Well it's unlikely we'll ever know for sure when the first humans made it to the Americas (currently the two theories make first arrival over any time between 20, 000 years ago to 40, 000), but it would have certainly been across the Bering Strait. It's believed there was a land bridge we call "Beringia" after the strait, but even if there wasn't, to this day the Bering Strait freezes solid, connecting eastern Asia to northern America. But I'm sure someone can shed more light on this.