Did the Black Death change our genetics?

Archaeologists and geneticists have been looking at the ancient DNA of almost 300 people who lived in Cambridgeshire before and after the Black Death - the plague that repeatedly rampaged across mediaeval England between the 14th until the 17th Centuries. They’ve been able to put to the test a theory that’s been been around for decades that, because the plague was so vicious, carrying off as it did maybe maybe a third of the European population at the time, it had the effect of altering the genetics of the population, selecting for genetically fit survivors, and weeding out the genetically more vulnerable.

The results suggest that’s not true. The study also offers intriguing new details about the  evolving social landscape at the time. Some of the victims studied in the research were buried in mass graves, one of which was found under an earlier entrance to one of Cambridge’s oldest colleges. Chris Smith ventured there - on the coldest day of the year so far - to meet researcher Christiana Scheib, and Corpus Christi college’s Fiona Gilsenan…

Fiona - We're standing at what was the original entrance to Corpus Christi College. The college was founded in 1352 in the wake of the Black Death, the Plague, and the original entrance to the college ran across this passageway here from Bene't Street, past St. Bene’t's Church and into what's now called Old Court.

Chris - Indeed, this does look a bit like the tradesman's entrance. I can see why you moved it. It's much more prestigious now. But what's special about this entrance?

Fiona - Well, some years ago, we had to do some work on the building of what is now the Taylor Library. So we excavated part of this pathway and when we did that, the archaeologists found that there were some skeletons down there that were buried in an unusual way.

Chris - Is it relevant that right next door to where we're standing there is also a church.

Fiona - It's very relevant and this in fact is the oldest church in Cambridge and there are lots of burials here, but they were standard burials. The ones that we're talking about today were found stacked and not properly buried as individuals. And so that's an indication that they were probably buried quickly. The thinking by archaeologists who've worked on them was that they were probably victims of the Plague. And I think that there's some evidence that came out to indicate that that was the case.

Chris - Standing next to me is one of those researchers who worked on this. Tell us who you are and why you've come along today.

Christiana - I'm Dr. Christiana Scheib, I'm a fellow at St. John's College and a group leader at the Department of Zoology here in Cambridge.

Chris - You had access to some of the material that came out from under our feet. What were you looking for?

Christiana - I had access to four individuals, plus a fragment of cranium, so five individuals probably. And what I was looking for was, one, the human genome - we wanted to know more about mediaeval Cambridge, but also I was looking for evidence of plague.

Chris - There's been this question about the Plague which is, people say that because it killed so many people, it had an effect on the genetics of the people that are around today. It killed off a vulnerable group, left a survivor group. Were you able to get at any of those sorts of questions with this?

Christiana - That has been a longstanding question in the area, in the field. You would expect that anytime you have a pandemic, you will have a pressure on the population that's affected to adapt. And with the Plague, we know that the mortality rate was 30 to 50/60%. It was very high. So you would expect there would be a huge impact on the genetics of the people who survived. And that was what this project was about.

Chris - Was it just here you looked at or did you have material from other burials around the town?

Christiana - So the project itself comprised more than 10 sites across Cambridge. We were going all the way back to the Neolithic, so the earliest individuals are from an early Neolithic monumental burial in Trumpington. And the most recent are from the mid 19th century, from Holy Trinity Church just a few doors down. We wanted to get an idea of the overall population structure of Cambridge through time and how this might have affected them.

Chris - And the benefit of that huge time window is of course that straddles the time when the Plague circulated in Europe, doesn't it? Because it had gone by the 1800s and you predated it a bit. So I guess you can then ask, is there a before and after effect in terms of any impact on genetics?

Christiana - Exactly. What we were primarily looking at initially was the potential genetic impact of the Black Death or the second pandemic. That started in 1347 and came in recurring waves up until the 18th century.

Chris - How did you actually do it?

Christiana - So we screened a lot of dead people for both human genome, what their immune systems looked like, as well as what diseases they had and whether or not they did have plague. We had two sets, people who had plague and then people who did not have plague. And really for this study, we ended up not looking so much at the people who had plague because we know that those people actually died from plague. What we're interested in are the people who were living before the general population, before the second pandemic or the Black Death, and then the people who were definitely born afterwards, so therefore they were children of survivors.

Chris - And I guess you can then ask the question, are there any genes different between those two groups? Because if there was some effect of the plague killing off vulnerable people or preserving people who were more immune, I suppose you'd expect to see them more or less represented in those populations?

Christiana - That's exactly what we were looking for. Is there a difference in the genetics of the population before the Black Death and the people who were born, presumably of survivors?

Chris - And what did you find?

Christiana - We actually found that there was no difference. I mean there were minor differences, but nothing that you would call statistically significant.

Chris - So that sort of blows out of the water this idea that plague did have a big moulding effect on the genetics of Europe?

Christiana - The social impact was huge and the impact on the individual's lives would've been huge. But from a population standpoint, probably because we had so much mobility afterwards, we don't see a strong genetic impact, at least in Cambridgeshire. And it could also be because of the way that the plague kills. It's not maybe targeting a specific gene or something like that, it's maybe a more complicated route to death, if you will. And so therefore there's a lot of things being worked on and therefore there's not one particular target. It sort of muddies the waters.

Chris - What about the social question as in, because you were looking at a range of individuals from a range of different backgrounds, did you see that any particular groups were more susceptible? Because one of the things we see with disease, we often say people who are in poor conditions or poor health, generally, they don't have good food, don't have good living conditions, are more vulnerable. People who were better fed, et cetera, less so. Was the plague taking no prisoners or did it follow that pattern?

Christiana - People have long said plague is an indiscriminate killer and that seems to be reflected in our data. It doesn't matter how rich you are or how poor you are, you are still susceptible to the Plague. Now, whether you survived, if you had good resources that might have helped you survive. And unfortunately with this kind of study, we're only looking at the dead people that we can see. So we're seeing people who we know died of plague or people who we don't know whether they were ever exposed. We can assume they were exposed, but perhaps they never were. Maybe they moved to the country and they managed to avoid it, which did happen amongst people who could afford it.

Chris - Can you answer a question that's outside the scope of your present paper, but one which has baffled people for a long time, which is why plague just disappeared?

Christiana - Yes. This is a really interesting question and lots of people are working on it. It could have to do with the vectors. So we believe that it's carried in fleas on rats and perhaps something changed in the environment or the way that the bacterium was infecting the vectors and maybe improvements in sanitation helped eventually for it to die out, but it seems unlikely that it went away simply because everybody became immune.

Chris - And are there any more plague pits that you can dip into in Cambridge? Or have you run out of resources now?

Christiana - I think there are plague pits everywhere. In fact, my other work is on the first pandemic, the earlier plague, which came to England, that I've worked on as part of the 'After the Plague' project, but it's also known as the Plague of Justinian. So during the early Anglo-Saxon period, we also had a pandemic that came to England and affected people in very much the same way.

Chris - You're working on that now?

Christiana - I am, yes.

Chris - You'll have to tell us what you find.

Christiana - I will, I'll keep you updated.

Chris - And hopefully it'll be a warmer day.

Christiana - I'll publish in the summer.