Well, I'm willing to point out some explanation is given, but frankly it still doesn't make sense, after a recent re-watch and an extended debate with some of my more xenobiological savvy friends (I'm an Aerospace Engineer so that's who I hang out with).
The fauna of Pandora are ALL hexapods, meaning six limbs (not counting tails) and all have two sets of eyes. They all have black obsidian like teeth, and all breath through two or more large "ducts," where their chests meet their collar bones and neck.
The Na'vi on the other hand, are tetrapods (four limbs), have only one set of eyes, white teeth, and breath through their mouths and nostrils. This, doesn't make any fucking sense. Every macroscale land animal on EARTH is a tetrapod, has one set of eyes, and all breath in basically the same matter. The Na'vi are completely unlike any other life-form on their planet...
...save one. Early in the film they point out "Blue Lemurs," which are hexapods, but their two front limbs have fused together at the elbow. They have two large yellow eyes, and no breathing ducts in their chests. They also have only one "bond," thingy on the back of their heads. This leads me to conclude that Cameron was hoping this would settle any concerns over the evolution of the Na'vi, but the problem is that most major components of anatomy like that, typically evolve for good reason, and are constant across the native species of any given era So this leaves a big problem in the evolution of the Na'vi.
Phew! I apologize for the long winded answer, but I felt you deserved a fair answer.