The evolution of Yersinia pestis and the origin of bubonic plague

The question of the historical background and origin of a mind-stretching epidemic disaster such as the Black Death has fascinated scholars and amateur historians alike and inspired a great variety of suggestions or assertions. This question of origin is put much too simply and really contains several questions: (1) where and (2) when did the evolution of the bacterium of plague disease, Yersina pestis, begin? (3) From which microbe did it evolve? (4) When did the evolutionary process produce the original version of the bacterium of bubonic plague, and (5) where did plague originate as a human bubonic disease? Lastly and centrally in this book: (6) where did the strain of the Black Death evolve? (7) Where did the Black Death first break out in epidemic form and what circumstances allowed it to translate into the largest and most deadly pandemic in the history of human disease?

Plague disease does not leave any mark on the human skeleton. Only a couple of decades ago, the study of the history of plague and plague epidemics of the past was entirely dependent on historical sources. This was still the case when the part on the history of plague before the Black Death in the first edition was written around 2000. Since then, the new scientific discipline of paleobiology has arisen (see above) and provided much valuable new information also on these matters.

It was early shown that Yersinia pestis is a clone evolved from another bacterium of the same genus, Yersinia pseudotuberculosis. Also the progenitor of plague is pathogenic, that is, it causes disease but it mainly infects humans by contaminated food and water, displays a mainly different clinical panorama, and causes low mortality.

A comprehensive genetic material of specimens or strains of Y. pestis from many parts of the world had been stored and accumulated over a long period in various research laboratories on infectious diseases, much of it before paleobiology arose and turned it into the source material for Yersinia pestis’ evolutionary history. It has provided the opportunity to outline the evolutionary history of the plague bacterium, to construct a phylogenetic tree that shows how the various strains with their specific mutational characteristics are linked and form branches relating to various parts of the Old World.