KELLAMS: This is Ozarks at Large. I'm Kyle Kellams.
Yersinia pestis is an adaptable survivor. It is the bacterium that causes the bubonic plague. It's very deadly, very infectious, and it's been around for millennia. The bubonic plague redesigned human existence when as many as one-third of people in Europe died from the disease in the 14th century. Other pandemics before and since have been responsible for millions more fatalities.
Taylor Hermes, assistant professor of anthropology at the University of Arkansas, is part of an international team of researchers finding out more about the origins of the plague. Hermes says we're still learning more about the plague's history.
HERMES: It wasn't until just a few years ago that we learned that it was, in fact, the bubonic plague that caused the Justinian plague in the sixth century, which ultimately led to the collapse of the Byzantine Empire and the Roman Empire. Later, we know that the Black Death—what happened in Europe during the 14th century—was also caused by Yersinia pestis. But what's super interesting about our latest findings in the paper we published in Cell is that we are drilling deeper into understanding the mysterious alternative form of the plague.
KELLAMS: Interesting, because what a lot of us learned in school was rats, fleas. humans- that's how it spread. But if it's going back even further than the Black Death, there may have been other ways it was transmitted.
HERMES: You are right on the money.
So the bubonic plague, as we know from history textbooks, spread via fleas and rats. There's a fascinating adaptation that the bacterium does to fleas that makes them so effective in transmitting it. It causes a blockage in the digestive system, starving the fleas. The fleas become crazed for food, and they bite more and more, putting their saliva into the wounds and spreading the bacterium.
But this earlier alternative form of plague did not have the genetic toolkit to transmit via fleas. The mystery is how did this plague transmit across Eurasia—thousands of miles and potentially thousands of people?
We only have approximately 200 genomes of humans infected with this earlier form, but we assume the numbers were much higher. We don't know exactly the epidemiology. Until our findings, we didn’t understand how it spread. It wasn’t able to transmit via fleas, so the enormous question was: how did this thing move and infect so many people?
KELLAMS: Was there at least a suspicion that livestock would be involved, since livestock and humans were close to each other?
HERMES: This is a hypothesis I had been dabbling with for quite a few years. Eurasia is a gigantic region. Most of the environments across the middle latitudes are grasslands—the Eurasian steppes. The people who lived there tended to be nomadic pastoralists. They moved herds of animals across the landscape and derived most of their food and identity from herding.
Looking at the map of ancient Yersinia pestis genomes, you see it concentrated along grassland environments. So I thought, maybe it was the nature of spending so much time close to animals, and animals had some factor in infecting people.
KELLAMS: Great to have this hypothesis. How do you begin to see if you can prove this?
HERMES: The way to test this is to look in the zooarchaeological record for domesticated animals infected with Yersinia pestis. As part of another study on the origins of domesticated livestock in this part of the world- that is how they moved into this part of the world from the Near East- we screened hundreds of ancient animal remains—teeth and bones from cattle, sheep, horses, and goats.
We often find all sorts of interesting genetic evidence in these samples. The samples are sort of like genetic soups full of DNA from not only the organism itself but anything that had infected it. Anything that was in the ground for the past thousands of years that had colonized the bone structure and little small bits and pieces inside. It’s quite an effort to filter out and identify what DNA fragment belongs to what source.
Through that screening process, we started getting interesting hits for Yersinia pestis from one sheep sample from an important site in the central Eurasian steppes.
KELLAMS: I want to be romantic about this and think there's a eureka moment where you see a reading that says: this tooth from this sheep has this with it. Is it instantaneous or does it come over time?
HERMES: It comes over time. The moment those data files are produced and we have the actual sequences from the individual samples, it takes a few hours for them to filter through automatic screening processes. It’s really looking to align any of these fragments that we get of DNA from the sample that range in size from 50 to 100 base pairs, if you’re luck. Just to remind the listener, the human genome is composed of 3.2 billion base pairs.
We’re dealing with these small itty bitty fragments because they just break down into these pieces that have very little information in them and of themselves. We’re really working with this incomplete, degraded system trying to identify which molecule belongs to what organism.
Once this process is churning in this giant computer cluster, effectively a super computer, we start getting hits. It’s not dramatic—no flashing lights. It’s more trivial. It’s looking at a data table for individual organisms with molecules confidently assigned to them.
KELLAMS: Where did this tooth come from?
HERMES: This tooth came from a very important site in the Eurasian steppes called Arkaim. Arkaim dates to about 2000 BC—4,000 years ago. It’s associated with this archaeological culture, Sintashta culture, who were the first people to practice intensive equestrianism. Most likely, they domesticated the lineage of horses we have today. This lineage came from the region between the Ural mountains and where Russia and Kazakhstan share a border.
