In Keys's startling thesis, a global climatic catastrophe in A.D. 535-536--a massive volcanic eruption sundering Java from Sumatra--was the decisive factor that transformed the ancient world into the medieval, or as Keys prefers to call it, the "proto-modern" era. Ancient chroniclers record a disaster in that year that blotted out the sun for months, causing famine, droughts, floods, storms and bubonic plague. Keys, archeology correspondent for the London Independent, uses tree-ring samples, analysis of lake deposits and ice cores, as well as contemporaneous documents to bolster his highly speculative thesis. In his scenario, the ensuing disasters precipitated the disintegration of the Roman Empire, beset by Slav, Mongol and Persian invaders propelled from their disrupted homelands. The sixth-century collapse of Arabian civilization under pressure from floods and crop failure created an apocalyptic atmosphere that set the stage for Islam's emergence. In Mexico, Keys claims, the cataclysm triggered the collapse of a Mesoamerican empire; in Anatolia, it helped the Turks establish what eventually became the Ottoman Empire; while in China, the ensuing half-century of political and social chaos led to a reunified nation. Huge claims call for big proof, yet Keys reassembles history to fit his thesis, relentlessly overworking its explanatory power in a manner reminiscent of Velikovsky's theory that a comet collided with the earth in 1500 B.C. Readers anxious about future cataclysms will take note of Keys's roundup of trouble spots that could conceivably wreak planetary havoc.