The journey of the ancestral Paleo-Indians across the land bridge between Siberia and Alaska, more than 12,000 years ago, was the final stage of a process of migration and colonization that had begun 1.5 million years earlier. Continual northward and eastward movements of hunter-gatherer bands had resulted in the extension of the range of human habitation from the tropical savannas of southern and eastern Africa to the cold, dry tundra-steppes of northern Asia. In adapting to the diverse environments of this vast region, humans had changed both physically and culturally. In this chapter we will briefly review the major evolutionary developments that occurred prior to the arrival of early humans at the threshold of the New World. Some familiarity with the prehistory of Eurasia is necessary, both to see the Paleo-Indian migration in a broader perspective, and in order intelligently to evaluate arguments concerning the date and nature of the initial colonization of the Americas.
EARLY HOMINIDS OF AFRICA
On the basis of recent fossil discoveries and comparative studies of proteins and DNA, a consensus has been reached among most anthropologists that the ancestral human line diverged from that of the chimpanzee in Africa, some time between 10 and 5 million years ago. The earliest hominid species that is well-represented by fossil remains is Australopithecus afarensis, which dates from about 3.5 million years ago. Fossils found in Ethiopia and Tanzania show that A. afarensis walked bipedally, like later humans. However, their brains were still ape-sized, and there is no evidence that they hunted or used tools.