http://www.newsweek.com/new-evidence-theory-how-ancient-humans-first-came-america-950068

Humans have always been good at spreading out and covering ground, but we can’t tackle every obstacle in our path. Giant ice sheets that spread for miles in every direction, for example, tend to stop all but the most adventurous of us quite efficiently—and they likely shaped the path humans took to first enter North America from Asia.

Scientists have been pondering two possible routes for decades. One crawls along the Pacific Coast, weaving between the islands of southern Alaska; the other slips between two giant ice sheets, much further east and inland. In order to help figure out how plausible the first route is, a team of scientists wanted to measure how long ago ice sheets retreated from the coastline—and now they've done just that, as they report in a new paper published in the journal Science Advances. The new research suggests the route would have been accessible about 17,000 years ago.