It's hard to comprehend the impact of the Black Death. The "Great Pestilence" is believed to have originated somewhere in Northern Asia in the 1330s before hitting Europe in 1347. It killed an estimated 75 million people worldwide - that's around 25 per cent of all humans in existence at the time. Now in an effort to better understand modern infectious diseases, scientist have sequenced the entire genome of the Black Death.

The research is based on analysis of skeletal remains from the East Smithfield "plague pits" in London using a previously developed methodology for extracting degraded DNA fragments of the bacteria that caused the plague - Yersinia pestis.

The international team, led by researchers from McMaster University and the University of Tubingen, say this is the first time a reconstructed genome of any ancient pathogen has been drafted.

So why is this relevant in the 21st Century?

Descendants of the specific variant of the Yersinia pestis found in the 14th Century remain exist today, killing an estimated 2,000 people a year. By drafting the reconstruction of the genome scientist are able to see changes in the pathogen's evolution - which in this case have been minor - and gain a better understanding of such deadly infections.

"The genomic data show that this bacterial strain, or variant, is the ancestor of all modern plagues we have today worldwide," says Hendrik Poinar, associate professor and director of the McMaster Ancient DNA Centre. "Every outbreak across the globe today stems from a descendant of the medieval plague. With a better understanding of the evolution of this deadly pathogen, we are entering a new era of research into infectious disease."

There are still questions that remain unanswered about why the Black Death was so devastating however.

"We found that in 660 years of evolution as a human pathogen, there have been relatively few changes in the genome of the ancient organism, but those changes, however small, may or may not account for the noted increased virulence of the bug that ravaged Europe," says Poinar. "The next step is to determine why this was so deadly."

In other words, the research may have knocked out one of the main contenders in establishing why the Black Death had such a deadly impact. The Paper states that because the Black Death may not have been due to bacterial phenotype, "factors other than microbial genetics, such as environment, vector dynamics and host susceptibility, should be at the forefront of epidemiological discussions regarding emerging Y. pestis infections."

It has also been shown that the variation of Yersinia pestis that caused the medieval plague originated sometime around the 12th Century, which means that it was not the cause of the Justinian plague that struck the Byzantine Empire around 540 CE was not caused by the same pathogen as once thought.

"Using the same methodology, it should now be possible to study the genomes of all sorts of historic pathogens," says Johannes Krause of the University of Tubingen Krause. "This will provide us with direct insights into the evolution of human pathogens and historical pandemics."

The research is published online in the scientific journal Nature

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