Climate, technology, and glaciers: The settlement of the Western Hemisphere

Abstract

A major problem in human prehistory is the late settlement of the Americas, which were not occupied until after 15,000 years ago from Beringia. The most likely barriers to earlier settlement are: 1) high-latitude environments (characterized by low biological productivity and extreme winter temperatures); and 2) the North American ice sheet complex, which probably blocked both coastal and interior migration routes between Beringia and NW North America at various times before 15,000 years ago. Here we argue that each was a barrier to earlier settlement, but to different human taxa. Neanderthals, Denisovans, and other nonmodern representatives of Homo did not occupy latitudes above 60° North, which precluded occupation of Beringia and migration to the Western Hemisphere. The critical variables probably were: 1) resource scarcity, reflecting low plant and animal productivity; and 2) low winter temperatures. Homo sapiens, expanding out of Africa after 75,000 years ago, occupied virtually all terrestrial habitats in Eurasia no later than 32,000 years ago, including year-round settlement of the Arctic. Adaptation to high-latitude environments was achieved primarily with technologies of functional and structural complexity comparable to those of recent hunter-gatherers in similar settings. There is evidence of human settlement in Arctic Beringia before and during the Last Glacial Maximum (28,000–16,000 cal BP), and we conclude that the North American ice sheet complex is the only credible barrier to modern human occupation of the Western Hemisphere before 15,000 years ago.