The ancients had the physical technology for worldwide travel. The problem is that proving that travel was actually done is pretty difficult.
Why would the Central Americans carve in stone on their monuments "we got the idea for this pyramid from stories told by travelers"?
Lewis and Clarke traveled a tremendous distance using technology that was basically available to ancient humans thousands of years ago. Yes, they had firearms, but this only reduced the hunting workload. The ancients still could have done such things over a slightly longer time frame, or with more hunters.
This is all merely speculation, but I have little doubt it's true.
Nope. All their ancestors (on all the continents) came from the same place...Sundaland. (it probably was Atlantis too)