Archaeologists have fought for decades over how and when people first arrived in the Americas. Did they walk down a corridor between the great glaciers of western Canada about 13,500 years ago? Or did they travel by boat even earlier? New research supports an early sea arrival, by way of the Pacific coast. By dating rocks and animal bones, scientists conclude that the coast of southeastern Alaska was largely ice-free and full of plant and animal life some 17,000 years ago—a welcoming environment for people venturing south into a new world.
“It’s the kind of study we need more of,” says Quentin Mackie, an archaeologist at the University of Victoria in Canada who was not involved in the work. Ancient landscape reconstructions like this, he says, provide a good starting point for imagining how ancient peoples would have come down the coast, and where archaeologists should look for their settlements.
Until 20 years ago, the debate itself was settled: Researchers were certain that the first people to enter North America walked down an ice-free corridor in western Canada some 13,500 years ago. But many archaeological sites in the Americas have been dated even earlier, including Page-Ladson in Florida and Monte Verde in Chile. Now, most archaeologists think the first Americans left Beringia, the now-drowned land between Siberia in Russia and Alaska, about 16,000 years ago—likely before the ice-free corridor opened—and traveled by boat down the Pacific coast. But direct evidence for such a journey is lacking.
Alia Lesnek, a geologist at the University at Buffalo, part of the State University of New York system, wanted to figure out when the trip would have been possible. So she spent the summer of 2015 helicoptering between remote islands off southeastern Alaska, seeking rocks exposed to the sky. Such rocks are constantly hit by cosmic rays hurtling down from space, which change individual oxygen-16 atoms in quartz to beryllium-10 atoms, one by one. By measuring the concentration of beryllium-10, researchers can calculate how long the rock has been out in the open. When Lesnek dated the rocks from four islands along the coast of southeastern Alaska, she found that the ice covering them had melted away about 17,000 years ago—just in time for the hypothesized coastal migration, her team reports today in Science Advances. “Making the actual measurement is very, very difficult, so each one of these data points, to me, is a diamond,” says Derek Fabel, an expert in this dating technique at the Scottish Universities Environmental Research Centre in Glasgow.
Lesnek then turned her attention to nearly 200 animal bones that had been previously excavated from caves in southeastern Alaska. The bones, from carnivores and their prey, had been dated using radiocarbon in the 1990s, but their ages hadn’t been corrected for the fact that most of the animals lived in the ocean or ate lots of seafood, which complicates radiocarbon estimates. Lesnek and her colleagues used the latest research to account for the effect of marine diets on radiocarbon. They dated their finds between 45,000 years ago and the present. But there was a notable gap between about 20,000 years ago and 17,000 years ago, implying that the area was covered in ice during that time.
That ice “may have been in and out quite quickly,” says Duane Froese, a geologist at the University of Alberta in Edmonton, Canada, who wasn’t involved in the research. “Ten, 20, 50 years after the glaciers retreat, you could have a kind of tundra,” Mackie adds. “And that makes it more attractive for someone to get out of their boat.”
Still, John Ives, an archaeologist also at the University of Alberta, warns against counting out the ice-free corridor just yet. New earth science research shows that the land in the corridor was exposed 15,000 years ago, and more work is forthcoming. Even if people first entered the Americas along the coast, the ice-free corridor still could have been used by another wave of travelers, perhaps even people moving from south to north. “We really ought to expect both the coastal and corridor pathways to have been significant, at different times and in different ways,” Ives says.