The peopling of the Americas began when Paleolithic hunter-gatherers (Paleo-Indians) entered North America from the North Asian Mammoth steppe via the Beringia land bridge, which had formed between northeastern Siberia and western Alaska due to the lowering of sea level during the Last Glacial Maximum (26,000 to 19,000 years ago).[2] These populations expanded south of the Laurentide Ice Sheet and spread rapidly southward, occupying both North and South America, by 12,000 to 14,000 years ago.[3][4][5][6][7] The earliest populations in the Americas, before roughly 10,000 years ago, are known as Paleo-Indians. Indigenous peoples of the Americas have been linked to Siberian populations by proposed linguistic factors, the distribution of blood types, and in genetic composition as reflected by molecular data, such as DNA.[8][9]
While there is general agreement that the Americas were first settled from Asia, the pattern of migration and the place(s) of origin in Eurasia of the peoples who migrated to the Americas remain unclear.[4] The traditional theory is that Ancient Beringians moved when sea levels were significantly lowered due to the Quaternary glaciation,[10][11] following herds of now-extinct Pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets.[12] Another route proposed is that, either on foot or using boats, they migrated down the Pacific coast to South America as far as Chile.[13] Any archaeological evidence of coastal occupation during the last Ice Age would now have been covered by the sea level rise, up to a hundred metres since then.[14]
The precise date for the peopling of the Americas is a long-standing open question. While advances in archaeology, Pleistocene geology, physical anthropology, and DNA analysis have progressively shed more light on the subject, significant questions remain unresolved.[15][16] The "Clovis first theory" refers to the hypothesis that the Clovis culture represents the earliest human presence in the Americas about 13,000 years ago.[17] Evidence of pre-Clovis cultures has accumulated and pushed back the possible date of the first peopling of the Americas.[18][19][20][21] Academics generally believe that humans reached North America south of the Laurentide Ice Sheet at some point between 15,000 and 20,000 years ago.[15][18][22][23][24][25] Some new controversial archaeological evidence suggests the possibility that human arrival in the Americas may have occurred prior to the Last Glacial Maximum more than 20,000 years ago.[18][26][27][28][29][30]
During the Wisconsin glaciation, the Earth's ocean water was, to varying degrees over time, stored in glacier ice. As water accumulated in glaciers, the volume of water in the oceans correspondingly decreased, resulting in lowering of global sea level. The variation of sea level over 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. A drop of eustatic sea level by about 60 to 120 metres (200 to 390 ft) from present-day levels, commencing around 30,000 years Before Present (BP), created Beringia, a durable and extensive geographic feature connecting Siberia with Alaska.[31] With the rise of sea level after the Last Glacial Maximum (LGM), the Beringian land bridge was again submerged. Estimates of the final re-submergence of the Beringian land bridge based purely on present bathymetry of the Bering Strait and eustatic sea level curve place the event around 11,000 years BP (Figure 1). Ongoing research reconstructing Beringian paleogeography during deglaciation could change that estimate and possible earlier submergence could further constrain models of human migration into North America.[31]
The onset of the Last Glacial Maximum after 30,000 years BP saw the expansion of alpine glaciers and continental ice sheets that blocked migration routes out of Beringia. By 21,000 years BP, and possibly thousands of years earlier, the Cordilleran and Laurentide ice sheets coalesced east of the Rocky Mountains, closing off a potential migration route into the center of North America.[32][33][34] Alpine glaciers in the coastal ranges and the Alaskan Peninsula isolated the interior of Beringia from the Pacific coast. 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 18,000 BP.[35][36] Coastal alpine glaciers started to retreat around 19,000 BP[37] while Cordilleran ice continued advancing in the Puget lowlands up to 16,800 BP.[36] Even during the maximum extent of coastal ice, unglaciated refugia persisted on present-day islands, that supported terrestrial and marine mammals.[34] As deglaciation occurred, refugia expanded until the coast became ice-free by 15,000 BP.[34] The retreat of glaciers on the Alaskan Peninsula provided access from Beringia to the Pacific coast by around 17,000 BP.[38] The ice barrier between interior Alaska and the Pacific coast broke up starting around 16,200 BP.[35] The ice-free corridor to the interior of North America opened between 13,000 and 12,000 BP.[32][33][34] Glaciation in eastern Siberia during the LGM was limited to alpine and valley glaciers in mountain ranges and did not block access between Siberia and Beringia.[31]
The paleoclimates and vegetation of eastern Siberia and Alaska during the Wisconsin glaciation have been deduced from high resolution oxygen isotope data and pollen stratigraphy.[31][39][40] Prior to the Last Glacial Maximum, climates in eastern Siberia fluctuated between conditions approximating present day conditions and colder periods. The pre-LGM warm cycles in Arctic Siberia saw flourishes of megafaunas.[31] The oxygen isotope record from the Greenland Ice Cap suggests that these cycles after about 45,000 BP lasted anywhere from hundreds to between one and two thousand years, with greater duration of cold periods starting around 32,000 BP.[31] The pollen record from Elikchan Lake, north of the Sea of Okhotsk, shows a marked shift from tree and shrub pollen to herb pollen prior to 30,000 BP, as herb tundra replaced boreal forest and shrub steppe going into the LGM.[31] A similar record of tree/shrub pollen being replaced with herb pollen as the LGM approached was recovered near the Kolyma River in Arctic Siberia.[40] 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 archaeological sites in that region dating to the LGM.[40][41] 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. Diverse, though not necessarily plentiful, megafauna were present in those environments. Herb tundra dominated during the LGM, due to cold and dry conditions.[39]
