Scientists found evidence of Yersinia pestis (more commonly referred to as Y. pestis) in the teeth of human skulls dating back to the Bronze Age. Y. pestis is the microbe that is responsible for the plague.

There have been many plagues over time and each time a plague fell over the land, it was catastrophic.

Going all the way back to the year 541, there was the Plague of Justinian.

The Plague of Justinian caused over 25 million deaths over roughly two hundred years, according to the Center for Disease Control (CDC).

In 1334, the most well-known plague outbreak began: the Black Death. Around 60 percent of the European population was completely wiped out, though that version of the plague did originate in China.

The next outbreak also began in China and occurred in the 1850s. It lasted until the end of the century and took over 10 million lives.

It’s easy to think of the plague as a disease of the past, but Y. pestis is still causing episodes of the plague to appear. Most new cases of the plague are reported from sub-Saharan Africa and Madagascar and the most recent severe outbreak was seen in the 1970s, during the Vietnam War.

Scientists have believed that Y. pestis was transmitted to victims of some of the more well-known plagues through fleas. In fact, it is a common belief. The question asked by the authors of the study “Early Divergent Strains of Yersinia pestis in Eurasia 5,000 Years Ago” is not how the microbe reacted in known history, but what it was like before that.

The genetic study was published Thursday in the research journal Cell. As part of the study, researchers tested samples of teeth from 101 Bronze Age human remains that were found throughout Europe and Asia. Seven of the humans tested were found to have been infected with Y. pestis.

The Bronze Age skulls were found to be from anywhere between 2794 BCE and 951 BCE, and finding Y. pestis among their remains was significant. It means that humans were being infected by the plague long before scientists originally believed. In fact, it means that humans have been dealing with Y. pestis for more than 3,000 years longer than we thought, and there was no way it was always transmitted by fleas.

According to Discovery, the form of Y. pestis found in the Bronze Age teeth lacked a gene that would allow it to exist inside the flea.

“Bronze Age Y. pestis lacked a gene called Yersinia murine toxin (ymt). Ymt allows the bacteria to survive in the gut of a flea, a critical disease vector. Beginning between 3,700 and 3,000 years ago, this gene turns up in the DNA of Iron Age individuals, which suggests human transmission was possible in that era.”

The study’s authors do not believe this means that scientists have been wrong about what caused the frighteningly quick spread of the plague during the time of Black Death. Instead, they believe that the plague that could be spread through fleas is a plague that evolved from what was found in the Bronze Age remains.

“Our findings suggest that the virulent, flea-borne Y. pestis strain that caused the historic bubonic plague pandemics evolved from a less pathogenic Y. pestis lineage infecting human populations long before recorded evidence of plague outbreaks,” wrote the authors of the study.

The researchers that were involved with the study are unsure of just how deadly Y. pestis was during the Bronze Age. Nevertheless they are hopeful that they will be able to use the information learned to figure out how pathogens, like the plague, evolve.

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