North America’s First People May Have Arrived on Sea Ice 24,000 Years Ago

Some of the earliest North Americans may have arrived on the continent using a “sea-ice highway” as far back as around 24,000 years ago.

The findings of the research, which was presented on Friday at the American Geophysical Union Annual Meeting (AGU23) in San Francisco, shed new light on the peopling of the Americas—one of the hottest debates in archaeology.

For decades, experts argued that people first arrived in North America around 13,000 years ago by crossing a land bridge known as Beringia that connected what is now Alaska with the northeastern tip of the Asian continent. According to the hypothesis, these people then spread further south through an ice-free corridor that briefly opened up between two massive ice sheets that covered most of the northern parts of North America at that time.

But mounting archaeological and genetic evidence now suggests that humans had arrived in North America at least around 16,000 years ago, and possibly as early or even prior to the Last Glacial Maximum (LGM). This was a period in Earth’s history when ice sheets and glaciers reached their greatest extent during the last ice age.

Evidence indicating human settlement of the Americas earlier than 16,000 years ago has often been met with skepticism, although recent finds, such as the dating of preserved human footprints in New Mexico to between 23-21,000 years ago, are helping to rewrite the story.

People arriving from northeast Asia between around 26,000 and 14,000 years ago would likely have gradually moved down into North America along the Pacific Coast, following the bountiful resources found in the coastal waters.

This is because the interior route would have been blocked in this period. The ice-free corridor between the two massive sheets is not thought to have completely opened until around 13,800 years ago, according to a 2022 study published in the journal PNAS.

However, environmental conditions along the Pacific Coast route would have been “challenging” during the late Pleistocene—the geological epoch that lasted from around 2.58 million years ago to 12,000 years ago—according to the authors of the research presented on Friday.

Research published in 2020, for example, suggested that freshwater from melting glaciers in the latter part of the Pleistocene may have created a strong current that would make it difficult for people to travel along the coast.

“It remains unclear whether the coastal route was passable throughout this period, or if there were times when movement was blocked by marine-terminating glaciers, strong ocean currents, and/or prolonged sea ice conditions,” the researchers wrote in their study.

In order to shed light on this crucial period of human migration, Summer Praetorius of the U.S. Geological Survey and colleagues recreated the ancient climate of the Pacific Northwest during the late Pleistocene using computer simulation models. Their aim was to assess viable time periods in which humans could have traversed the coastal corridor.

The findings of the study indicate that sea ice may have been one way for people to move farther south.

In creating their computer models, the researchers used data collected from ocean sediments along the coast, which served as climate proxies. Most of the data came from tiny, fossilized plankton, which helped the scientists reconstruct ancient ocean temperatures, salinity and sea ice cover.

Praetorious and colleagues found that ocean currents were more than twice the strength they are today during the height of the LGM around 20,000 years ago. While it would not have been impossible to paddle against these currents, the conditions would have made traveling by boat along the coastal route very difficult, according to the study.

But the findings also suggest that the coastal region would have been home to sea ice in winter until around 15,000 years ago. Early Americans may have used this “sea ice highway” to get around and hunt marine mammals, gradually making their way south in the process. This practice is similar to that of modern Arctic people who travel along sea ice on dog sleds and snowmobiles.

“Rather than having to paddle against this horrible glacial current, maybe they were using the sea ice as a platform,” Praetorius said in a press release.

The researchers found that conditions in the periods spanning 24,500-22,000 years ago and 16,400-14,800 years ago would have been most conducive to early migration along the Alaskan coast, possibly aided by the presence of winter sea ice.

Finding concrete evidence that people were using the sea ice to travel will be challenging given that any archaeological remains potentially left behind by this migration will now be underwater. However, the latest findings provide a new framework for understanding how humans may have arrived in North America without a land bridge or easy ocean travel.

The team’s models also showed that the currents of the coast of Alaska had likely calmed down by around 14,000 years ago, meaning it would have been easier to travel along the coast after this period.

“Nothing is off the table,” Praetorius said. “We will always be surprised by ancient human ingenuity.”