system
Given the following post, respond as if you're commenting on Reddit:
human
[James Camerons Avatar] Navi Anatomy.

Why are they biped with only two eyes, when a majority of the Pandoran wildlife have six legs and more than one set of eyes. And is it ever explained how and why they can merge or bond with other life forms?
gpt
I've used this theory before but:

The Navi are not really native to Pandora. Neither is the world-tree Eywa. 

The Eywa is a species that launches it's seeds into interstellar space. When they finally fall into a gravity well and find a planet they do one of two things. Either they start building a biosphere there, or they adapt any existing biosphere to their needs. Their needs, of course, are going to be dictated by the biosphere they evolved in (it had four limbed creatures)

This particular Eywa found Pandora. It grafted it's access ports into all the existing plant life and existing six limbed creatures. And it built it's chosen servitor people, the Navi. For it's servitors it wanted a humanoid body plan, four limbs, two arms, two legs, and a certain level of independent intelligence. Not too much though, because that might threaten it. Can't have your servants too independent or intelligent.

From this evidence we could surmise that an Eywa world-tree actually landed and failed on Earth (Garden of Eden, The Tree of Knowledge of Good and Evil). It didn't even get as far as adding the networking features, but it had an influence. That's why we have a nearly identical body plan to the Navi, that's why we have central nervous systems that are even closely compatible.