Speculated is, that the first point of entry for humans into America was the Bering land bridge, connecting what is nowadays Russia and Alaska thanks to the low sea level back then (between 40000 and 17000 years ago). After the sea level rose, its not like we couldn't find a way in anymore, right? Were there just not enough trade routes / too little people to keep the contact between Asia and America going and after some time we just forgot? Did Chinese/Japanese know America existed?