What we think was happening is that these people rode horses, built large circular settlements, but did not practice agriculture. No remains of wheat or barley have been found, which would have been expected in a large, Bronze Age city site. Instead we have people riding horses and herding sheep, goats, and cattle.
Riding horses allowed them to manage much larger flocks or herds of cattle, by a factor of ten. Maybe, with our untrained ability, we could handle about 100, 150 sheep. You put one person on a horse and you can manage up to 1,000 or 1,500 because you can move so much faster and you are up high and can see better. Throw in a well-trained herding dog and this number gets even higher.
This means you can produce more food. You also need to move more to get grass for these animals to eat.
This consistent interaction with animals, larger herds, and larger human populations, which we think contributed to more infections with Yersinia pestis.
KELLAMS: Those of us who aren't scientists want to connect dots sometimes. What can this help us connect with certainty?
HERMES: Always important to understand the past—for where we came from and how we developed as a global society. It’s also important to understand the evolutionary history of diseases. This has absolute relevancy today. We all lived through the pandemic. We know what that was like. It was a disease that emerged very suddenly through a combination of social factors and ecological factors.
It’s important to understand disease dynamics—how our society intersects with the environment and how encroaching on ecosystems increases the chance for diseases to spill over. It’s very important to understand and respect that process so that we can mitigate these things from happening again.
Showing examples from 4,000 years ago is important to get people interested in science and to understand what can be known about our past and what we can do to mitigate future problems.
KELLAMS: And understanding that these things evolve and could further evolve.
HERMES: Absolutely. These pathogens are in an evolutionary arms race with us and other organisms. They are constantly evolving, and we are constantly developing new treatments and technologies. It’s an ongoing battle.
KELLAMS: So we have this tooth from a sheep thousands of years old. How did the sheep get it?
HERMES: That is a great question. Your question speaks to the ongoing mystery of how the sheep got it in the first place, because this bacterium wasn’t transmitting via fleas. The short answer: we still do not know.
There are a few hypotheses that are leading ongoing work, also that I’m a part of.
The first hypothesis is that the natural reservoir- where a pathogen exists and persists in nature- for this form Yersinia pestis was in rodents. But how did the sheep get it from rodents? From biting sheep, maybe.
It’s also documented in contemporary times that sheep will interact with roadkill or dead animals on the landscape. I’m not sure why. It could be just curiosity. Deer, sheep, and ruminant animals will often find dead animals and nibble on them; nudge them around. If there were rodents infected then it gets to sheep and then humans.
Another possibility is migratory birds were the natural reservoir. What species is unclear. There are several potential options of flyaway migratory birds that spend winters in warm Africa and migrate throughout all Euraise. Their droppings could contaminate grass eaten by animals. But it’s difficult to test—bird remains are rare or hard to date. People also didn’t have as many birds that they consumed in comparison.
Rodents are also hard to test. When a city is abandoned, they love to come in and colonize the settlement. So, it’s hard to tell where exactly the rodent bones are from. You always have to assume that they are intrusive to the sight.
KELLAMS: So there is a chance this mystery may never be solved. Is that okay?
HERMES: That is the fundamental premise of science—there are things we don’t know. This drives us to do the work. Costs for ancient DNA work will come down, and I hope this motivates people to keep trying. I see opportunity on the horizon.
We will be motivated to answer this question. I still think it's very interesting. It will eliminate some of these ancient dynamics of how Yersinias pestis worked and will help us understand how our modern society is vulnerable and can be less so to disease.
KELLAMS: So Yersinia pestis existed thousands of years before the Black Death. And today there are still 1,000 to 1,500 cases worldwide.
HERMES: It’s a nasty bacterium and it’s tough. It still causes infections and mortality. In Mongolia, it’s sometimes linked to undercooked rodent meat. It’s prevalent in marmots and ground squirrels in Eurasia, and also in North America. I saw signs at the Grand Canyon warning people not to get close to rodents because they carry the plague.
Luckily it’s treatable with antibiotics, so it tends not to be a problem in societies with access to medical care.
KELLAMS: Well, thanks so much for your research and thanks for coming in.
HERMES: Thank you so much. I really appreciate it.
KELLAMS: Taylor Hermes is an assistant professor of anthropology at the University of Arkansas and is part of an international team of researchers examining how learning more about ancient plague genomes can help us better understand mutation forms over long periods of time. Our conversation took place in the Anthony and Susan Hui News Studio late last month.
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