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The physical landscape was the primary factor influencing the initial peopling of the North American continent during the Late Pleistocene epoch. This was the ending stages of the Pleistocene; a geologic time period that began 2.58 Ma and ended around the terminus of the Younger Dryas climatic period around 11.8 ka BP. During this time, the North American continent experienced the maximum extent of ice sheets during its most recent glaciation, the Wisconsin Glacial Episode, with glaciers extending from the northern Canadian Arctic south to present-day Washington state, Minnesota, and Wisconsin. This northern ice was part two massive ice sheets, the Laurentide ice sheet and the Cordilleran ice sheet. During this glacial period, it's thought the North American continent was absent human populations. The questions of when humans first arrived in North America, how they got here, and what their cultures were like has long been a field of intense research and speculation. Researchers have highlighted 4 primary environmental factors that affected the migration and landscape viability of Late Pleistocene peoples in both Beringia and those migrating southwards: variations in climate, extent of ice sheets and glaciers, changes in sea level, and landscape vegetation.

Landscape-Human Interaction in the Late Pleistocene
Further Information: The Holocene: An Environmental History

Beringia
See also: Beringia

See also: Ancient Beringian It's long been understood that one source of initial human populations into North America was Northeast Asia. During the Last Glacial Maximum sea levels were roughly 100 meters lower than present-day, exposing dry land that connected Asia and North America in what is now Siberia and Alaska. This landscape was known as the Bering Sea Land Bridge, or Beringia. This connected landscape enabled early humans in northeast Asia, with domestic populations evident back to at least 40 ka BP, to settle in the Beringian tundra. Current findings indicate that the first presence of human habitation in this newly exposed land was from around 24.9 ka BP. The environmental landscape of Beringia would have been ideal for supporting resident human populations as well as those migrating southward beyond the ice sheets. A transition from glacial tundra following retreat of the ice sheets to shrub-tundra with dispersed trees between 20 and 14 ka BP was likely the first landscape change that enable the environment to support human migration. This transition was aided by large amounts of exposed land relative to today, giving large portions of Beringia a continental climate. This would have helped support large, diverse populations of animal species of sufficient concentrations to support Pleistocene human communities. Starting around 14 ka BP and lasting relatively briefly until the Younger Dryas cold period around 11.8 ka BP was a sudden increase in Birch shrub and tree species, as evident from pollens in multiple Alaskan sediment core.

Elements of the Landscape
See also: Postglacial vegetation

See also: Glacial Climate in Last Glacial Maximum

Vegetation
During the Late Pleistocene, at the time of the Last Glacial Maximum, the northwestern part of North America was dominated by dry herb tundra and grasslands. Research conducted in 2001 on a sediment core from the Seward Peninsula in southern Alaska, with Late Pleistocene vegetation preserved in a volcanic ash layer, indicated the presence of grasses, sedges, forbes, and various mosses around this time. Around 11 ka BP there was a shift in this vegetation profile however as the landscape shifted from the predominately tundra-grassland to a more subalpine forest environment. Multiple studies indicate the existence of forests in glacial refugia located in ice-free pockets along the Pacific coast, which likely increased the spread of forests following deglaciation. This change in landscape vegetation would have been critical to supporting the migration of more modern animal species, such as moose, moving beyond the southern terminus of the ice sheets. Additionally, this increase in fauna, as well as increased availability of wood products for fires, dwelling construction, and tools, would have aided in the migration of greater numbers of human groups.

Ice Sheets
See "The Ice-free Corridor" and "The Pacific Corridor" below.

Megafauna
See also: Pleistocene Megafauna

See "Resource Availability" below.

Environmental Factors on Migration Routes and Access
See also: Settlement of the Americas

Much of the debate around the arrival of early human communities and cultures to North America centers around the timing, access, and specific migration routes of the first inhabitants.

The "Ice-free Corridor"
See also: Interior Route in Settlement of the Americas

Accessibility
For much of the early debate surrounding first human migration to North American from Asia, a proposed gap between the Laurentide and Cordilleran Ice Sheets in western Canada (Alberta) was viewed as the most likely route. Around the time of the Last Glacial Maximum at 26 ka BP, both the Laurentide and Cordilleran ice sheets were joined together at their northwestern and northeastern edges respectively. This created a complete blockage of overland passage outside of Alaska and Beringia to the rest of North America before the late Holocene. As the two ice sheets began decreasing in mass volume and extent, mountain tops and ridges, like those of the Mackenzie Mountains, became exposed starting around 19.3 to 18.5 ka BP. As the two ice sheets melted, they separated in a "zipper-like" fashion from their northern extents southeastwards, creating and then draining a series of proglacial lakes. A lot of research and debate surrounds when this separation of the two ice sheets was completed and the creation of a relatively ice free corridor first appeared. A general consensus around 15.6 to 14.8 ka BP is thought to be when the two ice sheets complete separated from contact with one another. An area of active research is investigating the width of this opening, or "bottle-neck", between the two ice sheets and its viability for human migration. Current estimates of the gap's width range from 110 km around 14.8 ka BP, 430 km around 12.8 ka BP, to about 700 km around 12.5 ka BP. Much of the age discrepancy center round the dating source, with the former date coming from 14C dating of wood samples from lake clays and the latter from using foliage remains in similar type sediments. There is consensus however in that following this separation, a large proglacial lake called Glacial Lake Peace was formed, blocking a migration route through this gap for an estimated 2 ka. It's thought that the lake finally drained and the passage was navigable between 13.55 and 12.6 ka BP.

