New dating of rocks and reanalysis of animal bones from islands along the shore of southeastern Alaska suggests that a narrow corridor between the Pacific Ocean and the Cordilleran Ice Sheet (CIS) in Alaska may have enabled the migration of humans to the Americas as early as 17,000 years ago.
While the data imply that this Pacific coastal corridor – a pathway exposed following deglaciation – was physically and environmentally viable for early human migration to the Americas, the authors say that archaeological evidence of human activity is still necessary to confirm that this pathway played an important role in the peopling of the Americas.
The Beringia route was widely believed by many in the 20th century to be the course of human migration to the Americas from Asia. Recent analyses, on the other hand, have suggested that early human migration took place approximately 16 thousand years (ka) ago through a deglaciated corridor to the west of the CIS along the North Pacific coast.
Yet, whether or not the coastal region’s environmental conditions – such as biological productivity, availability of food resources and the presence or absence of physical barriers – were fit to support humans at that time remains unknown.
Here, Alia J. Lesnek and colleagues reconstructed deglaciation using two different proxies. First, they applied 10Beryllium-dating of ten rock surfaces – five perched boulders and five bedrock samples – from three proposed areas: Dall Island, Suemez Island, and Warren Island. Second, they utilized previously published 14Carbon-dated mammal and bird bones found in caves on nearby islands.
Their analysis showed that this area was glaciated from around 20 to 17 ka ago and thus not open for human migration. However, the islands were deglaciated after 17 ka ago and hosted robust terrestrial and marine ecosystems that could support humans during southward migration into the Americas.
When and how did the first people come to the Americas?
The conventional story says that the earliest settlers came via Siberia, crossing the now-defunct Bering land bridge on foot and trekking through Canada when an ice-free corridor opened up between massive ice sheets toward the end of the last ice age.
But with recent archaeological evidence casting doubt on this thinking, scientists are seeking new explanations.
One dominant, new theory: The first Americans took a coastal route along Alaska’s Pacific border to enter the continent.
A new geological study provides compelling evidence to support this hypothesis.
By analyzing boulders and bedrock, a research team led by the University at Buffalo shows that part of a coastal migration route became accessible to humans 17,000 years ago. During this period, ancient glaciers receded, exposing islands of southern Alaska’s Alexander Archipelago to air and sun — and, possibly, to human migration.
The timing of these events is key: Recent genetic and archaeological estimates suggest that settlers may have begun traveling deeper into the Americas some 16,000 years ago, soon after the coastal gateway opened up.
“People are fascinated by these questions of where they come from and how they got there,” says lead scientist Jason Briner, PhD, professor of geology in UB’s College of Arts and Sciences. “Our research contributes to the debate about how humans came to the Americas. It’s potentially adding to what we know about our ancestry and how we colonized our planet.”
“Our study provides some of the first geologic evidence that a coastal migration route was available for early humans as they colonized the New World,” says UB geology PhD candidate Alia Lesnek, the study’s first author. “There was a coastal route available, and the appearance of this newly ice-free terrain may have spurred early humans to migrate southward.”
The findings do not mean that early settlers definitely traversed Alaska’s southern coast to spread into the Americas: The project examined just one section of the coast, and scientists would need to study multiple locations up and down the coastline to draw firmer conclusions.
Abstract The route and timing of early human migration to the Americas have been a contentious topic for decades. Recent paleogenetic analyses suggest that the initial colonization from Beringia took place as early as 16 thousand years (ka) ago via a deglaciated corridor along the North Pacific coast. However, the feasibility of such a migration depends on the extent of the western Cordilleran Ice Sheet (CIS) and the available resources along the hypothesized coastal route during this timeframe. We date the culmination of maximum CIS conditions in southeastern Alaska, a potential bottleneck region for human migration, to ~20 to 17 ka ago with cosmogenic 10Be exposure dating and 14C dating of bones from an ice-overrun cave. We also show that productive marine and terrestrial ecosystems were established almost immediately following deglaciation. We conclude that CIS retreat ensured that an open and ecologically viable pathway through southeastern Alaska was available after 17 ka ago, which may have been traversed by early humans as they colonized the Americas.
