Researchers think an ancient flea preserved in amber with tiny bacteria could be the oldest evidence of an ancient ancestor of the bubonic plague, as per a report of the Journal of Medical Entomology. The flea in amber could be about 20 million years old.

According to the researchers, the bacterium is a kind of Yersinia pestis, a facultative anaerobic bacterium that can infect animals and humans. Yersinia pestis had caused the bubonic plague in the past, according to historians. The disease that spread in 14th century had killed more than 30 million people in Europe.

The researchers did not identify the bacterium in the preserved flea, but after studying the flea’s size, shape and location, they believed that it is linked to deadly Yersinia pestis. The amber, where the ancient flea was present, was found in Dominican Republic’s amber mines. There was a time when the Dominican Republic was a dense tropical forest. The researchers predicted that the flea was likely trapped in amber after feeding on an infected rodent.

The researchers used advanced imaging analysis and found that the strands of the coccobacillus bacteria were made up of a mix of rod and semi-spherical shapes.

George Poinar, Jr, an entomology researcher in the College of Science at Oregon State University, said, “Aside from physical characteristics of the fossil bacteria that are similar to plague bacteria, their location in the rectum of the flea is known to occur in modern plague bacteria”.

"If this is an ancient strain of Yersinia, it would be extraordinary," Poinar said. "It would show that plague is actually an ancient disease that no doubt was infecting and possibly causing some extinction of animals long before any humans existed. Plague may have played a larger role in the past than we imagined."