I still want to know why every other creature on Pandora has four arms and four eyes, while the Na'vi are humanoid. They're improbable even within the logic of the movie.
It's not improbable; it's impossible. The Na'vi not having 6 limbs and 4 eyes means that they did not evolve from the same base creature that every other animal on Pandora evolved from.
Maybe I'm being generous, but it's still just improbable. Among land creatures on Earth we have both quadra-limbed endomorphs (including oddballs like snakes) and arthropods who have all sorts of eye and limb configurations. It is damn weird that one and only one creature, especially one complex enough to evolve an intelligent nervous system, would be the oddball, but it's certainly not impossible.
Also, I'm hoping for a crossover. I want to see a reincarnated Sigourney Weaver Na'vi taking on Xenomorphs.
The vast majority of macro non-insectoid life on earth is based off the same basic tetrapod design. Even snakes are tetrapods, although their four limbs have devolved away into almost nothing.
All Amphibians, reptiles, dinosaurs, birds, and mammals are tetrapods, having all evolved from the
Sarcopterygii. Every single amphibian, reptile, dinosaur, bird and mammal on this planet is descended from the same subclass of fish.
This is why we all share the same basic design. The fact that the vast majority of Pandora's life has 4 eyes and 6 limbs suggests a similar route of evolution, which means that the Na'vi should have 4 eyes and 6 limbs as well. The fact that they do not is simply creative liscence, nothing more.
In theory it
might be possible for a
single four limbed, two eyed creature to evolve alongside all the other four eyes, six limbs creatures. But it's highly unlikely, especially considering the Na'vi had mind tentacles as well. The Na'vi also had hair, something all other Pandoran life was missing, again this can be put down to creative liscence.
Ultimately though; we don't know enough about evolution and the process by which life comes to occupy the shapes it does to tell if it would be possible or not, but given the evidence i
highly doubt it.