The Black Death was once just a stomach bug- how did it evolve?

The great plague wiped out nearly half of Europe’s population in medieval times. The bacterium behind the plague is widely regarded as one of the worst killers in history. Recently, new research into the diseases that afflicted ancient peoples suggests that humanity’s great plague has been tormenting us for some 3,300 years longer than previously thought.

Current evidence shows that the earliest appearance of the plague was around 540 AD. But Danish researchers now claim that the plague first appeared in 2,800 BC.

“We were very surprised to find it 3,000 years before it was supposed to exist,” said Simon Rasmussen of the Technical University of Denmark, one of the study’s authors

Hoping to discover how deadly diseases evolve, scientists drilled into the teeth of over 100 skeletons from people who lived in Central Asia and Europe 2,800-5,000 years ago. In particular, the scientists were seeking evidence of bacterial infections during these times. Seven bodies produced viable DNA samples that showed they suffered from plague-causing bacteria.

The bacterium known as Yersinia pestis was responsible not only for the Black Death of the 14th century that killed 30 to 50 percent of Europe’s population but also the Plague of Justinian in 541 AD that killed around 100 million and a third pandemic that affected China from the 1850s all the way up until the mid-20th century. It’s ancestor, Y. pseudotuberculosis may have been responsible for even more deaths.

“The plague bacterium is very similar to its ancestor, the bacterium Y. pseudotuberculosis,” said Rasmussen. “But that ancestor isn’t so harmful. If you eat it, you’ll feel bad, but you don’t die from it. So how could a bacterium that’s harmful but not deadly evolve into one of the most deadly that’s ever existed for humans?”

According to the paper published in the journal Cell, sometime during the Bronze Age,Y. pseudotuberculosis evolved a way to survive on fleas, those increasing its ability to spread throughout human populations and ultimately becoming Y. pestis.

“Here we’re able to directly identify when this happened, when this bacteria went from not being able to live in fleas to this very important part of its lifestyle. That tells us a lot about how a pathogenic bacteria evolves into becoming even more dangerous,” said Rasmussen.

“These Bronze Age strains couldn’t cause bubonic plague, but they caused septicemic plague in the blood and pneumonic plague in the lungs, which you can transmit through the air whenever you sneeze or cough.”

If Rasmussen is correct, a number of histories mysterious may be solved. For instance, the Plague of Athens, an epidemic that killed around 100,000 people in 430 BC, may very well have been an early strain of the plague that eventually caused the Black Death.

“People have been speculating about what this was, like was this measles or typhus, but it could well have been plague,” said Rasmussen.

Rasmussen’s team is hoping to learn how the plague developed from an intestinal infection to one of the worst diseases of all time. If the plague’s evolution can be charted, it may shed a light on the development of other deadly illnesses.

“We continue to learn that small genetic changes can have huge impact on human health and disease,” said Wyndham Lathem, a microbiologist at Northwestern University Feinberg School of Medicine. “We have here a bacteria that was able to infect humans much earlier than we thought, but it was missing a couple of key factors that might have restricted its spread to smaller populations. But then it just picks up one gene and a couple of mutations and suddenly we’ve gone from small local outbreaks to global pandemic infections.”

“It may be that some disease, Ebola for example, could acquire a new gene or piece of DNA and then be spread through the air rather than just through bodily fluids. In that kind of example, we’d then have a much bigger problem on our hands.”

Fortunately, today the bacterial infection caused byY. pestis is easily treated.

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