Black Death Is 'Grandmother' of All Modern Plague

Black Death victims, genome of black death bacteria sequenced — Dental pulp taken from Black Death victims, like these, excavated from the East Smithfield cemetery in London, allowed scientists to sequence the genome of the bacterium that is believed to have killed them about 660 years ago.

(Image credit: Museum of London)

The bacterium blamed for the Black Death that wiped out more than a third of Europe's population within about five years of the 14th century looks an awful lot like the modern versions of the plague-causing bug, new genetic research indicates.

By taking the now-powdery black pulp out of the teeth of plague victims buried in London's East Smithfield Cemetery — a cemetery established solely to handle the onslaught of the Black Death once it arrived in the city in 1348 — researchers have managed to reconstruct the entire genetic blueprint, or genome, of the bacterium blamed for the devastation.

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This doesn't fit well with earlier work published this September in the journal the Proceedings of the National Academy of Sciences by many of the same researchers, who reported unique mutations in the medieval DNA. However, that was a limited study; they had yet to sequence the entire Black Death genome. These changes did not show up in the more-recent work and were likely the results of damage to the DNA sequence, a problem the researchers corrected for this time, according to Krause.

In the more recent paper, Krause and the others constructed a family tree based on variations seen in the medieval bacteria and many modern ones, and placed the medieval sequences nearly at the root of the tree. They also calculated that all human-plague causing bacteria diverged from a common ancestor between 1282 and 1343, making the Black Death, which arrived in Europe in 1347, the original plague pandemic.

"There is always some uncertainty in the calculations because evolution is not constant, there might be spikes in evolution during a human (plague) outbreak and other periods in time when plague resides at a lower evolutionary rate in its natural rodent host population," said Eppinger, who is a researcher at the Institute for Genomic Sciences at the University of Maryland School of Medicine. [10 Deadly Diseases That Hopped Across Species]

This newly sequenced genome will allow scientists to study the evolution of the plague bacteria by looking at the ancient killer itself. All studies of the Black Death have had to rely on snippets of ancient DNA and modern strains no more than a few decades old, Eppinger said. "But this time, it is really the blueprint of the ancestral genome that is actually responsible for the deaths of people back then," he said.

Instead, the team proposes that circumstances played out just right to allow the disease to explode through the population. The genetic data indicate that Europeans had never encountered it before, so they had no immunological or behavioral adaptations (such as quarantine). Tree-ring data indicates the beginning of a cold, wet period, which would have hurt food supplies, Poinar said during a press conference Tuesday (Oct. 11). A hungry population would be more vulnerable to illness.

The discrepancies between modern plague and the Black Death have led some to suggest that other pathogens were responsible, including an Ebola-like hemorrhagic virus. While there is no physical evidence so far of another pathogen, it's possible that just such a virus joined forces with Y. pestis to decimate medieval Europe, according to Sharon DeWitte, one of the researchers and an assistant professor of anthropology at the University of South Carolina.

Wynne was a reporter at The Stamford Advocate. She has interned at Discover magazine and has freelanced for The New York Times and Scientific American's web site. She has a masters in journalism from Columbia University and a bachelor's degree in biology from the University of Utah.