Looking at ancient Roman plagues through an environmental lens

A pit of human bones, potential evidence of a catastrophic epidemic that struck Constantinople in 541 A.D. Sulfur deposits trapped in polar ice, showing traces of a series of massive volcanic eruptions. For Brandon McDonald, these seemingly incongruous findings—one in Turkey, the other in Greenland—hold clues to a puzzle about the interplay of environment and disease.

By combining ancient written records with physical archives like archaeological records and ice core samples, he is helping create a new understanding of how climate shaped landscapes, environmental change influenced disease patterns, and humans themselves transformed ecosystems—with cascading effects on ancient economies and public health.

An environmental archaeologist, McDonald arrived at Tufts this fall from a postdoc at the University of Basel in Switzerland, by way of the Connecting the Community of Tufts Scholars initiative that hired a cluster of faculty members with expertise in climate change. At Oxford, where he got his Ph.D., he focused on the interplay of climate and pathogens in the Roman Empire.

He is now the Rumsey Family Assistant Professor in the Department of Classical Studies and holds a secondary position in the Environmental Studies Program. His first semester teaching includes the undergraduate seminar Pestilence in Antiquity: Ecology of Infectious Disease in the Graeco-Roman Mediterranean. The course takes direction from a web of interrelations summed up in one word: ecology.

“The concept of ‘ecology’ perfectly captures my research focus: the intricate relationships between humans, climate, environment, and disease in the Greco-Roman world,” McDonald said. “You can think of ecology as ‘The Puzzle’—a complex one that not only illuminates the ancient Mediterranean world but also offers valuable insights for understanding our own environmental challenges.”

Tufts Now recently sat down with McDonald to learn more about his profession, what we’re learning about virulent pandemics, and what lessons antiquity might offer to us today.

What got you interested in this field?

As an undergrad at Columbia, I took a course on the environmental history of the ancient Mediterranean. At the time, there were some monographs on the influence of environment on the health of the Roman Empire, but not a lot was being done. I thought that it needed more attention, so I moved over to archaeology.

I wanted a fuller picture of the range of evidence, and in archaeology you have more opportunities to be interdisciplinary. I love to collaborate; it gets ideas flowing. That’s why I started working with paleoclimatologists, people who reconstruct past climate, and paleogeneticists, people who analyze ancient DNA. We’re all using our interdisciplinary brains to solve outstanding questions about the past.

Your study of antiquity covers four devastating plagues, including the Justinianic Plague, which struck first in Egypt in 541 and is believed to have been the first pandemic of bubonic plague. It killed an estimated 15 to 50 million people and reached nearly every corner of the Roman/Byzantine Empire by 544. What conditions contributed to its spread?

The main condition was the interconnectivity across the region, with Egypt as the nexus for cross-cultural trade routes, which, at the same time, acted as a gateway for the passage of pathogens into the ancient Mediterranean.

In recently published research, Sabine Huebner and I elaborate on how Egypt, and more so the lands south of it, had a “pestilential reputation” that goes back to Thucydides’ account of the Plague of Athens in 430 B.C. By the time of Emperor Justinian, and with Constantinople now a thriving port city, trade is even more fluid, bridging ecologies never bridged before.

As for disease transmission, for a long time we surmised, based on written accounts of symptoms, that the Justinianic Plague was bubonic plague—caused by the bacterium Yersinia pestis and transmitted by fleas carried by rodents.

The first irrefutable evidence that the Justinianic Plague was indeed bubonic plague only came a little more than a decade ago, when archaeogeneticists were able to extract DNA from human bones in multi-person graves and find evidence of the bacterium.

Ancient DNA (aDNA) research is answering questions about what diseases may have caused death in certain groups or communities. It’s also allowing us to build mathematical models and reconstruct pathogenic genomes based on aDNA extracted from human remains. We pool these data in global databases, and similar aDNA research gives a good picture of migration as well.

Ancient DNA evidence confirms that bubonic plague spread across the Mediterranean region all the way to western Europe and Britain right around the mid-sixth century. And now we can model things like transmission and mortality because we know specifically what the disease was. We don’t have to speculate anymore.

Coinciding with the Justinianic Plague was long-lasting cooling, from 536 to about 660, known as Late Antique Little Ice Age. Some areas of Eurasia may have cooled by about 4.5°F. Can you talk about what triggered that climate change?

The cooling is attributed to two massive volcanic eruptions, two of the biggest eruptions in the past 2,500 years. The first eruption happened at 536 somewhere in the northern hemisphere, followed by an even bigger one in 540. They both spewed sun-blocking aerosols into the stratosphere and threw climatic systems into a tailspin.

