About 100 or so years after a WORLDWIDE Flood. Probably was a land bridge btwn the Russian landmass and Alaska. When the waters settled the land bridge went underwater.
If one accepts the Biblical account in Genesis, the continents were all together in the days of Peleg, who lived sometime between Adam and Noah.
Even if one does not, the various continental drift theories of human migration leaves the continents somewhat less clearly defined than they are today. The horse, widely thought to originated in America, certainly wasn't here in the immediate pre-Columbian era.
And the Bering Straight theory of migration, while certainly likely in its explanation of the existence of some pre-Columbian peoples, doesn't explain them all. Otherwise, we would have evidence of the most advanced civilizations in oldest parts of the migration route getting more rudimentary as you moved south rather than the mirror opposite.