The three plague waves of history, the 6th century plague that reached Constantinople during the reign of Justinian and is thought to have killed perhaps half the population of Europe and to have eased the Arab takeover of Byzantine provinces in the Near East and Africa, the “Black Death” that began in Europe in 1347 and carried off an estimated 30 percent or more of the population, and the Manchurian Pneumonic Plague epidemic that reached Hong Kong in 1894, and from there San Francisco in March 1900, have been tied in a common family tree by a team of medical geneticists led by Mark Achtman of University College Cork in Ireland, as reported by the New York Times.
By looking at variations in living strains of “Yersinia pestis”, the bacterium first identified by the Swiss and French Pasteur bacteriologist Alexandre Yersin in Hong Kong in 1894 in his study of the bubonic plague epidemic there, the team has reconstructed a family tree of the bacterium and dated the branch points of the tree, correlating the major branches with historical events.
They conclude that all three of the great waves originated in China, where the root of their tree is situated. Plague would have reached Europe across the Silk Road. But then apparently it separated in two branches according to a team of biologists led by Barbara Bramanti of the Institut Pasteur in Paris and Stephanie Haensch of Johannes Gutenberg University in Germany. Analyzing ancient DNA and proteins from plague pits, the mass burial grounds across Europe were plague victims were interred, they have been able to distinguish two strains of the bacterium and they inferred that medieval Europe has been invaded by two different sources of Yersinia pestis, one reaching the port of Marseilles in 1347 and progressing rapidly north, and the other which arrived from Norway into Bergen op Zoom in the Netherlands.
These epidemic waves had momentous political and economic consequences: the Justinian plague arguably weakened in a decisive manner an already over-extended Byzantine empire, reducing its defense capacity by curtailing the number of soldiers it could mobilize against Eastern conquerors as well as its tax revenues, also opening the way to several major “barbarian invasions” of what was left of the Roman empire, while the Black Death radically changed the land/labor ratio in Western Europe, raising agricultural productivity and consequently wage rates, and stimulating the commercial revolution of the end of the Middle Ages, events beautifully analyzed by William McNeill in his pioneering and now classic book of 1976, Plagues and Peoples.
These new discoveries also provide and excellent example of the trend towards a “consilience” of various sciences (here biology, geography, economics and history) that McNeill first envisioned.