Three times the plague has appeared in deadly force. And all three times, scientists have found, the disease-bearing bacteria originated in China and spread across the world through different routes.
The plague’s most famous appearance came as the Black Death in 14th century Europe, when it wiped out nearly a third of the population. But it also struck as the Justinian Plague in the Byzantine Empire of the 6th century, and a less severe outbreak spread around the world and reached the American mainland in 1900 (see map above). This week in the journal Nature Genetics, Mark Achtman and colleagues rebuilt the evolutionary history of the bacterium Yersinia pestis, the cause of bubonic plague, and traced all three major waves of plague back to a starting point in China.
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. [The New York Times]