[nkoren]

Also, the fact that many Polynesian languages used nearly the same word for "sweet potato" as some South American languages (kuumala -- Polynesian vs. kumara/cumal -- Quechua) is kind of a clue, yes?

The fact that the "vegetation raft" theory persisted for more than a few seconds in the face of this knowledge always astonished me.

[eythian]

Not just nearly the same. It's literally kūmara in Māori.