Evidence from the Cooper's Ferry archaeological site in Western Idaho shows that people lived in the Columbia River Basin around 16,000 years ago. That's well before a corridor between ice sheets opened up, clearing an inland route south from the Bering land bridge. That suggests that people migrated south along the Pacific coast. Stone tools from the site suggest a possible connection between these first Americans and Northeast Asian hunter-gatherers from the same period.

Route closed due to ice

A piece of charcoal unearthed in the lowest layer of sediment that contains artifacts is between 15,945 and 15,335 years old, according to radiocarbon dating. More charcoal, from the remains of an ancient hearth pit, dated to between 14,075 and 15,195 years old. A few other pieces of bone and charcoal returned radiocarbon dates in the 14,000- to 15,500-year-old range. In higher, more recent layers, archaeologists found bone and charcoal as recent as 8,000 years old, with a range of dates in between.

This makes clear that people had been using the Cooper's Ferry site for a very long time, but it's hard to say whether they stuck around or just kept coming back. "Because we did not excavate the entire site, it is difficult to know if people occupied the site continuously starting at 16,000 years ago," Oregon State University archaeologist Loren Davis told Ars. "I expect that this site was used on a seasonal basis, perhaps as a base camp for hunting, gathering, and fishing activities."

Either way, the local Nimiipuu (Nez Perce) people know the site as the location of an ancient village called Nipéhe. "We worked with archaeologists and student interns from the Nez Perce Tribe who visited to get tours of the excavation and to participate in excavations at the site," said Davis.

Davis and his colleagues used a statistical model to calculate how old the very oldest layers of artifacts at the site should be. "The Bayesian model makes predictions about the age of the lower portion of [the excavated layers] based on the chronological trend of known radiocarbon ages in the upper and middle third," Davis explained. According to the model, the very oldest artifacts at Nipéhe are probably between 16,560 and 15,280 years old.

That's about 2,000 to 1,500 years before the great continent-spanning ice sheets of the Pleistocene began to break up. That break-up opened an ice-free corridor southward from the Bering land bridge between the towering sides of the Cordilleran and Laurentian ice sheets. According to computer simulations, that corridor was closed and buried under several kilometers of ice until at least 14,800 years ago, and possibly even later. And that has some important implications for when, and how, people first set foot in the Americas.

The coastal route

If the ice-free corridor wasn't open, the only way to get south of the ice sheets would have been to skirt along the Pacific coast on foot or by boat, moving among locations where the edges of the 4km (2.5 miles) thick glaciers didn't quite reach the Pacific Ocean. Much of Ice Age coastline is now underwater, largely thanks to the melting of those huge glaciers. But there have been a few recent archaeological finds that support the idea that the first humans in the Americans moved south along the coast much earlier than previously thought.

Genetic evidence, which uses predictable rates of genetic mutations to tell how long ago populations separated from each other, suggests that sometime between 17,500 and 14,600 years ago, the people living south of the ice sheets split up into two major groups, which moved generally northward and generally southward. That lines up well with the timing at Nipéhe.

At this point, there's not really much debate about whether people had arrived in the Americas before the rise of the Clovis culture, the collection of tools and weapons once thought to represent the oldest human activity in the Americas. Clovis appears starting around 13,250 years ago, so some groups were clearly present earlier. Most of the debate now is focused on the route these earlier people took to reach the thawed, habitable parts of North America.

Davis and his colleagues say Nipéhe is strong evidence for the coastal route. "This does not preclude subsequent human migrations through the [ice-free corridor] at a later time, as suggested by paleogenomics," they wrote, "but such possible population movements do not represent the initial peopling of the Americas."

A Japanese connection?

Buried in the Ice Age layers at Nipéhe, Davis and his colleagues found animal bones and discarded stone tools, including bifaces (two-sided handaxes; think of them as prehistoric multi-tools), blades, sharp stone flakes, and fragments of two projectile points. The tool collection didn't look a thing like the fluted projectile points that have become the archaeological calling card of the Clovis culture.

To make a Clovis-style projectile point, the flint-knapper has to chip off a flake from one or both faces at a point right at the base of the object. That creates a small groove (also called a flute), which makes it easier to fit the point onto the shaft of a spear or arrow. But at Nipéhe (and at a few other pre-Clovis sites in the Americas), people took the opposite approach: they shaped the base of the point into a stem to attach to the spear or arrow shaft. Some of the younger stone tools from Nipéhe are about the same age as the Clovis culture, but they're clearly a separate technology.

Stemmed projectile points aren't a recent technology, even by archaeological standards; people figured out that stems made points easier to haft by around 50,000 years ago in Africa, Asia, and the Levant. But there are different ways to shape a chunk of flint into a stemmed point, and the ones at Nipéhe look strikingly similar to stemmed points from Northeast Asia. Similarities are especially strong with items from the Japanese island of Hokkaido, which have turned up at sites dating between 16,000 and 13,000 years ago. (As an interesting side note, stemmed projectile points from a 13,500-year-old site in Kamchatka, in east Russia, were made with a distinctly different style.)

Davis et al. 2019

Loren Davis

Loren Davis

Davis et al. 2019

Loren Davis

Loren Davis

Loren Davis

Loren Davis

Loren Davis

Other aspects of the stone tools at Nipéhe also resemble the ones being made and used on Hokkaido at around the same time and slightly earlier. Davis and his colleagues claim that similarity is no coincidence. They suggest that the similar stone tool technology is evidence of a cultural link between the earliest Americans—who arrived on the Pacific coast and migrated southward before moving inland south of the ice sheets—and people in Northeastern Asia.

The dates line up well; many of the Hokkaido sites with stemmed points are older than Nipéhe, while others are around the same age. That suggests that it's possible for the culture to have originated in Japan and then spread to North America—although it's impossible to guess how many generations removed the people of Nipéhe may have been from their relatives in Hokkaido by the time they dug their hearth pit in Western Idaho.

Davis told Ars that archaeologists need to consider the possibility that two distant cultures happened to come up with the same stone-tool-making techniques at around the same time. But that seems unlikely. "These archaeological patterns require further study," he and his colleagues acknowledged in their paper.

Science, 2019. DOI: 10.1126/science.aax9830 (About DOIs).