In an ancient grave in Sweden, scientists have unearthed the oldest known strain of a deadly bacteria that has killed millions of people over thousands of years.
They call it Yersinia pestis. You may know it as the plague.
The new discovery suggests that the microscopic bacteria has been wiping out great swaths of the human population for more than 5,000 years -- destroying empires, spurring political uprisings and leaving a permanent mark on regional gene pools.
"What we found in the Swedish grave site is not only the oldest sample of the Y. pestis genome but also the oldest version of the genome," said Simon Rasmussen, a metagenomics researcher at the Technical University of Denmark, who led the work. "Think of it as the root of the tree."
The oldest recorded plague pandemic, known as Justinian's Plague, dates to 541 AD. Over the course of 200 years, it killed more than 25 million people across the Byzantine Empire, hitting the capital city of Constantinople especially hard.
The next major plague pandemic, known as the Black Death or the Great Plague, started in China in 1334 and spread along trade routes to Constantinople before reaching Europe in the 1340s. It also claimed the lives of an estimated 25 million people, including about half the population of Europe, according to researchers at the Centers for Disease Control and Prevention. Some towns weren't left with enough survivors to bury the dead.
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The third major outbreak, known as the Modern Plague, took root in China in the 1860s. It popped up again in Hong Kong in 1894 and spread to port cities around the world over the next 20 years, carried by stowaway rats on steamships. It was during this pandemic that scientists discovered the bacterial source of the disease, and that it is spread by fleas that pick it up from rats and pass it on to humans. Even with that knowledge, however, plague still managed to cause 10 million deaths.
Rat-associated plague can still be found in populations of ground squirrels and other small mammals in the Americas, Africa and Asia. It is now under control in most urban areas across the globe, and if it's caught early enough, it can be treated with antibiotics. From 2010 to 2015, there were 3,248 human cases of plague reported worldwide and 584 deaths from the disease, according to the World Health Organization.
The new study, published Thursday in the journal Cell, reveals that the relationship between humans and plague goes back even further than scientists had realized. The bacteria identified by Rasmussen and his colleagues may represent a previously unknown outbreak of plague that struck Europe as much as 5,700 years ago.
The researchers already knew that the population of Europe plummeted 5,000 to 6,000 years ago, after it had grown for thousands of years. This sudden plunge is known as the Neolithic decline, and its cause is still up for debate.