The post-Roman centuries in Europe have a bit of an identity crisis. If we defined the period from when the Western Emperor was abolished in 480 to the crowning of Charlemagne as Holy Roman Emperor on Christmas day 800 AD, what would you call it? At times I’ve used all of these names, and a good argument can be made for all of them. I’m curious what my readers prefer.
Monica H. Green (firstname.lastname@example.org,@MonicaMedHist) is a historian of medieval medicine. An elected Fellow of the Medieval Academy of America, she teaches both global history and the global history of health. She was the editor in 2014 of Pandemic Disease in the Medieval World: Rethinking the Black Death, the inaugural issue of a new journal, The Medieval Globe.
Boris Schmid (@BorisVSchmid) is a theoretical biologist at the University of Oslo, Norway, and specializes in disease ecology and epidemiology. He recently described a link between climate fluctuations in medieval Central Asia and what looks like repeated introductions of plague into Europe’s harbors, a hypothesis that can be tested by the analysis of ancient DNA samples of Y. pestis. He works in a multidisciplinary team of theoreticians, archeologists, microbiologists and historians, led by Nils Chr. Stenseth.
In our previous blog post, Monica and I discussed how different lineages of plague – Yersinia pestis – collected their own genetic signature (SNP profile) as they diversified from a common ancestor. Monica also summarized in broad terms what ancient DNA samples of Y. pestis (extracted from plague victims) are now available from the initial Black Death outbreak and how they are related, using the latest plague studies of Haensch, Bos and Spyrou.
In this blog post Monica will delve into the nitty gritty details of these aDNA plague studies, and give an example of how to transform those details into a new understanding of the past rodent reservoirs and global mobility of plague, one of the deadliest diseases of our collective past. And I close the post by reflecting on the potential of aDNA to connect the fields of history and biology.
Thanks, Boris. It might be important to remind readers that we don’t have any aDNA evidence from past rodent populations yet. All the samples to date have been retrieved from human victims. But the SNP study that Seifert et al. published earlier this year from samples in Brandenburg, an inhumation from the time of the 30 Years War (1618-1648); the whole genome study that Bos and Herbig et al. also published this year, reporting on the samples from 18th-century Marseille; and the sample from Ellwangen included in the new study by Spyrou et al., all document that Branch 1A (see tree in our previous post) “focalized,” that is, it set up shop in some rodent population(s) and happily continued to proliferate for another 400 years. But all that happened, it is clear, separately from what was going on with Branch 1B, what I have taken to call the pestis secunda.
In their most recent study, the Tübingen/Jena team headed by Krause give us further insight into the early stages of Branch 1B. The beginning of Branch 1B was first documented in the 2011 London study, though it was only earlier this year that I realized that London sample 6330 likely dates from the 1360s and does not come from the initial Black Death outbreak. (It comes from a different burial ground, St Mary Graces.) In Bos and Herbig et al. 2016, it was reported that sample 6330 differed from the 1348-50 London Y. pestis genome by two SNPs. In the present study, interestingly, Spyrou et al. report something slightly different. Sample 6330 does indeed differ from the London Black Death genome by two SNPs (p3 and p4), but a third SNP in sample 6330 they are reporting here for the first time (p5) seems to be unique, a ‘G’ to ‘T’ switch at position 4,301,295 not found in any other historic genome or in the reference strain, CO92. (Spyrou et al. did not include London 6330 on their Table 1, so we offer a modified version of it here in fig. 3.)
First of all, we might say that those three SNPs are significant for the time gap they suggest between the Black Death and the 2nd wave of plague to hit London in 1361-63; for the sake of argument, we’ll say “21 years,” to use your formula (3 x 7 years), Boris. But think about the implications of that: plague arrives in London at the end of 1348 as a new disease, and a new strain (with 3 new SNPs) is causing a major new outbreak in 1361-63, which is when this burial seems to date from. 21 years have not passed since the previous outbreak. So what gives? Obviously, the “time-to-SNP” calculus we’re using is an average, not an absolute. But it does make us stop and wonder: did all this really happen so fast? And did it really happen in western Europe?
