There are so many movies about extraterrestrials that it has become a norm, but rarely does anyone ask how and why they look the way they do. This is the reason why:
First, lets start at Pandora. The reason that the Na'vi, Titanotheres, Thanators, and Ikran are the way they are is because of evolution. In Pandoras history, there could have beeb multiple mass extinctions. The time place of these extinctions is the reason for the appearance of many differant aliens. Scientists have concluded that if it were not for the mass extinction at the end of the Cretacious Period on Earth, that Troodon, and other intellegant species would evolve into a humanid with a distinct resemblance to the "Little Green Men" that we have all come to known. This is because evolution will always follow a similiar path unless interrupted by a cataclismic event. So the reason for E.T., Na'vi, and little green men is that the evolutionary paths on their respective planets was interupted at a certain point. Thus Eywa would be the result of the last mass extinction on Pandora. She would be the common ancestor of all life on Pandora, and so explains the reason for such strong connection between the life of Pandora. Many people on Earth believe that if another cataclismic event like the one 65 million years ago were to happen it would be the end of Earth. This is not true, as another intellegant organism will pick up the pieces where we left off. Just like it did 65 million years ago. If you don't believe in Evolution, I'm sorry for what I wrote. But evolution is afterall only a Theory.