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In glacial ages more of the earth's water was locked in polar and mountain ice caps, lowering sea levels. The lower sea levels exposed additional land area along the coasts of the present continents. This coastal land, now below the surface of the ocean, was home to populations of plants and animals which, in the normal course of their lives, came to be distributed over wide areas, some of them separated by water today.
The Bering Strait is today a relatively shallow body of water between Russia and Alaska. It partially dried up in some of these periods, producing a vast stretch of land that united northeast Asia with the Americas. The region was home to a wide range of cold-adapted animals and, at some periods, human beings.
This body of land is referred to as Beringia. (External Link) At times of lowest seas, it was about a thousand miles from north to south (roughly the distance between Ottawa and Winnipeg or between San Diego and Seattle).
Because this land lay across the modern divide between Eurasia and the Americas, it is also called the "Bering Straits Land Bridge." Some of the plants and animals living there (including people) are said to have "migrated to the Americas from Siberia" by this "route," even though they themselves probably had no sense that they were doing anything more than minding their own business and foraging on the now-vanished landscape where they lived.
Scientists are now striving to understand the paleoenvironments of Beringia. Such research should eventually help us to figure out what animals and plants were found there during the various times when it was above sea level, and what environmental factors might have been conducive to long-term "transcontinental" movement of plants and animals (and microbes and viruses, for that matter). Much attention has been paid to the possibilities of steppe or tundra environments in this region. (Click for more about steppe environments or tundra environments.)
Most early human populations of the Americas are descended from the peoples who lived in Beringia and, over the course of generations, "crossed" Beringia, so that when the waters eventually rose again, their descendants were dwelling on the American side. Part of our reconstruction of the peopling of the New World therefore depends in part upon knowing the periods when Beringia was above water and available for habitation.
(Increasing evidence suggests that some small North American populations, perhaps earlier ones, may have entered the Americas from elsewhere, since some early human remains in this continent do not seem to bear the expected close physical resemblence to subsequent pre-Columbian populations. Unfortunately relevant evidence has been systematically suppressed as many of the critical specimens found in the United States have been reburied by their modern "descendants," preventing further study. This is possible because a loophole in American law provides for modern descendants to have authority over their ancestors' remains, but does not recognize the possibility of ancient populations that are not ancestral to any modern groups. The specimen receiving most press attention in recent years is called Kennewick Man.)
The same cold weather that lowered the sea levels as much as 120 meters below their present level also produced glaciers over much of northern North America. During some periods when Beringia itself was available, however, a wide unglaciated "corridor" extended southwestward on the east side of the Canadian Rockies, even though the area along the coast of Alaska and British Columbia was covered with ice. The ice-free inland corridor followed the Mackenzie River basin in Northwest Territories and is therefore called the "Mackenzie Corridor." It could in theory have provided a possible inland route for southward human migration, although there is no evidence that it was in fact used that way. (The important Charlie Lake Cave site in northeasterm British Columbia [external link] dates to about 8,800 BC, so humans were in the area then, but archaeological evidence seems to suggest that they more likely came from the south rather than from the north.)
At other periods glaciers covered the Mackenzie Corridor, but not the western coast of the North American continent, which provided a potential coastal route for possible migration, either on foot, or in part using simple water craft and living on coastal sea resources. Once again, there is no evidence that such a route was in fact used. In the case of the coastal route, we might expect that any scant remains of prehistoric coastal settlements would today be under water, since the modern higher seas would have covered the old coasts, so it is hard to imagine that we will ever have solid evidence for this path of migration.
One attempt to study the coastal terrain of this area as it was about 10,000 BC used a bathymetric map to locate areas for submarine sample collection which could be Carbon-14 dated. This provided evidence of coastal forests that had covered the area by that time, even though a couple of thousand years earlier it would have been frozen. One stone tool was even recovered dating from about 8,000 BC from a level 53 meters below the present sea level. These finds suggest continuing human populations in these coastal areas, and increase the probability of a model that would see the coast as also having been an early "migration route" into the Americas.