Coastal environments during the Last Glacial Maximum were complex. The lowered sea level, and an isostatic bulge equilibrated with the depression beneath the Cordilleran Ice Sheet, exposed the continental shelf to form a coastal plain.[42] 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.[39] The now-submerged coastal plain has potential for more refugia.[39] Pollen data indicate mostly herb/shrub tundra vegetation in unglaciated areas, with some boreal forest towards the southern end of the range of Cordilleran ice.[39] The coastal marine environment remained productive, as indicated by fossils of pinnipeds.[42] The highly productive kelp forests over rocky marine shallows may have been a lure for coastal migration.[43][44] Reconstruction of the southern Beringian coastline also suggests potential for a highly productive coastal marine environment.[44]
Pollen data indicate a warm period culminating between 17,000 and 13,000 BP followed by cooling between 13,000 and 11,500 BP.[42] Coastal areas deglaciated rapidly as coastal alpine glaciers, then lobes of Cordilleran ice, retreated. The retreat was accelerated as sea levels rose and floated glacial termini. It has been estimated that the coast range was fully ice-free between 16,000 and 15,000 BP.[42][34] Littoral marine organisms colonized shorelines as ocean water replaced glacial meltwater. Replacement of herb/shrub tundra by coniferous forests was underway by 15,000 BP north of Haida Gwaii. Eustatic sea level rise caused flooding, which accelerated as the rate grew more rapid.[42]
The inland Cordilleran and Laurentide ice sheets retreated more slowly than did the coastal glaciers. Opening of an ice-free corridor did not occur until after 13,000 to 12,000 BP.[32][33][34] The early environment of the ice-free corridor was dominated by glacial outwash and meltwater, with ice-dammed lakes and periodic flooding from the release of ice-dammed meltwater.[32] Biological productivity of the deglaciated landscape increased slowly.[34] The earliest possible viability of the ice-free corridor as a human migration route has been estimated at 11,500 BP.[34]
Birch forests were advancing across former herb tundra in Beringia by 17,000 BP in response to climatic amelioration, indicating increased productivity of the landscape.[40]
Analyses of biomarkers and microfossils preserved in sediments from Lake E5 and Burial Lake in northern Alaska suggest early humans burned Beringian landscapes as early as 34,000 years ago.[45][46] The authors of these studies suggest that fire was used as means of hunting megafauna.
The Indigenous peoples of the Americas have an ascertained archaeological presence in the Americas dating back to about 15,000 years ago.[47][48] More recent research, however, suggests a human presence dating to between 18,000 and 26,000 years ago, during the Last Glacial Maximum.[49][50][7]There remain uncertainties regarding the precise dating of individual sites and regarding conclusions drawn from population genetics studies of contemporary Native Americans.
!["Maps depicting each phase of the three-step early human migrations for the peopling of the Americas. (A) Gradual population expansion of the Amerind ancestors from their Central East Asian gene pool (blue arrow). (B) Proto-Amerind occupation of Beringia with little to no population growth for ≈20,000 years. (C) Rapid colonization of the New World by a founder group migrating southward through the ice-free, inland corridor between the eastern Laurentide and western Cordilleran Ice Sheets (green arrow) and/or along the Pacific coast (red arrow). In (B), the exposed seafloor is shown at its greatest extent during the last glacial maximum at ≈20–18,000 years ago [25]. In (A) and (C), the exposed seafloor is depicted at ≈40,000 years ago and ≈16,0000 years ago, when prehistoric sea levels were comparable. A scaled-down version of Beringia today (60% reduction of A–C) is presented in the lower left corner. This smaller map highlights the Bering Strait that has geographically separated the New World from Asia since ≈11–10,000 years ago."](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Journal.pone.0001596.g004.png/1280px-Journal.pone.0001596.g004.png)
In the early 21st century, the models of the chronology of migration are divided into two general approaches.[52][53]
The first is the short chronology theory, that the first migration occurred after the LGM, which went into decline after about 19,000 years ago,[37] and was then followed by successive waves of immigrants.[54]
The second theory is the long chronology theory, which proposes that the first group of people entered Beringia, including ice-free parts of Alaska, at a much earlier date, possibly 40,000 years ago,[55][56][57] followed by a much later second wave of immigrants.[53][58]
The Clovis First theory, which dominated thinking on New World anthropology for much of the 20th century, was challenged in the 2000s by the secure dating of archaeological sites in the Americas to before 13,000 years ago.[32][33][34][59][48]
The archaeological sites in the Americas with the oldest dates that have gained broad acceptance are all compatible with an age of about 15,000 years. This includes the Buttermilk Creek Complex in Texas,[47] the Meadowcroft Rockshelter site in Pennsylvania and the Monte Verde site in southern Chile.[48] Archaeological evidence of pre-Clovis people points to the South Carolina Topper Site being 16,000 years old, at a time when the glacial maximum would have theoretically allowed for lower coastlines.