We have written accounts from this period in which people describe a dust cloud that dimmed the sun. What’s fascinating is that we’re finding evidence of these volcanic eruptions persisting in polar ice cores. Sulfur dioxide and other gases are released during eruptions, and we can deduce the timing and magnitude of the sixth-century eruptions through annually resolved concentrations of sulfate in ice core samples. It’s another data source that helps us understand the environment of antiquity.

Is it possible that this pervasive cooling had an influence on the spread of disease in the Mediterranean region? Some researchers say that climate change did influence the spread of infectious diseases and, by extension, played a part in the fall of the Roman Empire.

While yes, climate change did most likely facilitate the spread of some infectious pathogens in antiquity, this a broad, generalizing view. I take a more focused approach. What I’m trying to do is zero in on what caused the specific societal decline we are observing in the archaeological record, or identify what we can or can’t say about a particular natural environment or disease event.

In the context of the relationship between climate change and disease outbreaks, the former does not always lead to latter, because not all pathogens and disease ecologies are hypersensitive to shifts in climate.

When you consider the ecology of bubonic plague, the sixth-century climate change we were just discussing very likely influenced the spread of the Justinianic Plague in various ways, but for other diseases—such as some types of viral hemorrhagic fever, like Ebola—there is little evidence that climatic factors play a major role.

The point is that not all pathogens are equal. While climate change can indeed influence disease ecologies, others exhibit less sensitivity. You have to take it on a case-by-case basis, because pathogens exist under very specific environmental conditions.

Our goal is to recreate, through a variety of means and as accurately as possible, an ecology of conditions. A good example is my study of the Negev Desert in the sixth century. Significant climate change was happening at the same time as the Justinianic Plague, and the Negev was in the path of both.

We have archaeological data showing that the region underwent an economic decline soon after the onset of those events. But what exactly was disrupting things? Flash flooding or rises in aridity could have played a role, but the paleoclimatic and paleo-environmental evidence do not support the idea that such climate change caused a decline as severe as we’re seeing in the archaeological record. It seems that the plague probably played the largest role, and environmental change less so.

What’s one big question about disease in antiquity that you’d love to answer?

That would be identifying the pathogen behind the Antonine Plague, which affected the Roman Empire from 165 to around A.D. 180. It caused a tremendous loss of life, with mortality estimates ranging from 5% to 25% of the Roman population. Most scholars working on the Antonine Plague think it was smallpox, or an ancient ancestor of smallpox, judging from the symptomatology gleaned from Roman-era authors. But we don’t know, and if it were a human poxvirus, it likely wasn’t smallpox as we know it.

The earliest evidence we have scientifically for a poxvirus that affected humans is an ancient strain discovered five years ago in Viking Age Europe that is genetically similar to, but in some ways different from, modern smallpox. If we were able to extract aDNA of the pathogen responsible for the Antonine pandemic, it could answer so many questions, including questions about its origin, transmission and virulence.

When you look at your profession, and it’s a relatively new discipline, what work lies ahead? What needs further inquiry?

I’m trying to champion methodology that properly preserves bones, both animal and human, unearthed during archaeological exploration so that we have a better chance of extracting aDNA when the need arises. We’re also now making sure that we keep track of rat remains. In certain cities, and particularly in ports that fell into decline right around the time of bubonic plague epidemics, we find rat bones.

In the past, they were not usually of much interest. But diseases are now on the minds of archaeologists, and anything that will help us gain evidence related to disease events, like the preservation of bones or teeth—or rat and other animal remains—is regarded as valuable information.

Are there links between ancient Greco-Roman plagues and what’s happening today with the coronavirus pandemic and carbon-induced climate change?

There is a certain resonance that makes my teaching an unusual opportunity for students to connect what we’re learning with personal experiences and ideas. That always enriches how I teach and how they learn.

The concept of contagion, though, especially given the immediacy of COVID-19, is a tricky subject. I spent two classes of my seminar talking with students about disease awareness in antiquity; they wondered what the Greeks and Romans knew and didn’t know.

I tell them that they knew that proximity to another diseased person wasn’t good, but that they could not explain why. Some people in antiquity even thought that certain diseases were spread by the way a person looked at you. They really didn’t know what they were dealing with. These diseases were an explosion that came out of nowhere. It is an example of what happens to societies that are unprepared.

As for climate change, the climate change brought about by volcanic eruptions is very different from the change that we’ve created. But I tell my students, climate change is climate change—regardless of when or how it comes about, it can shift natural environments and disrupt disease ecologies.

Our climate is getting warmer. As that shift continues, ecological guardrails keeping pathogens at bay will start to be lowered or broken; pathogens, perhaps some new to us, will emerge and eventually find us. Again, it’s all about being prepared. The ancient Greeks and Romans couldn’t do anything about their climate change, but we can do something about ours.