The following dates summarize the availability of unobstructed routes for human migration southward from Beringia during the ice age. During the "warm" periods of melted glaciers and high sea water, when Beringia itself was submerged, naturally both the coastal and inland routes were ice-free. Although the chart suggests a binary "open" and "shut" distinction, this is too simplistic: "Open"is not always equally inviting, as the variation in sea levels and coastal woodland ecology clearly shows.
From the paleoclimatological information given here, it looks as though the coastal route hypothesis is stronger than the inland route hypothesis, and as though the "smart money" would be on the period 22,000-15,000 BC as the most probable time for this initial migration, with 38,000-34,000 as a provocative early-end hypothesis.
| Dates BC | Beringia "Land Bridge" | Coastal Route | Mackenzie Corridor |
|---|---|---|---|
| 38,000-34,000 | accessible (open) | open | closed |
| 34,000-30,000 | submerged (closed) | open | open |
| 30,000-22,000 | accessible (open) | closed | open |
| 22,000-15,000 | accessible (open) | open | closed |
| 15,000 BC - today | submerged (closed) | open | open |
We have seen that clear archaeological evidence of such a migration has not been found in the Mackenzie Corridor, and that if it is in the Coastal Corridor it is under the sea. But paleoclimatological and archaeological data are not the only way to approach this issue. By the turn of the XXIst century, the most likely reconstruction, based on linguistic and genetic evidence, suggested that we should think of three separate "waves" of migration of pre-Columbian ancestors of populations in the Americas:
| Dates BC | Linguistic & Genetic Evidence |
|---|---|
| 30,000 | ancestors of Amerind speakers (now distributed over nearly all of the Americas) |
| 9,000 - 12,000 | ancestors of Na-Diné speakers (distributed over the northern half of North America) |
| 4,800 - 5,400 | ancestors of Eskimo-Aleut ("Eskaleut") speakers (distributed in the northernmost part of North America) |
(This listing does not include immigrants who, so far as we know, were not ancestral to anyone alive in North America today, possibly including Kennewick Man and similar specimens.)
If we combine the paleoclimatological dates with the linguistic-genetic dates, we notice that the inland route was "open" about 4,000 years earlier than the time when the linguistic and genetic "clocks" said these events would have happened. One view is that this might be the result of some kind of error in fixing dates for the linguistic and genetic evidence, or it could simply mean that a newly opened route wasn't immediately populated.
By 2010, DNA research was indeed pointing to somewhat earlier dates, with separate "Eskaleut" and Na-Diné migrations both occurring about 18,000 years ago (16,000 BC), when both the land bridge and the sea route were negotiable:
| Dates BC | Linguistic & Genetic Evidence (Revised) |
|---|---|
| 30,000 | ancestors of Amerind speakers (now distributed over nearly all of the Americas) |
| 16,000 | ancestors of Na-Diné speakers (distributed over the northern half of North America) |
| 16,000 | ancestors of Eskimo-Aleut ("Eskaleut") speakers (distributed in the northernmost part of North America) |
With these changes in our understanding of genetic evidence, the linguistic and genetic dates still fit well with the history of coastal route accessibility, but fit better with the history of land-bridge availability. Further genetic studies in 2012 suggested later dates, with more rapid spread once populations were established on the American side of the Straits. However, studies based exclusively on DNA evidence sometimes produce models that do not seem readily compatible with geological or linguistic evidence. (Example)
It seems fair to say that the balance of evidence (for the time being) points to the now submerged coast in combination with a nomadic foraging way of life on the land bridge itself as the most probable migration route for these populations.
As to dates, because these would have been extremely small, if expanding, populations, we would expect that archaeological evidence would appear to lag genetic or linguistic evidence slightly. Not surprisingly, the most compelling "earliest" archaeological material, discounting extremely controversial claims, seems to date from about three millennia later than the the second wave of migration postulated by the linguistic and genetic evidence (i.e., about 13,000 BC) when, we can imagine, originally coastal populations would have moved or expanded to higher ground inland.
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