The Black Death Bacterium Decoded

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Museum of London Archaeology

Wresting DNA from the 663-year-old teeth of victims of the Black Death, researchers have reconstructed the genome of the bacterium that originated in Asia and wiped out up to half of western Europe’s population. It’s the first time that scientists have successfully pieced together the genome of an ancient microbe from skeletal remains.

Surprisingly, the researchers report in the journal Nature, the DNA of the microbe — Yersinia pestis — has changed very little since the mid-14th century, when it decimated the populations of London, Paris, Marseilles, Barcelona and other cities. The researchers also believe that Y. pestis was a newly evolved pathogen at the time, having arisen from a harmless dirt-dwelling bacterium perhaps just 140 years before it wreaked havoc in Europe. Modern forms of the plague-causing microbe, however, don’t appear to be that virulent, which has led some scientists to believe that Y. pestis wasn’t the bug responsible for the Black Death at all.

The new report resolves that debate, confirming that the bacterium was indeed circulating during the Black Death, but it still doesn’t answer the question of why the bug proved so devastating in medieval populations. The authors think it may have had at least as much to do with the environment at the time as the microbe itself. Europe was then going through a cooling period known as the Little Ice Age, which reduced harvests and led to heavy rains that drowned out crops, in turn leading to famine and malnutrition. Economic downturns left large swaths of the population in poverty and squalor. The financial strains, the lack of public hygiene and sanitation, and an already weakened population made for ripe ground for a contagious and pathogenic bug like Y. pestis to flourish.

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“You had an immune-compromised population living in London under stressful conditions who were then hit with a new pathogen,” Hendrik Poinar, a co-author of the paper and an evolutionary geneticist at McMaster University, told USA Today.

The DNA of the ancient bacterium was extracted from the teeth of four skeletons buried in the East Smithfield cemetery in London, which had been specifically designated as a resting place for victims of the plague and was excavated by scientists at the Museum of London Archeology. The microbe’s DNA was fragmented and blended in an amalgam of genetic material that included the hosts’ genomes as well as the genes of bacteria that had settled in to degrade the bodies, which makes the extrication of the Y. pestis genome that much more of a technological achievement. The scientists used probes made of DNA from modern strains of the bug — knowing that complementary DNA would bind to it — to pull out nearly 99% of the bacterium’s genome.

The fact that Y. pestis was able to kill so effectively in what was essentially its debut in human hosts should serve as a warning about how virulent new pathogens can be, say the authors. Scientists had previously thought that because it was so deadly, the Black Death’s pathogen was a superstrain, a bug that had circulated and mutated repeatedly before hitting upon a particularly devastating form. The current analysis suggests that wasn’t the case.

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But fortunately, when it comes to plague, we’re in a better position now to confront Y. pestis than medieval populations were. Our public health systems are able to detect outbreaks earlier, before they pick up momentum, and antibiotics can stop the bacterial infection even after it takes hold. So it’s not likely that we’ll see another outbreak of Y. pestis like the one that occurred in the 1340s. But that doesn’t mean we won’t see a modern iteration of the Black Death triggered by an entirely different but equally virulent bug. That possibility always exists.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.