It was William Tuning, author of Fuzzy Bones, who took the space-traveling-Fuzzy theory and ran with it.

Lucas said in some print interview once that indeed, the ewoks were supposed to have been wookies originally.

Which does make some sense, as Wookie slave labour was used to construct the first Death Star. It certainly would have made the battle to capture the shield generator less uncertain. But as the films progressed, the inherent incongruity became apparent even to Lucas. Wookies are technologically-advanced beings, capable of piloting starships, wielding energy weapons, and repairing the delicate workings of overly-prissy protocol droids. Their civilization isn't likely to be stone-age.

So we ended up with a new race of primitive, fuzzy tree-dwellers on a forest world, just because Lucas didn't want to abandon the fanciful element of "cavemen vs. astronauts".