The Great Divide compares the development of humankind in the Old World and the New between 15,000 BC and AD 1500. Watson identifies three major differences between the two worlds—climate, domesticable mammals, and hallucinogenic plants—that combined to produce very different trajectories of civilization in the two hemispheres. Combining the most up-to-date knowledge in archaeology, anthropology, geology, meteorology, cosmology, and mythology, this unprecedented, masterful study offers uniquely revealing insight into what it means to be human.