COVER STORY / THE BLACK DEATH
“When the Black Death arrived in Europe, it was like striking a match in tinder”
The medievalist and historian of medicine Monica H Green tells Ellie Cawthorne how scientific advances have changed our thinking on what caused the Black Death – and why it was so devastating
On the HistoryExtra podcast
Ellie Cawthorne: As a leading expert in the field of Black Death studies, how would you say science has changed the picture for research into this dark episode in human history?
Monica Green: I have to be able to talk about the Black Death – it’s part of the remit of my job. But the problem with the Black Death was that, for 30 years, there was an intense debate about what caused it. And when you don’t know the biological cause of a pandemic, it becomes a black hole in the story that you can throw all kinds of theories into. So I had spent most of my career being frustrated about not being able to tell a coherent story about what was clearly a major historical event.
But in the last couple of decades there’s been an absolute sea change, caused by something called paleogenetics. You can now take modern laboratory techniques and apply them to the past – enabling the genetic analysis of molecular material from 100 years, 500 years, 10,000 years ago. This has allowed us to identify the pathogens that have caused diseases.
And the first pathogen that was completely sequenced from ancient DNA was the Black Death. Right in the middle of modern London, near the Tower of London, there is a cemetery that was created when they heard that a great plague was coming in 1348. When the epidemic passed, they closed it and sold the land. So the site itself is dated perfectly. And then the science swooped in and established what was in the bodies of the people buried there. What they found there was Yersinia pestis, the bacterium that causes plague.
As this huge historical question shifted entirely out of the field of history and into the field of science, I found myself sitting on the sidelines, watching the scientists at work. But then I had to ask – where do we as historians come in? Scientists can reveal what was happening at a bacterial level – but what are the ramifications for human populations? How do we read the science and interpret it? We historians needed to keep up with the geneticists and figure out how this changes the story. And what I realised is, it changes a lot.
So how does the identification of Yersinia pestis as the cause of plague help historians?
As a medievalist, I’ve been used to building a story around written sources. But the trouble was, medieval writers used modes of description that were never going to satisfy us – they described the disease in terms of miasmas [unhealthy vapours], carbuncles [boils on the skin] and fevers – without, of course, using a thermometer. But that’s all changed. Now that we have the biological criteria for Yersinia pestis, we know what symptoms we should be looking for, and we can identify potential vectors [carriers of disease]. We can even think about what kind of ecological shifts might have been going on that would allow this disease to proliferate.
Is there a particular breakthrough moment in the scientific research that you’d like to point to?
The big transformation came in a 2011 study using genetic material from London that published an entire Yersinia pestis genome [an organism’s complete set of genetic material].
When you have a portion of the genome, you can say what the organism is – whether that’s a banana, a gorilla or a single-celled pathogen. However, once you have the entire genome, you can compare that to every other genome [from the same species] you have from anywhere in the world and you can create a phylogenetic tree, which is essentially just a family tree.
Every time a new genome is published, whether modern or ancient, it gives you more information about the evolutionary history of the whole species of the pathogen. For me, that has transformed what we can now say about the history of the Black Death. For example, we now know that in both California and Arizona today there are strains of plague that are descendants from strains around at the time of the Third Plague Pandemic, which peaked in the late 19th century.
Have these advances enabled us to establish how old plague is?
Us historians working with our written documents have traditionally talked about three plague pandemics: the Justinianic Plague [c 541–750AD]; the 1340s Black Death and the outbreaks that followed it; and a modern plague pandemic coming out of Hong Kong in the late 19th century and extending into the 20th century.
But in 2015, something happened that nobody had anticipated: Yersinia pestis was retrieved from Bronze Age samples. This Bronze Age study was transformative for historians, as it serves as evidence of plague from before systems of writing. Now we can identify plague moving back and forth across northern Eurasia for millennia. The oldest sample we have of Yersinia pestis in a human body is about 5,000 years old, and it is estimated that the organism itself is probably about 7,000 years old.
Almost all of the late Neolithic and Bronze Age recoveries that have been found so far are individuals. No mass graves – which would indicate a sudden and intense outbreak – have been discovered yet. So we cannot say at this point whether we are looking at evidence for pandemics (where there’s really massive human morbidity all at the same time), or whether we’re dealing with some kind of slow, creeping thing that’s moving through rodent populations and only once in a blue moon getting into a human population.
Do we know where Yersinia pestis originated?
So far, most of the late Neolithic and Bronze Age samples are coming from northern Eurasia – areas in modern-day China and Russia. But there’s a new Bronze Age sample that’s come from England, so it was already widely disseminated in this period.
Interestingly, the strains of plague alive today that are most closely related to both the Black Death and the Justinianic Plague are found in a single area – the Tian Shan mountains and the Junggar Basin. On a modern map, that’s at the juncture between China and Kyrgyzstan. Why might that be?
We can assume for the moment that strains of plague persist in that particular ecology because of marmots, which are among the biggest rodents in the world. [Marmots living in this border region today still have strains of Yersinia pestis closely associated with strains found in the bodies of historical plague victims].
As we’ve traditionally thought about it, the Justinianic Plague happened in the Mediterranean, and the Black Death happened in the Black Sea and the Mediterranean, before spreading to north-west Europe. So how does a marmot disease from central Eurasia end up killing humans in western Europe? That’s the big challenge to explain historically: how we get from Kyrgyzstan to London before there are aeroplanes.
“Then something happened that no one had anticipated: Yersinia pestis was found in Bronze Age samples ”
So what’s your theory? How did the Black Death travel from central Asia to northwest Europe?
