic. I ask her what she thinks it all means for a world today still grappling with a pandemic. She replies, with a harrowing, centuries-look ahead, “The story I have reconstructed about the Black Death is 100 percent an emerging infectious disease story. ... an ‘emerging’ disease lasted for 500-600 years!!!”

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