Tiny bacteria found on a flea that became trapped in amber 20 million years ago could be the most ancient evidence ever found of the historic killer the Black Death — bubonic plague.

If further investigation confirms the bacteria's relation to the plague bacteria — Yersina pestis — it would mean the disease that descended on humanity in the 14th century and wiped out a third of Europe's population had already been in existence in one form or another for millions of years, a researcher says.

Samples of the bacteria were found on the flea's proboscis, its sucking mouthparts, and in its rectum, suggesting it was consuming and then excreting or possibly regurgitating the bacteria.

"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," says George Poinar, an entomology researcher at Oregon State University. "And in this fossil, the presence of similar bacteria in a dried droplet on the proboscis of the flea is consistent with the method of transmission [regurgitation] of plague bacteria by modern fleas."

The flea in its surrounding amber was found in mines in what is known today as the Dominican Republic, a place that was likely a tropical forest millions of years in the past.

Poinar has authored a study on the flea and the bacteria published in the Journal of Medical Entomology.

The findings suggest the bacteria existed for millions of years, has shown up in many parts of the world and possibly predates the existence of the human race, Poinar says.

The study brings into question modern genomic investigations that have suggested the flea-plague-vertebrate link that inflicted the Black Death on the world evolved just in the last 20,000 years, not millions.

However, Poinar points out, Yersina pestis has existed as several different and distinct strains, several of which are extinct at this point in time.

While strains capable of infecting humans may be more recent, ancient strains of Yersinia that appeared as rodent-afflicting parasites — possibly like those on the flea in amber — could have evolved long before humans, he says.

Such an ancient strain of Yersina would be extraordinary, he notes. "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."

It is evidence that plague may have had a larger role in the past than had been thought, he says.