How did early humans first enter America?
After crossing into Alaska, the Ice Age adventurers may have hiked in two ways: either walking through the interior of today's Canada through a grassy passage between two large ice sheets, or they were moving south along the Pacific Coast.
Scientists have discussed 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, 17,000 years ago, geologists exploring boulders and bedrock on the southeastern islands of Alaska found traces of an ice-free route that would have allowed human travel.
"We definitely do not say that they have taken the coastal path," said Alia Lesnek, a graduate student at the University of Buffalo and lead author of the study. "We have some of the first direct evidence that this is something that could be done."
The result, published Wednesday in the Science Advances journal, supports the theory that the first people to populate America were sailors, island-to-island travelers.
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In the summer of 2015, Miss Lesnek jumped from a helicopter into a grassy valley on Baker Island in southeastern Alaska. There she discovered a large gray boulder that seemed inconspicuous to most people. But for Mrs. Lesnek, the smooth surface and rounded edges of the rock were clues to his ancient past: she'd been flopped onto the landscape thousands of years ago by giant glaciers.
She took a chainsaw with such a big blade out of a grapefruit and cut it into the rock with two hands. "It's exciting, but a bit nerve wracking, because it's spinning very fast and making a loud noise like & # 39; reeeiininnnn & # 39; Ms. Lesnek said.
After making a chip a few inches deep, she used a sledgehammer and a chisel to strike the surface piece. It was one of the many rock and rock samples she and her colleagues collected from four different islands in the Alexander Archipelago in southeastern Alaska.
Back in the lab, the team decided How long have the rock samples been trapped by ice sheets? Glaciers are like slow-moving rivers that pick up and move rocks. When the ice melts, the boulders fall. When they sit on the surface of the earth, they are exposed to the cosmic radiation that the scientists can analyze.
"It's like a sunburn," said Mrs. Lesnek.
The team concluded that the islands were covered with ice about 15,000 to 17,000 years ago. The result suggests that the glaciers that covered this part of the Pacific coast had melted and possibly created a path for humans at the right time.
The dating agrees with recently discovered archaeological and genetic evidence suggesting that the first impulse of human migration in America occurred about 16,000 years ago, the team said. The ice cover that covered Canada's Inland Corridor melted only 13,000 to 14,000 years ago, according to Jason Briner, a geologist at the University of Buffalo and author of the study.
"Our data suggest that the coastal route arose 17,000 years ago" Dr. Briner said: "That's 4000 to 3000 years earlier than at the opening of the domestic route."
Earlier Reports of the Colonization of America
The team also dated some seal bones, previously in a coastal cave, noting that the animals were present about 17,000 years ago. The bones suggested that if humans had taken the coastal road, they would have found food.
Dr. Briner said her study will only look at about 10 percent of the entire coastal corridor and that future work will aim to apply the same dating methods to other parts of the route.
"The data is consistent with other lines of evidence and reliably interpreted," said E. James Dixon, an anthropologist and professor emeritus at the University of New Mexico who was not involved in the paper. "Although research does not prove the coastal migration hypothesis, it does strengthen it."