Which brings us to the Bolgar City sample. It, clearly, is a “descendant” of the same strain as London 6330: it has the two new SNPs, p3 and p4. (It doesn’t have that unique p5 SNP of London 6330, but that may have arisen as little as three days before this person died. We cannot attach any evolutionary significance to it until we see it documented somewhere else.) But note this: the Bolgar City strain has evolved further. It now has the p6 SNP that will define all the rest of Branch 1B and it has its own unique SNP, p7. And again, we have a problem of time compression: the Bolgar City sample (if we can trust the dating of the coins which were said to have been found with the body) may date as early as the late 1360s.
Remember what we need to have an outbreak in humans from a new lineage of Y. pestis: not simply does that new lineage have to arise from a single change in a single cell of Y. pestis, but that new SNP needs to proliferate enough in a reservoir rodent population to cause a new epidemic in humans. So looking over all these SNPs, p1-p7, we can see that they cluster into two “founder effect” phenomena: one that creates the initial Black Death lineage (Branch 1A) and one that creates the pestis secunda lineage (Branch 1B).
Where did those two lineage foundations happen? Let’s go back to Caffa, the “hurling bodies over the walls” scenario. Clearly, if we can believe that story (and remember, we have only one account of it, and that from a non-eye-witness), it tells of an already proliferating plague outbreak. By October 1346, Y. pestis was multiplying by the millions in rats and mice and rodents of whatever kind that lived in and around Caffa.
One, and only one, of those gazillions of Caffese offspring gave rise to Branch 1B. It, too, needed to find a place to set up shop and proliferate to make gazillions of (nearly) identical copies. And where was that place? Was it (as Krause seemed to imply in his April lecture) in London? Maybe it was in or near Bergen op Zoom (NL), where we find a sample with the same SNP profile as the London 6330 sample (Haensch et al. 2010)? Or was it near the same place where Branch 1A had already established its original home, before it reached the Black Sea? Haensch et al. had already proposed in 2010 a “northern” route for the introduction of the pestis secunda strain that reached the Netherlands. I’ll admit, I was skeptical for the longest time. But now I see that this possibility might bear more analysis. At the very least, the question shifts our focus away from western Europe and back to the areas around the Black and Caspian Seas. And that’s exactly where our Bolgar City sample is from, the one that is already showing two SNPs of further evolution beyond London 6330 but might not be a whole lot younger than it. As we said, jetting out of Heathrow wasn’t yet an option in the 14th century. But there was plenty of activity in these central Eurasian areas dominated by the Mongol Golden Horde to connect lots of rodent reservoirs to a bacterium looking for a new place to call home.
Thanks Monica! The amount of information that follows from a few different nucleotides between aDNA samples is quite amazing, and learning how to interpret this data historically is rightly one of the transformative processes now happening in Biology (and if I say so, in Medical History as well).
Monica’s interpretation of plague’s past mobility is based on the same genetic data as the one sketched out in Spyrou 2016, and highlights the challenge of interpreting ancient DNA, given that the ancient DNA sequences of plague are still so sparsely sampled across time and space. One thing that strikes me as especially important is how much the argument of “favor the most simple, parsimonious explanation” changes based on whether you think of plague largely in terms of a human epidemic (which Wagner 2014, and by extension Spyrou 2016 appear to do), or as a disease that spread through human and wildlife both, as Monica and I do. If you include the possibility of new wildlife reservoirs of plague (and plague has created numerous new wildlife reservoirs in time), say near Bolgar City, the logic of how plague moved across Eurasia changes.
As more aDNA data becomes available, it will be very interesting to see the geographic range that a lineage of plague bacteria can spread without collecting changes in its SNP profile. Once we have a good idea of that, and a more complete view of the SNP profiles that existed during the past pandemics, SNP profiles might be used to shed light on the actual source of a historic plague outbreak, and thus offer an independent way of checking the reliability of historic sources that blame particular smugglers, ships, refugees or clothing as the source of a plague outbreak.
Wow, thanks, Monica, for this great discussion. This is an example on how history and biology can intertwine, and while we are all waiting for more revelations from aDNA and historic sources, it seems prudent to start more interactions between historians and biologists. There is an inherent bias to doubt your own data too much, and trust another fields’ data too blindly, leading to mistakes at both sides: we blindly pick some historic report as authoritative, or put too much faith in a report on the (in-) efficiency of plague transmission by different flea species, whilst a single mutation that causes the loss of a gene can have drastic effects on how well the disease transmits (Hinnebusch, 2016). The only practical way to avoid falling into such pitfalls is by investing in cross-talk between scholars of the humanities and natural sciences!
