First Americans: no ice-free entry

Thousands of years ago, scientists believe, the first peoples migrated across a broad land bridge connecting Siberia to Alaska. From there, they spread southward throughout the Americas.

But how, during the height of the Ice Age, did they get from Alaska to points south? The land bridge between Siberia and Alaska (often called Beringia, because much of it is now beneath the Bering Sea) was ice free. But to the south was a giant wall of ice, spreading all the way across Canada. 

This icy barrier, however, had a weak link, a seam between its eastern and western ice sheets that was known to have “unzipped” late in the Ice Age, creating an “ice-free corridor” through which archaeologists presumed ancient peoples could have migrated down the east side of the Canadian Rockies.

A study in today’s Proceedings of the National Academy of Sciences, however, appears to put the kibosh on that theory. 

The ice-free corridor, reports a team led by Jorie Clark, a geologist at Oregon State University, US, didn’t open up until 13,800 years ago – 1,800 years before archaeologists believe people were living south of the ice, in Idaho. The only way they could have gotten there via the ice-free corridor would have been with the help of a time machine. 

The find came by measuring the amount of beryllium-10 in boulders that had been ripped from their bedrock by the glaciers, then dropped on the surface when the ice melted. 

Called glacial erratics, these rocks would have been sheltered from cosmic rays during the time they were entombed in the ice. But once the ice melted, cosmic rays – a type of high-energy radiation coming in from outer space – would have begun to pelt them.

Cosmic rays penetrate quartz grains in the boulder and produce beryllium-10, so the longer the boulder has been exposed, the greater the number of these elements. “We measure the concentration of these elements in the laboratory,” Clark said. “Since we know how many new elements are produced every year, then we can calculate the time since the boulder was first exposed by retreat of the ice sheet.”

Other scientists applaud the finding. “I think it’s really important because there have been conflicting views on when the ice-free corridor was available for people to travel through,” says Jennifer Raff, an anthropological geneticist at the University of Kansas, US, and author of Origin: A Genetic History of the Americas, who was not part of the study team. 

Frank Hoffecker, an archeologist and paleoanthropologist at the University of Colorado, Boulder (also not part of the study team) agrees. “I think cosmogenic beryllium-10 dating is the way to go,” he says. “It looks as though it will finally put the ‘ice-free corridor’ to rest as a credible route for the early sites in mid-latitude North America.”

But if the first Americans didn’t come down the ice-free corridor, how did they get there? 

Most likely they came down the Pacific coast. This would have required boats, for which archaeologists have yet to find traces. But, Raff says, if the peopling of the Americas happened at about that time, the only way would have been down the coast. 

But, there’s that little word “if.” As in “if” it occurred 15,600 years or so ago, when people are known to have been in Idaho, that’s not the only option. Maybe the first people came here far earlier, before the glaciers slammed shut the overland route. If so, they wouldn’t have needed boats… but would have been on the continent 10,000 years or more before archaeologists are sure they were.

“That’s one of the main outstanding questions in the field,” Raff says: “whether or not people were in the Americas before the height of the glacial maximum.”