in 1940, they could treat wood to make it water proof, had nails, sails, and food preservation. Cave man era had nothing of the like.
The only civilization that, IMO, could make it to the americas was the phoenicians. The only possible route would have to be off an island in west africa then over to the point of brazil or to florida. But then why didnt the phoenicians or egyptians try and conquer and settle the lands?