Got no source at hand, but i remember reading something about the old maps which Columbus & Co. used. On the other hand: while analyzing the body of one of the Pharaos the traces of "coca" have been found, - a plant originated ( only ) from South America. Those are two infos coming to my mind, with no scientific sources or support material, though. And iirc, the knowledge about the fact, that the Earth is a ball is quite old, and that information would be enough encouragement for some intrepid explorers of the ancient times to accept the challenge and find out, what´s out there, imho.