During the most recent of the Ice Ages, lasting from 30,000 to 10,000 years ago, an undersea ridge between Siberia and Alaska emerges from the sea.
Known as the Bering Land Bridge, it lies partly south of the ice cap.
Archaeological evidence shows that by about 15,000 years ago the central plains of America are widely inhabited.
Traces of human activity at this time are preserved in the remarkable La Brea tar pit in Los Angeles.
In the desert regions of the southwest human subsistence depends on smaller animals and gathered seeds.