It has often been suggested that an ice-free corridor, in what is now Western Canada, would have allowed migration before the beginning of the Holocene. However, a 2016 study has argued against this, suggesting that the peopling of North America via such a corridor is unlikely to significantly pre-date the earliest Clovis sites. The study concludes that the ice-free corridor in what is now Alberta and British Columbia "was gradually taken over by a boreal forest dominated by spruce and pine trees" and that the "Clovis people likely came from the south, not the north, perhaps following wild animals such as bison".[60][61]An alternative hypothesis for the peopling of America is coastal migration, which may have been feasible along the deglaciated (but now submerged) coastline of the Pacific Northwest from about 16,000 years ago.
Pre-LGM migration across Beringia has been proposed to explain purported pre-LGM ages of archaeological sites in the Americas such as Bluefish Caves[56] and Old Crow Flats[57] in the Yukon Territory, and Meadowcroft Rock Shelter in Pennsylvania.[53][58] The oldest archaeological sites on the Alaskan side of Beringia date to around 14,000 BP.[40][62] It is possible that a small founder population had entered Beringia before that time. However, archaeological sites that date closer to the LGM on either the Siberian or the Alaskan side of Beringia are lacking. Biomarker and microfossil analyses of sediments from Lake E5 and Burial Lake in northern Alaska suggest human presence in eastern Beringia as early as 34,000 years ago.[45] These sedimentary analyses have been suggested to be the only possibly recoverable remnants of humans living in Alaska during the last Glacial period.[46]
At Old Crow Flats, mammoth bones have been found that are broken in distinctive ways indicating human butchery. The radiocarbon dates on these vary between 25,000 and 40,000 BP. Also, stone microflakes have been found in the area indicating tool production.[63] However, the interpretations of butcher marks and the geologic association of bones at the Bluefish Cave and Old Crow Flats sites, and the related Bonnet Plume site, have been called into question.[29] No evidence of human remains have been discovered at these sites. In addition to disputed archaeological sites, support for pre-LGM human presence has been found in lake sediment records of northern Alaska. Biomarker and microfossil analyses of sediments from Lake E5 and Burial Lake in suggest human presence in eastern Beringia as early as 34,000 years ago.[45][46] These analyses are indeed compelling in that they corroborate the inferences made from the Bluefish Cave and Old Crow Flats sites.
In 2020, evidence emerged for a new pre-LGM site in North-Central Mexico. Chiquihuite cave, an archaeological site in Zacatecas State, has been dated to 26,000 years BP based on numerous lithic artefacts discovered there.[64] However, there is scholarly debate over whether the artifacts should be considered evidence of human activity or if they were formed naturally.[65][28] No evidence of human DNA or hearth have been unearthed.[66]
Pre-LGM human presence in South America rests partly on the chronology of the controversial Pedra Furada rock shelter in Piauí, Brazil. More recently, studies at the archaeological sites Santa Elina (27000-10000 years BP)[67] in the midwest, and Rincão I (20000-12000 years BP)[68] in southeastern Brazil also show associations of evidence of human presence with sediments dating from before the LGM. A 2003 study dated evidence for the controlled use of fire to before 40,000 years ago.[69] Additional evidence has been adduced from the morphology of Luzia Woman fossil, which was described as Australo-Melanesian. This interpretation was challenged in a 2003 review which concluded the features in question could also have arisen by genetic drift.[70] In November 2018, scientists of the University of São Paulo and Harvard University released a study that contradicts the alleged Australo-Melanesian origin of Luzia. Using DNA sequencing, the results showed that Luzia's ancestry was entirely Native American.[71][72]
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 around 130 thousand years ago.[73] No human bones were found and expert reaction was mixed; claims of tools and bone processing were called "not plausible" by Prof. Tom Dillehay.[74]
The Yana River Rhino Horn site (RHS) has dated human occupation of eastern Arctic Siberia to 31,300 BP.[75] That date has been interpreted by some as evidence that migration into Beringia was imminent, lending credence to occupation of Beringia during the LGM.[76][77] However, the Yana RHS date is from the beginning of the cooling period that led into the LGM.[31] A compilation of archaeological site dates throughout eastern Siberia suggest that the cooling period caused a retreat of humans southwards.[40]