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Stones described as probable tools, hammerstones and anvils, have been found in southern California, at the Cerutti Mastodon site, that are associated with a mastodon skeleton which appeared to have been processed by humans.The mastodon skeleton was dated by thorium-230/uranium radiometric analysis, using diffusion–adsorption–decay dating models, to 130.7 ± 9.4 thousand years ago. Waters commented that "To demonstrate such early occupation of the Americas requires the presence of unequivocal stone artefacts.However, archaeosites that date closer to the Last Glacial Maximum on either the Siberian or the Alaskan side of Beringia are lacking.Recent studies of Amerindian genetics have used high resolution analytical techniques applied to DNA samples from modern Native Americans and Asian populations regarded as their source populations to reconstruct the development of human Y-chromosome DNA haplogroups (y DNA haplogroups) and human mitochondrial DNA haplogroups (mt DNA haplogroups) characteristic of Native American populations.Current understanding of the settlement of the Americas derives from advances in four interrelated disciplines: archaeology, Pleistocene geology, physical anthropology, and DNA analysis.While there is general agreement that the Americas were first settled from Asia, the pattern of migration, its timing, and the place(s) of origin in Asia of the peoples who migrated to the Americas remain unclear.
Coastal alpine glaciers and lobes of Cordilleran ice coalesced into piedmont glaciers that covered large stretches of the coastline as far south as Vancouver Island and formed an ice lobe across the Straits of Juan de Fuca by 15,000 Prior to the Last Glacial Maximum, climates in eastern Siberia fluctuated between conditions approximating present day conditions and colder periods.
As water accumulated in glaciers, the volume of water in the oceans correspondingly decreased, resulting in lowering of the eustatic sea level.
The variation of the eustatic sea level with time has been reconstructed using oxygen isotope analysis of deep sea cores, the dating of marine terraces, and high resolution oxygen isotope sampling from ocean basins and modern ice caps.
The abandonment of the northern regions of Siberia due to rapid cooling or the retreat of game species with the onset of the LGM has been proposed to explain the lack of archaeosites in that region dating to the LGM.
The pollen record from the Alaskan side shows shifts between herb/shrub and shrub tundra prior to the LGM, suggesting less dramatic warming episodes than those that allowed forest colonization on the Siberian side.
While much of the coastal plain was covered with piedmont glaciers, unglaciated refugia supporting terrestrial mammals have been identified on Haida Gwaii, Prince of Wales Island, and outer islands of the Alexander Archipelago.