Resource Availability
Aside from the accessibility of the ice-free corridor is the debate about its feasibility as a human migration route. The movement of people requires supporting resources from access to drinkable water and food sources, to hospitable climatic conditions and wood products. Research studies centered around the discovery and analysis of paleo-bison remains indicates the interior passage was supportable of human migration starting around at least 13 ka BP. Horse and bison species were established in the south central part of the corridor 13.45 ka BP, which could have provided food and other resources to migration populations. Additionally, several sources indicate substantial populations of mammoth (Mammut americanum) and mastadon (Mammuthus spp.) within the corridor and regions further south. It's thought that the high-mountain refugia within the Mackenzie Mountains and those of eastern British Columbia supported populations of mountain goats which were likely present following deglaciation. Evidence from charcoal remains in glacial lakes indicate rapid migration of forests and other large shrubs into the ice-free corridor around this same time, possibly providing building and tool-making materials.

Consensus
Despite a clear, ice-free passage and evidence of migration-supportable landscapes, most researchers believe this interior access route from Beringia, while clearly used by early human populations, was not the route of first migration into North America. The timing of the corridor's opening and ice-free date were too late, and evidence indicates the presence of human populations south of this corridor prior to the estimated ice-free dates of the corridor. Additionally, it's thought that the formation of glacial lakes and subsequent blockage of access southward might have occurred on a cyclical seasonal timeline. If early populations did use this corridor, it's thought that these populations might have migrated in waves using alternative routes first until the viability and navigable of the corridor became apparent. Mikkel Pedersen at the University of Copenhagen, along with other researchers, found that "[early human populations] must have descended from a population that entered the Americas via a different route than the ice-free corridor".

The "Pacific Corridor"
See also: Pacific Coastal Route in Settlement of the Americas

Accessibility
The latest research conducted in 2018 indicates the most likely access route of first peoples to North America was along the "Pacific Corridor" on the coast of present-day Alaska, British Columbia, and Washington state. Expansion of the western edge of the Cordilleran Ice Sheet in present-day British Columbia reached its maximum extent along the Pacific coast 22-19 ka BP. This terminal edge of the ice sheet created tidewater glaciers in Pacific coast fjords that had regular iceberg pulses making navigability difficult and dangerous. It's thought that this coastal terminus of the ice sheet began retreating soon after the Last Glacial Maximum, creating initial ice-free areas at northern reaches as early as 19.4 ka BP. This retreat of the ice sheet, which contributed greater volumes of fresh water to the nearby ocean environment, along with global-scale changes in Pacific ocean currents and shifts in the Intertropical Convergence Zone (ITCZ), accelerated coastal ice melting. Geographic and archaeological evidence shows that the Pacific corridor, with exposure of outlying islands, was a viable migration route as early as 18.4 to 18.1 ka BP. This is almost 5 ka sooner than agreed-upon dates for the viability of passage of the interior ice-free corridor.

Resource Availability
The establishment of human migration-supporting environmental resources, principally forests and coastal marine mammal species, was thought to have occurred quite rapidly after the retreat of the Cordilleran Ice Sheet from the Pacific coast; as quickly as 1 ka following deglaciation. For example, evidence of Ringed seal on Shuká Káa island on the central British Columbia coast were dated to around 17.2 ka BP. However, topographic changes caused by isostatic uplift of the land masses and increases in sea level, may have caused extirpation of these seal populations as soon as 15 ka BP. Despite this, It's thought that the influx of kelp beds and accompanying marine life around 17 to 14 ka BP further added to the availability of marine food sources for migrating human populations. It's also widely believed that extensive glacial refugia persisted throughout this latest glacial period. As the ice sheet retreated, these already established tundra and forest environments, which supported moderate-sized land mammals like deer and fox, helped sustain possible migrating groups.

Consensus
The present consensus, with a heavy concentration of past and present research efforts, is that the coastal waterways along the Pacific Northwest coast between southeast Alaska and Washington State were the likely route of first peoples to northern and central North America.