Study of the prehistory of an Americas has proceeded through successive cycles, in which archaeologists have assumed or postulated hypotheses of migration and development, have searched for evidence with which to test these hypotheses, and have abandoned them when the evidence proved to be contradictory. We are completing the current cycle and must seek new hypotheses that better fit our present evidence.

The available data are reviewed to show the inaccuracy of the current hypotheses and to develop new ones. It is suggested that the first Indians arrived during a Lower Lithic age, marked by the manufacture of irregular flakes, trimmed only on their edges. This age may have started as early as 30,000 BP.

South of the Laurentide ice sheet, the Lower Lithic age began to give way about 14,000 BP to a Middle Lithic Age. This was marked by a variety of types of bifacially chipped projectile points which seem to have developed locally: Folsomoid points in Anglo-America, Joboid points in the Circum-Caribbean region, and Magelloid points in western and southern South America.

North of the ice sheet, the place of the Middle Lithic age was taken by an Upper Lithic age, marked by microblades of Asiatic origin. These appear to have reached the Bering Strait region about 14,000 BP. The dichotomy between the Middle and Upper Lithic ages lasted until the retreat of the ice ca. 8000 BP.