To answer this question, you need to look at the phylogenetic trees of the Black Death. These show that, not only do we have the branch that gets to Europe, but we also have three other branches that are created at the same time. The bullseye of this proliferation is Kyrgyzstan. So, as a historian, you have to say, OK, what was happening there?
We also need to look at an earlier period. The geneticists call this moment the “Big Bang” –a sudden explosion of Yersinia pestis in all these different directions. But what caused this Big Bang and when did it occur? They say it might have happened in the 13th century, maybe even in the 12th century. And I, as a medieval historian, say: “Well, something big did happen in the 13th century.” That was the rise of the Mongol empire. The largest land empire in human history seems to have been quickly followed by the largest pandemic in human history. Can that be a coincidence?
How might the rise of the Mongol empire have been connected to the spread of plague across medieval Eurasia?
I’ve identified a possible mechanism of transmission, connected to major movements of populations triggered by the Mongols’ wars and conquest. And that is army provisioning. In the 1250s, the Mongols were moving through central Eurasia. They got as far as Syria and were even threatening the Mamluk regime in Egypt. Importantly, they brought some of their own provisioning from the foothills of the Tian Shan mountains. One of the texts that I found said the Mongols were bringing with them a special kind of the cereal crop millet.
I spent weeks looking up the history of the different kinds of millet. One was described by the sources as a kind of super food – they claimed you could feed a cup of it to a soldier in the morning, and it would sustain him for the rest of the day. After finding that one detail, all the other pieces of the puzzle fell into place. If you’re provisioning your army as it’s moving across swathes of land, you’re transporting huge sacks of grain. And what loves huge sacks of grain? Rodents.
This is one potential scenario. But the point is that it satisfies what needs to be satisfied in terms of historical explanation.
Does this mean we have to reconsider the geography of the Black Death? Should we be re-examining how far it spread?
There’s no explanation as to why the edges of the traditional Black Death map are where they are. That’s always troubled me. Why didn’t plague also move eastward? Why do we not have stories about the Black Death in China, Tibet or India? We still can’t answer those questions fully, but historians of China are now looking more closely at the records, and a study has recently suggested the possible presence of plague in Tibet. So changing the chronology does mean that we have to at least question the geography as well.
Along with Professor Nahyan Fancy [of DePauw University in Indiana], I’ve also gone through Arabic sources, chronicles, medical writings and so forth, and found clear evidence for plague in what is now Iraq, Syria and possibly also in Egypt in the 13th century. So almost 100 years before our regular narrative of the Black Death starts, plague has already moved across central Eurasia.
What I’ve documented for the 13th century are sporadic plague outbreaks. But what happened in the 14th century is clearly different – there’s a systematic spread of plague, and the results were devastating.
“Changing the chronology of the Black Death story does mean that we have to at least question the geography as well ”
How else has new research overturned previous thinking about the spread of the Black Death?
One of the oldest stories told about the Black Death is that of the Mongols besieging the Genoese-run merchant port of Kaffa. In this story, the Mongols had an outbreak of plague among their troops, and during the siege they hurled plague-infested bodies over the walls of the besieged city. The Genoese got infected and jumped on their boats back to Genoa, taking the disease with them to Europe.
But Professor Hannah Barker [of Arizona State University] has recently published a wonderful article reassessing this story. She was able to establish that there was no outbreak of plague associated with the siege itself. Kaffa was part of a trade infrastructure established between Italy and the Black Sea. And the big thing the Italians were importing was grain.
After the siege there were trade embargoes, and then in 1347, the embargoes were lifted. It was then that plague started moving to Europe, because the grain shipments back to Italy started again. What we can’t explain right now is why so many grain supplies were contaminated with plague all of a sudden, whether there might have been a flurry of rodents or a new kind of flea that’s involved in the transmission.
Once plague had reached the Black Sea and Mediterranean, the speed of movement was suddenly inflated, because now we have a maritime culture with ships involved. A ship hold full of grain is rodent-heaven, and if enough rodents survived the journey alive, they would have crawled off the ship, taking their fleas with them.
The Mongols were nomads. Whereas when plague arrived in Europe, it was coming into heavily populated, fixed urban environments that already had rodent infrastructures. It was like striking a match in tinder.
We have, as no one needs reminding, been living through a pandemic over the past two years. Has this changed the way historians view plague?
One debate that is very current about Covid is how pandemics end. I’ve come to accept that a pandemic lasts as long as the pathogen is still around in a way that can threaten human populations. That’s something that’s important to remember about the plague – that it didn’t just disappear in Europe after the 1340s. It came back in the 1350s and 1360s, and then in subsequent waves all the way through to the 17th century. It hit one town and then it hit another. It was terrifying. It was still incredibly lethal, and no one knew when it was going to strike again.
Plague and Covid are different diseases, but what defines a pandemic isn’t just the pathogen but also the human involvement in the process of transmission. Every pandemic – from cholera to HIV to flu – starts as a local disease. The question is, how does a local disease become a global disease? That can only happen if humans are involved. The successful pandemic pathogen will be the one that can most effectively exploit our mechanisms of travel, migration and exchange, transportation, connection and communication.
What has Covid done? It’s exploited aviation. What did plague exploit? The fact that so many different societies were majorly invested in the grain trade. So that’s the thing that we need to look at – the way that we as humans are creating the mechanisms to allow the pathogens to spread.
Monica H Green is a historian and independent scholar, who was recently visiting professor of the history of science at Stanford University. Monica is one of the experts to appear on our new podcast series on the Black Death
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