The Four Black Deaths

Abstract The Black Death, often called the largest pandemic in human history, is conventionally defined as the massive plague outbreak of 1346 to 1353 c.e. that struck the Black Sea and Mediterranean, extended into the Middle East, North Africa, and western Europe, and killed as much as half the total population of those regions. Yet genetic approaches to plague’s history have established that Yersinia pestis, the causative organism of plague, suddenly diverged in Central Asia at some point before the Black Death, splitting into four new branches—a divergence geneticists have called the “Big Bang.” Drawing on a “biological archive” of genetic evidence, I trace the bacterial descendants of the Big Bang proliferation, comparing that data to historical human activities in and around the area of plague’s emergence. The Mongols, whose empire emerged in 1206, unwittingly moved plague through Central Eurasia in the thirteenth, not the fourteenth, century. Grain shipments that the Mongols brought with them to several sieges, including the siege of Baghdad, were the most likely mechanism of transmission. The fourteenth century plague outbreaks represent local spillover events out of the new plague reservoirs seeded by the military campaigns of the thirteenth century.