Researchers trace the roots of Europe's Black Death plague

— -- The Black Death of the medieval era, far from being just a more dangerous form of an already-occurring disease, was a newly evolved variant of a harmless bacteria that quickly began its death march across the globe.

That's the finding of researchers who have sequenced the genome, or genetic blueprint, of the plague bacteria and found that the strain arose no more than 140 years before the Bubonic Plague's start in 1347. The plague killed an estimated 50 million people in Europe in just five years.

Even more surprising, the bacteria, or Yersinia pestis, of that era don't appear to be that much more virulent than the plague of today. This leads the scientists, writing in Thursday's edition of the journal Nature, to believe that climate change played a role, too.

Europe was in the midst of a cooling trend called the Little Ice Age, which lowered temperatures and harvests and increased rains. People were ill-nourished and weak while rats flourished.

"You had an immuno-compromised population living in London under stressful conditions who were then hit with a new pathogen," says co-author Hendrik Poinar, an evolutionary geneticist at McMaster University in Canada.

Researchers were able to extract DNA from the teeth of four skeletons excavated at a cemetery in London where more than 2,500 people were buried during the plague years. They were able to reconstruct about 99% of the plague's genome.

The plague evolved from a soil-dwelling bacteria that acquired extra genetic material, which bacteria sometimes do, and became a killing machine.

It originated in Mongolia, infecting fleas that lived on rats and quickly came into contact with a nomadic population called the Golden Horde, which was sweeping along the Silk Road trading route. They brought the plague with them to Europe, where it rapidly spread, says Johannes Krause, one of the paper's authors and a paleo-archeologist at the University of Tübingen in Germany.

The lessons for today are two-fold, he says. New diseases tend to cross over when humans move into new areas and come into contact with animals and pathogens with which they previously didn't interact. And climate change is likely to cause movement of both people and animals.

The good news is that "we have very good medical facilities and lots of researchers that can deal with this much better than they could in 1348," Poinar says. Though the plague still exists today, killing an estimated 3,000 people a year mostly in the developing world, it is easily treated with antibiotics, Poinar says.

Charles Chiu, who directs the Viral Diagnostics Center at the University of California, San Francisco, says it's interesting that the Black Death wasn't simply a super-dangerous variant of many versions of the plague that were circulating among humans. Instead, the first time it appeared it killed close to half the Europeans then alive.

Knowing that the Black Death was a newly evolved bacterium when it began its spread reminds us that we need to be wary, he says. "We know this is going to happen. Some bacteria will mutate," and humans will face something we have no immunity to.