Prehistoric migration from Asia to the Americas
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). 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. 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.
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. The traditional theory is that Ancient Beringians moved when sea levels were significantly lowered due to the Quaternary glaciation, following herds of now-extinct Pleistocene megafauna along "ice-free corridors" that stretched between the Laurentide and Cordilleran ice sheets. Another route proposed is that, either on foot or using boats, they migrated down the Pacific coast to South America as far as Chile. 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.
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. 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. Evidence of pre-Clovis cultures has accumulated and pushed back the possible date of the first peopling of the Americas. 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. 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.
Scholarly debate.
Historically, researchers believed a single theory explained the peopling of the Americas, focusing on findings from Blackwater Draw New Mexico, where human artifacts dated from the last ice age were found alongside the remains of extinct animals in 1930s This led to the widespread belief in the "Clovis-first model," proposing that the first Americans migrated over the Beringia land bridge from Asia during a time when glacial passages opened. This model linked the first inhabitants to distinctive spear points, known as Clovis points, ranging in age from 13,250 to 12,800 years old.
Numerous claims of earlier human presence began to challenge the Clovis first model beginning in the 1990s, culminating in significant discoveries at Monte Verde, Chile, dating back 14,500 years. At Oregon’s Paisley Caves, fossilized human feces date back 14,300 years. In Texas, at Buttermilk Creek complex, stone tool fragments date back 15,500 years. At Arroyo Seco 2 in Argentina, archaeologists discovered 14,000-year-old butchered animal bones. Meadowcroft Rockshelter in Pennsylvania may have a history of at least 16,000 years. As research progressed in the 2000s, the narrative shifted from a single migration event to multiple small, diverse groups entering the continent at various points in time. This indicates that people might have populated North and South America as early as 15,000 to 20,000 years ago, which some believe support a coastal migration route.
The genetic history of the Indigenous peoples of the Americas has highlighted populations that adapted over tens of thousands of years. Geneticists discovered that a Beringian population split from Siberian groups about 36,000 years ago. Around 25,000 years ago, they became isolated, forming a new genetic group linked to today’s Indigenous populations, which divided into two main lineages between 14,500 and 17,000 years ago reflecting the dispersal associated with the early peopling of the Americas.
The Chiquihuite Cave in Mexico suggest that ancient stone tools were being utilized as long ago as 30,000 years ago. However, proving these tools were made by humans has faced criticism. In New Mexico, researchers uncovered fossilized footprints, dating back perhaps 23,000 years, showing humans and megafauna like mammoths co-existing, which is also under great debate. In addition, some argue that evidence points towards human presence extending back 130,000 years, though this is outright rejected by the vast majority of scholars across multiple academic fields. Scholars such as David J. Meltzer have emphasized the fact that there are no verified archaeological sites in the Americas older than 16,000 years accepted by the academic community at large, which questions claims of sites being 20,000, 25,000, or even 130,000 years old.
The environment during the latest glaciation.
Emergence and submergence of Beringia.
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 from present-day levels, commencing around 30,000 years Before Present (BP), created Beringia, a durable and extensive geographic feature connecting Siberia with Alaska. 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.
Glaciers.
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. 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. Coastal alpine glaciers started to retreat around 19,000 BP while Cordilleran ice continued advancing in the Puget lowlands up to 16,800 BP. Even during the maximum extent of coastal ice, unglaciated refugia persisted on present-day islands, that supported terrestrial and marine mammals. As deglaciation occurred, refugia expanded until the coast became ice-free by 15,000 BP. The retreat of glaciers on the Alaskan Peninsula provided access from Beringia to the Pacific coast by around 17,000 BP. The ice barrier between interior Alaska and the Pacific coast broke up starting around 16,200 BP. The ice-free corridor to the interior of North America opened between 13,000 and 12,000 BP. 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.
Climate and biological environments.
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. 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. 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. 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. 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. 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. 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.
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. 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. The now-submerged coastal plain has potential for more refugia. 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. The coastal marine environment remained productive, as indicated by fossils of pinnipeds. The highly productive kelp forests over rocky marine shallows may have been a lure for coastal migration. Reconstruction of the southern Beringian coastline also suggests potential for a highly productive coastal marine environment.
Environmental changes during deglaciation.
Pollen data indicate a warm period culminating between 17,000 and 13,000 BP followed by cooling between 13,000 and 11,500 BP. 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. 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.
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. 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. Biological productivity of the deglaciated landscape increased slowly. The earliest possible viability of the ice-free corridor as a human migration route has been estimated at 11,500 BP.
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.
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. The authors of these studies suggest that fire was used as means of hunting megafauna.
Chronology, reasons for, and sources of migration.
The Indigenous peoples of the Americas have an ascertained archaeological presence in the Americas dating back to about 15,000 years ago. More recent research, however, suggests a human presence dating to between 18,000 and 26,000 years ago, during the Last Glacial Maximum.
There remain uncertainties regarding the precise dating of individual sites and regarding conclusions drawn from population genetics studies of contemporary Native Americans.
Chronology.
In the early 21st century, the models of the chronology of migration are divided into two general approaches.
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, and was then followed by successive waves of immigrants.
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