The Black Death is arguably one of the most evocative events in human history: the mere mention of this plague conjures up images of widespread death and the inexorable spread of disease. Indeed, estimates of the human cost of the seven-year-long 14th-century epidemic range from 75 to 200 million deaths. The bacterium responsible, Yersinia pestis, has undergone evolution as every other organism on Earth has, and a new study published in the Journal of Medical Entomology has revealed details of its potential ancient ancestor.
A single flea, approximately 20 million years ago, managed to get itself trapped in amber, preserving with it several examples of bacteria. Although the bacteria cannot be extracted from the amber and subjected to genetic testing to confirm its evolutionary link to modern day Y. pestis, the researchers, led by entomologist Dr George Poinar, Jr., noticed several similarities. Firstly, the bacteria were attached to the proboscis (feeding organ) and rectum of the flea – two areas the contemporary equivalent is often found. Secondly, their physical characteristics, including their size and shape, closely resemble that of the modern Y. pestis. Lastly, they are both the same type of bacteria: coccobacillus.
If it turns out the bacteria are indeed ancestral to the contemporary bubonic plague, then remarkably this would mean that they predate the emergence of Homo sapiens – our own species – by at least 19 million years. Although the strain of bacteria infecting modern humans may have only evolved around 20,000 years ago, far more ancient strains of the pathogen could have evolved to exist within other animals, including rodents – the creatures originally thought to have brought the plague to the shores of Europe in the 14th century.
The flea itself is one of very few preserved in amber and has thus undergone very limited biological degradation. Consequently, not only are the ancient bacteria preserved within the amber prison of the flea, but their method of transmission from host to host is also possibly preserved.
After feeding off a plague-infected animal, Y. pestis forms a sticky mass within part of the flea’s stomach, the proventriculus. This blockage reduces the flow of sucked-up blood into the flea’s stomach, and they react by finding another animal to feed on. As their proboscis is inserted into another animal, the bacterial colony is often evacuated into the new host’s circulatory system. The dried droplets found at the tip of the fossilized ancient flea’s proboscis could indeed be the same sticky bacterial mass.
Although no humans were around 20 million years ago to be infected by this potential Y. pestis relative, there were plenty of other ancient organisms around. The amber-encapsulated flea specimen was found in a mine within the Dominican Republic, which millions of years ago was a tropical, moist forest home to rodents, birds and reptiles. If an ancient ancestor of Y. pestis did exist within this 20 million-year-old ecosystem, then it may have played a greater role in influencing it than previously thought, driving the extinction of specific species.
With four cases of the human plague diagnosed in the United States this year, the sheer longevity of the group of bacteria responsible is clear to see; an evolutionary branch of death, tens of millions of years in the making.