The strain of bacterium analyzed from the bones and teeth of a Hereford plague pit dug in 1349 is identical to that from a plague pit of 1348 in southern France, suggesting a direct route of travel. But a plague pit in the Dutch town of Bergen op Zoom has bacteria of a different strain, which the researchers infer arrived from Norway.
The Black Death is the middle of three great waves of plague that have hit in historical times. The first appeared in the 6th century during the reign of the Byzantine emperor Justinian, reaching his capital, Constantinople, on grain ships from Egypt. The Justinian plague, as historians call it, 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 third great wave of plague began in China’s Yunnan province in 1894, emerged in Hong Kong and then spread via shipping routes throughout the world. It reached the United States through a plague ship from Hong Kong that docked at Hawaii, where plague broke out in December 1899, and then San Francisco, whose plague epidemic began in March 1900.
The three plague waves have now been tied together in common family tree by a team of medical geneticists led by Mark Achtman of University College Cork in Ireland. By looking at genetic variations in living strains of Yersinia pestis, Dr. Achtman’s team has reconstructed a family tree of the bacterium. By counting the number of genetic changes, which clock up at a generally steady rate, they have dated the branch points of the tree, which enables the major branches to be correlated with historical events.
In the issue of Nature Genetics published online Sunday, they conclude that all three of the great waves of plague originated from China, where the root of their tree is situated. Plague would have reached Europe across the Silk Road, they say. An epidemic of plague that reached East Africa was probably spread by the voyages of the Chinese admiral Zheng He who led a fleet of 300 ships to Africa in 1409.