Thank you, Boris. This was great. And very special thanks to Michelle Ziegler, for hosting our discussion on her super blog, Contagions.
Bos, K. I., Herbig, A., Sahl, J., Waglechner, N., Fourment, M., Forrest, S. A., et al. (2016). Eighteenth century Yersinia pestis genomes reveal the long-term persistence of an historical plague focus. eLife, 5, 17837. http://doi.org/10.7554/eLife.12994
Cui, Y., Yu, C., Yan, Y., Li, D., Li, Y., Jombart, T., et al. (2012). Historical variations in mutation rate in an epidemic pathogen, Yersinia pestis. Proceedings of the National Academy of Sciences, 110(2), 577-582. http://doi.org/10.1073/pnas.1205750110/-/DCSupplemental/sd01.xls
Haensch, S., Bianucci, R., Signoli, M., Rajerison, M., Schultz, M., Kacki, S., et al. (2010). Distinct Clones of Yersinia pestis Caused the Black Death. PLoS Pathogens, 6(10), e1001134. http://doi.org/10.1371/journal.ppat.1001134.t001
Hinnebusch, B. J., Chouikha, I., & Sun, Y.-C. (2016). Ecological Opportunity, Evolution, and the Emergence of Flea-borne Plague. Infection and Immunity, IAI.00188–16–31. http://doi.org/10.1128/IAI.00188-16
Krause, Johannes (4-12-2016) Oral Presentation #S577: Ancient pathogen genomics: what we learn from historic pandemics. European Congress of Clinical Microbiology and Infectious Diseaseshttp://eccmidlive.org/#resources/ancient-pathogen-genomics-what-we-learn-from-historic-pandemics
Seifert, L., Wiechmann, I., Harbeck, M., Thomas, A., Grupe, G., Projahn, M., et al. (2016). Genotyping Yersinia pestis in Historical Plague: Evidence for Long-Term Persistence of Y. pestis in Europe from the 14th to the 17th Century. PLoS ONE, 11(1), e0145194-8. http://doi.org/10.1371/journal.pone.0145194
Spyrou, M. A., Tukhbatova, R. I., Feldman, M., Drath, J., Kacki, S., de Heredia, J. B., et al. (2016). Historical Y. pestisGenomes Reveal the European Black Death as the Source of Ancient and Modern Plague Pandemics. Cell Host and Microbe, 19(6), 874–881. http://doi.org/10.1016/j.chom.2016.05.012
Wagner, D. M., Klunk, J., Harbeck, M., Devault, A., Waglechner, N., Sahl, J. W., et al. (2014). Yersinia pestis and the Plague of Justinian 541–543 AD: a genomic analysis. The Lancet Infectious Diseases, 14(4), 1–8. http://doi.org/10.1016/S1473-3099(13)70323-2
Beyond Germs: Native Depopulation in North America. Edited by Catherine Cameron, Paul Kelton and Alan Swedlund. University of Arizona Press, 2015.
With the number of emerging infectious diseases climbing and new revelations about plague’s past, this book is a timely caution to the rhetoric surrounding so-called virgin soil epidemics. This book is the publication of an interdisciplinary conference held to discuss the causes of Native American depopulation hosted by the Amerind Foundation. Essays by David Jones, George Milner, Clark Spenser Larsen, Debra Martin, Gerardo Gutiérrez, Alan Swedlund, Catherine Cameron, Paul Kelton, Katheleen Hull, and James Brooks are included. Most of these essays are case studies in depopulation of specific areas.
David Jones opens the book with a discussion of the rhetoric surrounding so-called virgin soil epidemics that are credited with being the primary cause of native depopulation. Admitting the influence of his mentor evolutionary biologist Stephan J Gould, Jones favors social and contingent causes for depopulation over biological determinism. Perhaps the influential role that Gould played in my own formation as a biologist makes me sympathetic to Jones’ argument, but I can certainly live with that. Genetic determinism, in my opinion, is the easy way out to explain what can not be yet understood. His argument against the sweeping rhetoric of ‘virgin soil epidemics’ is, I think, very effective.
