Mikkel Winther Pedersen

Mikkel Winther Pedersen

Mikkel Winther Pedersen

Mikkel Winther Pedersen

Mikkel Winther Pedersen

The standard story of how humans arrived in the Americas is that they marched 1,500km across the Bering Land Bridge, a now-vanished landmass between Siberia and Northern Canada. From there, they moved through a passage between the ice sheets that covered most of Canada. This route emerged roughly 15,000 years ago near the end of the last ice age.

But for the past decade, evidence has been piling up that humans arrived in the Americas by traveling in boats along the Pacific coast. Some 14,000-year-old campsites like Oregon's Paisley Caves have been found near rivers that meet the Pacific, suggesting that early humans came inland from the coast along these waterways.

Now, a new study published in Nature provides more solid evidence the first humans to reach the Americas could not have come along this route. The passage between the ice sheets, it appears, was simply not open early enough.

A group of geoscientists, anthropologists, and biologists led a massive effort to study the "ice free corridor," an environment in Northern Canada near the Bering Land Bridge. If the first humans to arrive in the American interior had come by land, they would have walked through this area 15,000 years ago.

The researchers used a common method for sampling ancient environments called coring. Using hollow tubes, they drilled deep into the sediment at the bottom of two frozen lakes in British Columbia, looking for fossils of plant and animal life from the era when humans might have first arrived. They picked these two specific lakes—Charlie Lake and Spring Lake, to be exact—because they were in the ice free corridor, a passageway into the Americas that opened as the ice sheets melted. The very first humans to pass into the Americas would have had to cross through this area had their migration taken place entirely by land.

Carefully analyzing the layers of sediment, the researchers were able to determine what kind of life inhabited the region. Radiocarbon dating allowed them to recreate a timeline for the ancient ecosystem there, too. In their paper, the researchers write:

We obtained radiocarbon dates, pollen, macrofossils, and metagenomic DNA from lake sediment cores in a bottleneck portion of the corridor. We find evidence of steppe vegetation, bison, and mammoth by approximately 12.6 cal. kyr BP, followed by open forest, with evidence of moose and elk at about 11.5 cal. kyr BP, and boreal forest approximately 10 cal. kyr BP. Our findings reveal that the first Americans, whether Clovis or earlier groups in unglaciated North America before 12.6 cal. kyr BP, are unlikely to have travelled by this route into the Americas. However, later groups may have used this north–south passageway.

These observations pretty clearly show that the first humans in the Americas couldn’t have come through this passage. We already have ample evidence of humans living throughout the Americas by 14,000 years ago. But to get from Beringia to the Americas, humans would have had to pass through the ice free corridor, which could not have supported a mass human migration on foot until at least 12,500 years ago, when the area had enough animals and vegetation for the humans to use as food and shelter. Before 12,500 years ago, the area was either a sterile landmass, still recovering from its millennia beneath the ice sheets, or (a bit earlier) still covered in ice.

This means that the first humans almost certainly skirted the ice sheets by traveling along the coast using boats. And, if they did that part of the trip in boats, there’s a good chance that the earlier portion of the trip, along the Bering land bridge, also took place in boats.

Humans probably did move from the Bering Land Bridge into the Americas once the ice free passage became available 12,500 years ago, but they would have arrived on a continent that was already populated with people who probably came along the Pacific coast thousands of years before. While it may sound improbable that humans could take boats along the coast from Asia, consider that humans arrived in Australia by boat, island hopping from Asia about 50,000 years ago.

Boat technology is one of our most ancient inventions, and it would have worked admirably for people who were using the vessels to go short distances along the coast, carrying supplies. Considered in this light, it shouldn’t surprise us if humans reached the Americas partly because they had developed fairly sophisticated transportation technology.

Nature, 2016. DOI: 10.1038/nature19085

Note: This article was corrected to clarify the routes of migration involving the ice free corridor and Bering land bridge.

Listing image by Mikkel Winther Pedersen