When the Paleo-Indian hunters entered the New World, the last ice age was nearing its end. The ice sheets reached their maximum extent around 16,000 B.C.; they began to melt and recede northward 4,000 years later. The glacial retreat was interrupted by several episodes of minor readvance, but by about 8000 B.C., the ice caps were restricted to the far north, and warmer interglacial temperatures prevailed. Average annual temperatures in northern areas were now as much as 16°C higher than they had been during the glacial maximum. After 11,000 B.C., as vast quantities of water were released from the melting ice sheets, sea levels rose around the world. The Bering land bridge was severed by water about 8000 B.C., and the coastal shelves of North America were inundated. Postglacial changes in temperature and rainfall patterns had major effects on vegetation. Some Pleistocene environments, such as the game-rich tundra-steppe, disappeared. Other environments spread into new areas, like the deciduous forest of southeastern North America, which now extended its range into the Northeast. In South America, tropical forest may have replaced previously widespread grasslands in Amazonia. These and other changes in vegetation in turn affected animal populations. We have already examined the possible role of human predation in the extinction of the Pleistocene megafauna. However, the impact of Paleo-Indian hunting on large mammals probably would not have been so devastating if the animal populations had not already been weakened by stressful readjustment to changing and shifting habitats.