The question of when and how people first came to the Americas has been puzzling scientists for decades. New evidence is piling up quickly, but it does not all point to the same conclusion. The latest development involves evidence that one possible route first became passable 17,000 years ago, which happens to be close in time to the favored estimate of when the crossing was first made.
"People are fascinated by these questions of where they come from and how they got there," said Professor Jason Briner of the University at Buffalo in a statement. Considering the enormous challenges of crossing the Pacific Ocean, the arrival of the first peoples in the Americas has been a big part of that.
For a long time, it was accepted that people reached the Americas by crossing the land bridge that existed between Siberia and Alaska when sea levels were much lower and then passing through an ice-free corridor in Canada. However, multiple problems with this theory have recently emerged. In the search for alternatives, the idea of a sea crossing, followed by a coastal migration route, has gained credence.
However, at the peak of the last Ice Age, this route was also covered in glaciers. The timing of when it became accessible is crucial to the debate.
Briner and PhD student Alia Lesnek explored four islands in the Alexander Archipelago, 320 kilometers (200 miles) southeast of Juneau. The smoothed rock surfaces and boulder distribution make clear the area was once glaciated. Among other things, ice protects rocks from cosmic radiation, and once it is removed, the boulders' surface chemistry starts to change from the bombardment of cosmic rays. Analysis of the top layers reveals the length of the rocks' exposure.
Bedrock could have been exposed during previous interglacial periods, but boulders known as erratics, which were swept up by the glaciers and deposited in new locations, provide a reliable indication of the most recent ice-free period. Briner and Lesnek report this to have been 17,000 years ago in Science Advances. Coastal areas further south presumably became ice-free even earlier. Confirmation comes from previously discovered seal bones from a nearby cave being dated as the same age.
Since most estimates of the timing of Native Americans' arrival are now hovering around 16,000-18,000 years ago, these pieces seem to be coming together, at least if we ignore the anomalous evidence for our presence at one Canadian site 7,000 years earlier.
The findings don't prove that the 250 people from whom all Indigenous Americans descend took the coastal route, but they suggest it was possible. Meanwhile, with increasing evidence that Canada's “ice-free” inland corridor was not capable of supporting much life until substantially later, the coastal route theory looks like an increasingly likely alternative.