Isolationism is the theory that bodies of water have separated all cultures until recently. Diffusionism is the theory that Humans have spent a lot more time on the ocean than just for fishing.
For a long time is was thought that the first Boats were Canoes and then
from there people added extra walls to keep water out, and that
eventually became a hull and a deck etc. (like Noah's Ark) But then it
was discovered that the first boats were actually more like Rafts, and
the goal was not to keep water out, but just to be as Buoyant as
Then it was thought for a long time (and some people still think today)
that Oceans were a massive obstacle that could never have been crossed
until Navigation evolved and Boats became "Ships". But Thor Heyerdahl
proved that wrong. He took 3 trips across the Ocean, 1 failed, but 2
succeeded, and he did it on a Raft the 1st time and a Reed (Grass) Boat
the 2nd time. And he copied the designs of Ancient Boats.
Here is his book called "Early Man and the Ocean":
The Aboriginals in and around Australia made their way to Australia and
Papau New Guinea over 70,000 years ago, the Paupaun people have ancient
DNA from a species of people known as "Denisovan". There is also a Dog
there that is a species of its own known as a "Singing Dog".
Heyerdahl proved that people can cross the ocean in ancient boats, but
there was still (and sometimes still are) people that think "The Ocean
was still far too expansive for Ancient man to even TRY to test it". But
then there are things like the Kula Ring, which the Paupaun people
still do today. What this is is a trade ring where they make dangerous
trips in small boats just to trade trinkets, and as they trade trinkets
they get more status between tribes. So it is not a large leap to say
that someone might load up on trinkets and make their way as far as they
possibly can, then just getting there and teaching an entirely new
tribe about new things. Like Boats, or a New Metals, or Jewelry, or New
Animals, or Double Headed Axes, etc.
Yam/Sweet Potato is a perfect example of a Plant that made its way
around the entire planet, and no one knows how. It's just everywhere.