How did early humans first enter the Americas?
After crossing into Alaska, the ice age adventurers may have trekked along two routes: either by foot through the interior of present-day Canada through a grassy passageway between two large ice sheets, or they moved south along the Pacific Coast.
Scientists have debated the two theories, and in recent years support for the coastal route has grown from archaeological finds, such as 13,000-year-old footprints on an island in British Columbia.
Now, geologists studying boulders and bedrock on Alaska's southeastern islands have found evidence of an ice-free route some 17,000 years ago down the coast that would have allowed human travel.
"We're not definitively saying they took the coastal route," said Alia Lesnek, a graduate student at the University at Buffalo and lead author of the study. "We have some of the first direct evidence that that was something that could be done."
The finding, published Wednesday in the journal Science Advances, supports the theory that the first people to populate the Americas were seafarers traveling from island to island.
In the summer of 2015, Lesnek hopped out of a helicopter into a grassy valley on Baker Island in Southeast Alaska. There, she spotted a large gray boulder that to most people may have appeared unremarkable. But to Lesnek, the rock's smooth surface and rounded edges were clues to its ancient past: It had been plopped onto the landscape thousands of years earlier by giant glaciers.
She took out a power saw with a blade as wide as a grapefruit and with two hands cut into the rock. "It's exciting, but a little bit nerve-racking because it spins really fast and makes a loud noise like 'Reeeiiiinnnn,'" Lesnek said.
After making a chip a couple inches deep, she used a sledgehammer and chisel to knock the surface piece loose. It was one of the many boulder and bedrock samples she and her colleagues collected from four islands in the Alexander Archipelago of Southeast Alaska.
Back at the lab, the team determined how long ago the rock samples had been trapped by ice sheets. Glaciers are like slow-moving rivers that pick up rocks and move them. When the ice melts, the boulders are dropped. As they sit on the Earth's surface, they are exposed to cosmic radiation, which the scientists can analyze.
"It's kind of like a rock sunburn," Lesnek said.
The team concluded the islands had been covered by ice sheets up until about 15,000 to 17,000 years ago. The finding suggests that the glaciers covering that part of the Pacific Coast melted and possibly created a pathway for humans at the right time.
The dating coincides with recently discovered archaeological and genetic evidence that suggests the first pulse of human migration into the Americas was around 16,000 years ago, the team said. The ice sheets covering Canada's inland corridor did not melt until 13,000 to 14,000 years ago, according to Jason Briner, a geologist at the University at Buffalo and an author on the study.
"Our data suggest the coastal route became available 17,000 years ago," Briner said. "That's like 4,000 to 3,000 years earlier than when the inland route opened."
The team also dated some seal bones that had been previously discovered in a coastal cave and found that the animals were present around 17,000 years ago. The bones suggested that if people had taken the coastal route they would have found food.
Briner said their study only looked at about 10 percent of the entire coastal corridor, and that future work will aim to apply the same dating methods on other parts of the route.
"The dates are concordant with other lines of evidence and reliably interpreted," said E. James Dixon, an anthropologist and emeritus professor at the University of New Mexico who was not involved in the paper. "Although the research does not prove the coastal migration hypothesis, it certainly strengthens it."