I didn't see 6 limbs on the flyers--only 4. And the primate-like ro-lemurs (sp?) had only 4 limbs as well.

Vertebrate life on earth follows a generally 4-limbed body plan, with some few species sporting 2 limbs, or none. So why could Pandora vertebrates not sport a generally 6-limbed body plan, with some life forms sporting 4, 2, or none?

That the Na'vi are far too human is obvious, but their evolution is not implausible given the other life forms depicted.