RENEE MONTAGNE, HOST:
It was more than 600 years ago that the Black Death swept across Europe. Over some five years, millions of people died from the plague, wiping out at least a third of the population. Scientists have long wondered what pathogen caused this deadly outbreak and why it was so virulent. NPR's Nell Greenfieldboyce reports that researchers are now finding the answers to these questions hidden in the DNA of the plague's victims.
NELL GREENFIELDBOYCE, BYLINE: In 1348, after the Black Death reached London, bodies got piled in special cemeteries. Scientists later collected some of the skeletal remains, including teeth. Hendrik Poinar is a biological anthropologist at McMaster University in Canada. He explains that these teeth are like time capsules.
HENDRIK POINAR: Teeth are a great sort of conservator of material because the enamel is such a great protective coating.
GREENFIELDBOYCE: Inside there's the remains of blood and pulp and DNA.
POINAR: That extract that comes from these teeth and these bones that have been buried in these burial pits for roughly 700 years are a mixture, a complex mixture of the DNA of the human, the DNA of the pathogens that that human harbored, the DNA of any bacteria that were symbiotic to the human.
GREENFIELDBOYCE: So it's no easy task to find the DNA from the Black Death pathogen. Most experts blame the plague bacteria Yersinia Pestis, which is still around today. But previous attempts to find its DNA in victim's teeth have had mixed results. In this latest study, researchers used a new and very sensitive technique. Poinar says they took bits of modern plague DNA and used them as a kind of magnet to pull ancient plague DNA out of the tooth extract.
POINAR: And that's the way we actually could fish for a needle in the haystack, so to speak.
GREENFIELDBOYCE: In the journal Nature, he and his colleagues report that not only did they detect DNA from the Black Death bacteria; they figured out its entire genetic code.
POINAR: Really, this is the first ancient pathogen genome isolated from skeletonized remains.
GREENFIELDBOYCE: Poinar says it's very similar to modern strains of plague and looks like their common ancestor.
POINAR: So there is no smoking gun, so to speak, to say, a-ha, we've found the one mutation which caused this tremendous virulence and now we know why it killed fifty million people.
GREENFIELDBOYCE: He says it probably wasn't the make-up of the germ that made it so deadly, but rather the fact that it emerged for the first time right when Europe was dealing with a cooling climate, poor crops, and crowded, filthy conditions.
The new study has impressed Thomas Gilbert of the Natural History Museum of Denmark, who's tried to get plague DNA out of victims' teeth in the past.
THOMAS GILBERT: I mean it's great results. It looks very, very convincing. There's no reason why the data shouldn't be real.
GREENFIELDBOYCE: Gilbert does question the report's conclusion that the medieval Black Death was the first time plague infected humans. If so, it couldn't have caused the Justinian Plague. That's another famous epidemic blamed on this bacteria that devastated the Eastern Roman Empire in the 6th century.
GILBERT: But really, you know, the only way to find out what caused the Justinian Plague is to do the same analyses on the Justinian samples. And I guess that's going to be the next attempt.
GREENFIELDBOYCE: And he says besides looking at plague, other researchers have already started to use these same new genetic techniques to unravel the history of other important diseases, like tuberculosis.
Nell Greenfieldboyce. NPR News.