I always thought it was kinda obvious. I mean there is a big ocean that separates, but don't underestimate the power of what humans are capable of doing. Isn't there a theory out there that Chinese sailors made it to the Americas too? Like present day Mexico?
"Pre-Columbian trans-oceanic contact theories speculate about possible visits to or interactions with the Americas, the indigenous peoples of the Americas, or both, by people from Africa, Asia, Europe, or Oceania at a time prior to Christopher Columbus' first voyage to the Caribbean in 1492 (i.e., during any part of the pre-Columbian era)."
https://books.google.com/books?id=lmvUBAAAQBAJ&pg=PT9#v=onepage&q&f=false
Man Across the Sea