PLEISTOCENE PATHWAYS OF HUMAN DISPERSAL INTO AMERICA

In 1987, Steve Porter wrote a sweeping review of glacial and paleoenvironmental factors constraining the timing of first human entry into the New World. These factors included relative sea level changes in Beringia and along the Pacific coastline, in addition to the extent and timing of the Cordilleran and Laurentide ice sheets in western North America. Thirty years on, we’re still trying to figure out how and when the First Americans arrived. We know people were in the Americas by 14.6 cal kya, and ancient DNA (aDNA) points to their divergence from Asian ancestors ca. 23 cal kya. Geological, paleoecological, and aDNA evidence helps assess when in that chronological window humans traversed the Bering Land Bridge from Siberia to Alaska, and then made it south of the North American continental ice sheets. Although Bering Strait was inundated most recently between 12 and 11 cal kya, the land bridge may have been irrelevant to people crossing from Asia on foot in winter or in boats in summer. Once in Alaska, there were two possible routes south: interior or coastal. Genetic separation of northern and southern bison clades (Heintzman et al. 2016) suggests the interior ice free corridor had either yet to open or was ecologically impassable to large grazing animals between 23 and 13.4 cal kya BP. Pederson et al (2016) place the glacial opening slightly earlier, though their environmental aDNA data puts the arrival of a viable biotic community ca. 12.6 cal kya BP. Despite this convergence on 13 cal kya for the opening of the corridor, Potter et al. (2017) assert the corridor was ice free by 19 cal kya and suitable for human passage by 14.9 cal kya. Clearly, the timing of the interior corridor remains uncertain. Glacial geology of the Pacific coastline suggests ice advanced all the way to the outer edge of the continental shelf between the Alaska Peninsula and western Washington during the last glacial maximum (LGM); however, local glacial maxima were out of phase, ranging from 17 cal kya in southeast Alaska, to 14.5 cal kya in Puget Sound. Still, a coastal route was open early enough for people to have reached lower latitude North America by 14.6 cal kya, provided they possessed a maritime technology that allowed multi-day passages around remnant glacier tongues and a subsistence strategy based on marine mammals.