The earliest period in Iowa prehistory is sometimes referred to as the "Paleoindian" or "Big-Game Hunting" stage. It represents the first time that we find evidence of people living in the state. The remains of even earlier people have been found in North, Central, and South America and suggest that the Western Hemisphere was colonized by 20,000 years ago, and possibly earlier. It is believed that the very earliest immigrants entered the New World at various times throughout the last "Ice Age," or Pleistocene epoch, when vast ice sheets partially covered the northern parts of North America and Eurasia. So far, Iowa has not revealed the remains of the earliest people to migrate to North America from Eurasia. Evidence of their descendants, the people of the Paleoindian period, is itself somewhat limited and has come almost exclusively from finds of leaf-shaped projectile points discovered on the surface of the ground. While the maximum time ranges for the Paleoindian period in North America may extend from 15,000-8,000 BP, the earliest remains in Iowa are perhaps 13,000 years old. The way of life of Iowa's first inhabitants during the Paleoindian period is assumed to have been predominantly nomadic with a specialized economy that concentrated on hunting and herds of large game animals. From various sites outside the state we know that Paleoindian hunters often worked together to drive herds of bison over cliffs or into areas where they might be more easily killed and then buthchered. Although almost nothing is known of the social organization, housing, or more perishable artifacts of Paleoindian people, it is assumed that social interaction necessary for driving herds of game would have involved cooperations among hunters and an established pattern of social control. Housing would probably have been temporary, involving some sort of branch or bone frame with a covering of skins or mats. Clothes would likewise been made of skins. Unfortunately, these are the kind of items that don't withstand the test of time, and so we are left to speculate about them.