Introduction Despite decades of research, the debate surrounding the timing and route of the initial human colonization of the Americas continues. Early hypotheses about human dispersal into the Americas from Beringia, the land mass that joined Northeast Asia and northwestern North America during the Late Pleistocene, relied heavily on the discovery of Clovis points in the interior plains of North America, and the resulting “Clovis First” paradigm dominated archeological thinking for most of the 20th century. This hypothesis hinges on the presence of a corridor of unglaciated terrain in west-central Canada separating the Laurentide Ice Sheet and Cordilleran Ice Sheet (CIS) throughout the Late Pleistocene (the so-called ice-free corridor). It is now widely recognized that these ice sheets coalesced during the Last Glacial Maximum [LGM; 26 to 19 thousand years (ka) ago] and well into the deglacial period. Recently, research in the ice-free corridor has determined that while the corridor became physically open sometime between 15.6 and 14.8 ka ago, the region was not biologically viable (that is, able to support human life) until 13.0 ka ago or perhaps even 12.6 ka ago. Furthermore, the increasing antiquity of known American archeological settlements, including Clovis sites, as well as mounting evidence from genetic studies, has pushed back the timing of the initial migration into the Americas to 16 ka ago. It is therefore unlikely that this first pulse of human migration took place through the ice-free corridor, and attention has accordingly shifted to the Pacific coast as an alternative, viable entry route to the Americas.
The feasibility of a coastal migration depends on two environmental factors: (i) the biological productivity and availability of food resources along the coastal route during the colonization event(s) and (ii) the presence or absence of significant physical barriers to migration. The movement of sea-faring peoples from Asia to the Americas may have been facilitated by highly productive coastal kelp forests, but it remains unclear whether a continuous “kelp highway” existed during the latest Pleistocene. Perhaps of greater importance, along the northeastern Pacific coast, marine margins of the CIS could have posed a major physical challenge to migration. The presence of small mountain glaciers independent of the CIS may have also impeded coastal travel. However, if ice did not extend to the continental shelf break, lower sea levels during the latest Pleistocene may have exposed portions of the now-submerged continental shelf, potentially providing habitable terrain for early human migration along the outermost coast. The viability of the coastal route is therefore intimately tied to CIS and mountain glacier extent during the latest Pleistocene.
If they existed, continuously ice-free areas may have facilitated human migration through southeastern Alaska by serving as refugia for plant and animal populations, which could provide sustenance, resources, and shelter from relatively harsh conditions along the North Pacific coast. In addition, the occurrence of ice-free terrain along the outer coast may have hastened the development of viable ecosystems elsewhere in the region following the retreat of the CIS. While the presence of LGM refugia in southeastern Alaska is often invoked to support the coastal migration hypothesis, to date, there remains no direct evidence for their existence. Here, we test the refugia hypothesis and assess the viability of a bottleneck region of the coastal migration route by reconstructing the timing of latest Pleistocene glaciation in southeastern Alaska.
Discussion The lack of fossilized cave bones dating between 19.8 and 17.2 cal ka BP implies one of two scenarios: cover of Shuká Káa’s entrance by the CIS or nondeposition of fossils in the cave unrelated to CIS extent. In the 45-ka period that animals made regular use of Shuká Káa, the period from 19.8 to 17.2 cal ka BP is the longest interval in the record with no dated bones. Moreover, our 10Be ages suggest that retreat of the CIS from Dall, Suemez, and Warren islands occurred at 17.0 ± 0.7 ka ago. When these outermost islands were glaciated, the entrance to Shuká Káa, at only 125 m above sea level (asl), would have also been covered by ice. We note that Shuká Káa is a slightly up flowline (~30 km), and thus may have been covered sooner, and uncovered later than our study sites farther west. Regardless, given the close correspondence in timing of the 10Be ages and the end of the 14C hiatus, we interpret the gap in fossilized cave bone ages from 19.8 to 17.2 cal ka BP to reflect ice cover over Shuká Káa. Together, our 14C and 10Be chronologies suggest that the maximum extent of the CIS in southeastern Alaska occurred between ~20 and 17 ka ago.
Rising ocean temperatures in the North Pacific between ~17 and 14 ka ago may have further increased the favorability of coastal regions by allowing productive kelp forests to expand, and a study of postglacial vegetation on nearby Haida Gwaii suggests that the region supported abundant plant resources that could have been used by humans migrating south along the coast.
The multiproxy data set presented here provides constraints on the maximum latest Pleistocene extent of the CIS in a bottleneck region of the Pacific coastal corridor, and our results highlight the importance of generating direct records of ice margin change along the hypothesized coastal migration route. We demonstrate that currently exposed land surfaces on Dall, Suemez, and Warren islands were glaciated between ~20 and 17 ka ago and therefore did not serve as refugia during the LGM. However, deglaciation of these islands after 17 ka ago ensured that an open, productive corridor through the southern Alexander Archipelago was available for human coastal migration. This corridor may have been connected to other recently deglaciated landscapes elsewhere along the northeastern Pacific coast. Although evaluating hypotheses about the peopling of the Americas ultimately requires archeological evidence of human occupation along potential migration routes, our results support the hypothesis that the Pacific coastal corridor was a viable pathway for the initial colonization.
(Source: “Deglaciation of the Pacific coastal corridor directly preceded the human colonization of the Americas”, by Alia J. Lesnek, Jason P. Briner, Charlotte Lindqvist, James F. Baichtal and Timothy H. Heaton)
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