Prehistoric Alaska begins with Paleolithic people moving into northwestern North America sometime between 40,000 and 15,000 years ago across the Bering Land Bridge in western Alaska; a date less than 20,000 years ago is most likely. They found their passage blocked by a huge sheet of ice until a temporary recession in the Wisconsin glaciation (the last ice age) opened up an ice-free corridor through northwestern Canada, possibly allowing bands to fan out throughout the rest of the continent. Eventually, Alaska became populated by the Inuit and a variety of Native American groups. Trade with both Asia and southern tribes was active even before the advent of Europeans.