Corridor thought to have facilitated human migration to Americas was not habitable

DNA analysis shows the area lacked plant and animal life needed for human survival.

At the end of the last ice age, retreating ice sheets opened up a corridor in western Canada. This area was long thought to be the most likely migration route through which the first human inhabitants in the Americas arrived from Siberia. However, new DNA analysis of sediment along the route shows it was devoid of plants and animals, and thus unable to support human migration, until thousands of years later. We speak with study author Mikkel Winther Pedersen about his methods and findings:

ResearchGate: What was the state of knowledge about human migration to North America before you did this study?

Mikkel Winther Pedersen:
When humans came to America and which route they used to do so has been a long-standing debate. What we know is that humans were in America south of the ice-sheets at least 14,700 years ago – probably even earlier – and that they most likely used a passage along the Pacific coast or an interior corridor on the eastern side of the Rocky Mountains between two massive ice sheets. Recent geological studies suggest that the two ice-sheets, which covered most of what is now Canada, retreated and created a corridor about 14,000 to 15,000 years ago. However, as the ice melted, large parts of the landscape were flooded by huge ice-dammed lakes. These studies also suggest that it was the middle section of the approximately 1,500 km long corridor that opened last. Despite this, we knew little about the ecology in this ice-free “bottleneck,” although it would have had a significant influence on the possibility for migrating through the ice-free corridor.

RG: What motivated you to do this study?

Pedersen: I have long been fascinated with reconstructing past environments, and especially with how ancient DNA can contribute to this line of research. So when my supervisor, Eske Willerslev, asked whether I would find this study interesting, I immediately said yes. The possibility of combining my training as a physical geographer in glaciers and environmental change, the skills in ancient DNA analysis I acquired during my master thesis, and an interesting archaeological question was too good to pass up. And of course, the fact that this also could help bring us one step closer to understanding the colonization of America was also motivating.

RG: How did you go about determining past viability and colonization in the corridor?

Pedersen: We used lakes from the bottleneck area, specifically the sediments from the bottom of these lakes. Over time, the sediment accumulates and creates a stacked archive of material. The lakes we examined formed right after the ice retreated and therefore started recording the surrounding environment. To determine the age of the sediment layers, we used 14C radiocarbon dating on plant macrofossils imbedded in the sediments. We then performed pollen, macrofossil, and ancient DNA analysis to determine which plants and animals were in the environment at a given time. Although these methods overlap in terms of taxa identified, we wanted to make the results and interpretation more robust by combining them.

Mikkel Winther Pedersen
Pedersen and a colleague prepare to core lake sediments. Credit: Mikkel Winther Pedersen

While counting pollen grains and identifying macrofossils are old, established methods for reconstructing past environments, ancient environmental DNA analysis is a relatively new method that has the potential to detect additional organisms. But, because we found that the DNA were highly damaged, we couldn’t just use the regular DNA barcode approach to look for specific genes. We therefore took a new approach by analyzing the complete pool of DNA, a technique called shotgun sequencing, instead of looking for specific genes. This approach enabled us to identify organisms in all the trophic layers, from small bacteria to large mega fauna.

RG: What were your results?

Laminated lake sediments containing molecular and fossil evidence revealing the succession of plants and animals in the ice free corridor. Credit: Mikkel Winther Pedersen
Laminated lake sediments containing molecular and fossil evidence researchers used to date the emergence of plants and animals in the ice free corridor. Credit: Mikkel Winther Pedersen

Pedersen: First, we found that the ice-free corridor that had been suggested as a possible entry route for the first groups of humans simply could not sustain a migration that would have allowed humans to reach Monte Verde 14,700 years ago. In fact, we found that later groups associated with Clovis artefacts also couldn’t have used the corridor for entering America, as the bottleneck area was not biologically viable until 12,600 years ago. Before that time, it was flooded by ice-dammed lakes, and the landmasses surrounding the lake were bare, as vegetation and animals had not colonized the newly exposed soil.

12,600 years ago, a sagebrush tundra established itself, supporting mega-fauna such bison and mammoth. From this point on, humans would have been able to traverse the corridor. In fact, not long after this point, Charlie Lake Cave was used a seasonal dwelling, presumably for hunting. At later stages we see the vegetation change and become more forested, at which point elk and moose appear in the record. By 10,500 years ago, the forest had become denser, resembling today’s landscape in the area.

RG: What would be a more likely first migration route?

Pedersen: Although which route the first humans took is still debated, much evidence points to a passage along the Pacific coast, which became ice-free earlier than the corridor. However, our results only tell us about the environment in the bottleneck area of the corridor. I cannot rule out any of the other proposed routes.

Featured image: A present day view in the area where retreating ice sheets created an ice free corridor more than 13,000 years ago. Courtesy of Mikkel Winther Pedersen.