Milner’s chapter tackles the tricky problem of the pre-contact population collapse of the Mississippian culture centered at the mid-continent site of Cahokia, near St. Louis. This had been the largest and most (archaeologically) complex native culture in North America but it collapsed so entirely that the mid-continent was still sparsely populated when Europeans arrived a couple of centuries later. Medieval Cahokia had been more populous than contemporary London. The example of Cahokia must give us pause for assuming that unrecorded demographic collapses that lack signs of massive destruction must be due to epidemics.
Larsen, Martin, Gutiérrez, and Hull cover the effect of the Spanish mission system in Florida, the Pueblo of the Southwest, Mexico, and California respectively. Structural violence was seen throughout the Spanish mission system from hard, forced labor. Larsen discusses the changed how changed landscapes and lifestyles make natives more susceptible to infection. He sees a dramatic rise in cribra orbitalis/porotic hyperostosis as a sign of iron deficiency when it may actually be malaria. Wet Florida would have been susceptible to endemic malaria and a reservoir for it to spread through the southeast. Martin looks at the bioarchaeological evidence for a ‘creeping genocide’ in the peublos of the south west. Sporatic massacres of pueblo communities was used to enforce compliance by the wider culture. Cultural resilience was also stressed to the breaking point by a prolonged period of drought and environmental deterioration in the southwest that left them with marginal nutritional sufficiency. In this stressed environment, smallpox spread widely among the pueblo communities. Epidemics were part of a set of practices used to destroy or reorient native culture to make it acceptible, and more importantly controllable for the Spanish. Gutiérrez focuses on the methods and effects of identity erasure on demographics. The caste system that developed was very systematic “virtuous cycle” with the goal of eliminating native identiy and indeed native (and African) ‘blood’ while maximizing Spanish identity and ‘blood’.
Chapters by Swedlund, Cameron, and Kelton examine tribes that dealt more closely with the English colonies and early American states. Swedlund looks at the great smallpox epidemic of 1633-34 beyond coastal New England up into the Connecticutt River valley. Cameron reviews the demographic effects of warfare and captive taking had on the Southeast, the northern Pays d’en Haunt (Great Lakes region), and the Southwest tribes. Colonial politics and trade caused more intertribal warfare than warfare directly with European colonists. Kelton writes about the disastrous experiences of the Cherokee with warfare, famine, and disease during the American Revolutionary War.
On of the overarching problems is the difficulty in determining population size before contact and then for the first couple centuries of the colonial period. Problematically, in the past abandoned villages have been assumed to be extinct due to disease rather than simply relocation or the movement of refugees to other tribes. The reality is that many areas that are fertile with Old World methods and domestic livestock were very difficult to make productive with native resources.
Over arching themes that I noticed which point toward other factors than just “germs” causing depopulation:
- Use of starvation as an intentional weapon accomplished by burning fields and disrupting the agricultural cycle.
- Selective taking of women of reproductive age as captives/slaves causing a gender imbalance that prevented populations from rebounding.
- Selling captives to slavers rather than incorporating them into the tribe as pre-contact tribes often did to bolster their numbers and replace their dead. Sales were often to repay debts for European trade goods, especially weapons and ammunition.
- Use of tribes as proxy militias by European powers to create intertribal warfare and recruitment of tribes by the British during the American revolution and war of 1812. Set up an adversarial relationship with the young American nation.
One of the things I took away from this collection is an appreciation for how long it takes to develop a fully agricultural culture. Many Native American groups were still in transition. Lacking domestic animals other than the dog, they were very vulnerable to climate and social disorder disrupting their agricultural cycle and yield. Some tribes adopted domestic animals from Europeans quickly. Colonists were greatly alarmed at how quickly the Cherokee adopted raising hogs and European crops, bringing them nutritional stability. Of course, horses are were adopted so well by natives that its hard for many of us today to even think of Native Americans without them. The idea that domestic animals were worth the effort may have been what was missing most, rather than a lack of animals capable of being domesticated.
I highly recommend this book for anyone interested the dynamics of depopulation, “dark ages”, and most importantly for comparison to other “virgin soil” epidemic situations. I don’t think it’s a coincidence that other renowned virgin soil epidemics like the first and second plague pandemics occurred in times of ecological and social stress in addition to the epidemic in question. By opening the explanations for Native American depopulation up to other causes than disease, it also opens up views into colonial life that even contemporaries tried to ignore. Wether their demise to disease was believed by contemporaries to be divine will or by modern historians as biological determinism, it has diverted attention away from the very human causes of depopulation and in some cases genocide.