Is the Ice-Free Corridor an Early Pathway into Americas?

Robson Glacier view from Mumm Basin
Robson Glacier view from Mumm Basin near the Continental Divide in Alberta, Canada. Dubicki Photography / Getty Images

The Ice-Free Corridor hypothesis (or IFC) has been a reasonable theory for how human colonization of the American continents occurred since at least the 1930s. The earliest mention of the possibility was arguably the 16th-century Spanish Jesuit scholar Fray Jose de Acosta who suggested that Native Americans must have walked across dry land from Asia.

In 1840, Louis Agassiz put forward his theory that the continents had been covered by glacial ice at several points in our ancient history. After dates for the last time that occurred became available in the 20th century, archaeologists such as W.A. Johnson and Marie Wormington were actively seeking a way by which humans could possibly have entered North America from Asia when ice covered most of Canada. Essentially, these scholars suggested that Clovis culture hunters—then considered the earliest arrivals in North America—arrived by chasing after now-extinct large-bodied versions of elephant and buffalo following an open corridor between the ice slabs. The route of the corridor, since identified, crossed what is now the provinces of Alberta and eastern British Columbia, between the Laurentide and Cordilleran ice masses.

The Ice-Free Corridor's existence and usefulness for human colonization are not questioned: but the latest theories about the timing of human colonization have seemingly ruled it out as the first pathway taken by people arriving from Beringea and northeastern Siberia.

Questioning the Ice-Free Corridor

Map of the Ice-Free Corridor
Map outlining the opening of the human migration routes in North America revealed by the results presented in this study.  Mikkel Winther Pedersen

In the early 1980s, modern vertebrate paleontology and geology were applied to the question. Studies showed that various portions of the IFC were in fact blocked by ice from between 30,000 to at least 11,500 calendar years ago (cal BP): that would have been during and for a long while after the Last Glacial Maximum. Clovis sites in North America date to about 13,400–12,800 cal BP; so somehow Clovis had to arrive in North America using a different path.

Further doubts about the corridor began to arise in the late 1980s when pre-Clovis sites—sites older than even 13,400 years (such as Monte Verde in Chile)—began to be supported by the archaeological community. Clearly, people who lived in far southern Chile 15,000 years ago could not have used the ice-free corridor to get there. 

The oldest confirmed human occupation site known within the main route of the corridor is in northern British Columbia: Charlie Lake Cave (12,500 cal BP), where the recovery of both southern bison bone and Clovis-like projectile points suggest that these colonists arrived from the south, and not from the north.

Clovis and the Ice Free Corridor

Recent archaeological studies in eastern Beringia, as well as detailed mapping of the route of the Ice Free Corridor, have led researchers to recognize that a passable opening between the ice sheets did exist beginning circa 14,000 cal BP (ca. 12,000 RCYBP). The passable opening was likely only partially ice-free, so it is sometimes called the "western interior corridor" or "deglaciation corridor" in the scientific literature. While still too late to represent a passageway for pre-Clovis people, the Ice-Free Corridor may well have been the main route taken by Clovis hunter-gatherers moving from the Plains up into the Canadian shield. Recent scholarship seems to suggest that the Clovis big-game hunting strategy originated in the central Plains of what is today the United States and then followed bison and then reindeer northward.

An alternative route for the first colonists has been proposed along the Pacific coast, which would have been ice-free and available for migration for pre-Clovis explorers in boats or along the shoreline. The change of path is both affected by and affects our comprehension of the earliest colonists in the Americas: rather than Clovis 'big game hunters,' the earliest Americans ("pre-Clovis") are now thought to have used a broad variety of food sources, including hunting, gathering, and fishing.

Some scholars such as American archaeologist Ben Potter and colleagues have pointed out, however, that hunters could well have followed ice margins and successfully crossed ice: the viability of the ICF is not ruled out.

Bluefish Caves and its Implications

Horse Mandible from Bluefish Caves #2
This horse mandible from Bluefish Cave 2 shows a number of cut marks on the lingual surface. They show the animal's tongue was cut out with a stone tool.  Université de Montréal

All of the accepted archaeological sites that have been identified in the IFC are younger than 13,400 cal BP, which is the watershed period for Clovis hunters and gatherers. There is one exception: Bluefish Caves, located at the northern end, Canada's Yukon Territory near the border with Alaska. Bluefish Caves are three small karstic cavities which each have a thick layer of loess, and they were excavated between 1977 and 1987 by Canadian archaeologist Jacques Cinq-Mars. The loess contained stone tools and animal bones, an assemblage that is similar to Dyuktai culture in eastern Siberia which itself dates at least as early as16,000–15,000 cal BP.

Reanalysis of the bone assemblage from the site by Canadian archaeologist Lauriane Bourgeon and colleagues included AMS radiocarbon dates on cut-marked bone samples. These results indicate that the site's earliest occupation dates to 24,000 cal BP (19,650 +/- 130 RCYPB), making it the oldest known archaeological site in the Americas. The radiocarbon dates also support the Beringian standstill hypothesis. The Ice-Free Corridor would not have been open at this early date, suggesting that the first colonists from Beringia likely dispersed along the Pacific coastline.

While the archaeological community is still somewhat divided about the reality and characterization of many archaeological sites that pre-date Clovis, Bluefish Caves is compelling support for a pre-Clovis entry into North America along the Pacific coast.

Sources

Bourgeon, Lauriane, Ariane Burke, and Thomas Higham. "Earliest Human Presence in North America Dated to the Last Glacial Maximum: New Radiocarbon Dates from Bluefish Caves, Canada." PLOS ONE 12.1 (2017): e0169486. Print.

Dawe, Robert J., and Marcel Kornfeld. "Nunataks and Valley Glaciers: Over the Mountains and through the Ice." Quaternary International 444 (2017): 56-71. Print.

Heintzman, Peter D., et al. "Bison Phylogeography Constrains Dispersal and Viability of the Ice Free Corridor in Western Canada." Proceedings of the National Academy of Sciences 113.29 (2016): 8057-63. Print.

Llamas, Bastien, et al. "Ancient Mitochondrial DNA Provides High-Resolution Time Scale of the Peopling of the Americas." Science Advances 2.4 (2016). Print.

Pedersen, Mikkel W., et al. "Postglacial Viability and Colonization in North America’s Ice-Free Corridor." Nature 537 (2016): 45. Print.

Potter, Ben A., et al. "Early Colonization of Beringia and Northern North America: Chronology, Routes, and Adaptive Strategies." Quaternary International 444 (2017): 36-55. Print.

Smith, Heather L., and Ted Goebel. "Origins and Spread of Fluted-Point Technology in the Canadian Ice-Free Corridor and Eastern Beringia." Proceedings of the National Academy of Sciences 115.16 (2018): 4116-21. Print.

Waguespack, Nicole M. "Why We’re Still Arguing About the Pleistocene Occupation of the Americas." Evolutionary Anthropology 16.63-74 (2007). Print.