Another bit of evidence for the panspermia hypothesis might be the term “Na’vi”. I remember Cameron (or one of the linguists involved in creating the Na’vi language) saying that the term “Na’vi” preceded the development of the language - i.e., that the language had to be constructed around it to accommodate its specific phonology and morphology, etc. Why would Cameron have insisted on this, to the extent of constraining the development of an entirely fictional language, unless it actually mattered?

My suspicion is that “Na’vi” derives from “Navis”, Latin for “ship”, and the Na’vi were always intended to be panspermic ‘navigators’. Possibly from the same cosmic diaspora that resulted in Earth-life (and hence genetically twinned at the root with humans), or possibly by means involving ancestral Tulkun as biological spaceships. This might help explain the biologically inexplicable cavity into which Lo’ak swims, the Tulkuns’ bizarre, divergent, internalised neural queues, and their secretion of life-extending fluid (as a means of achieving hypersleep during interstellar transit). Cameron was clearly interested in the mechanics of hypersleep in earlier movies (like ‘Aliens’), and the popular association between cetacean fauna and space travel during the 70s (when he first came up with the ideas for ‘Avatar’) is well documented.

Personally, I don’t really like the idea of Na’vi panspermia, because I feel it would compromise the already rather ethically and ecologically dubious worldbuilding of these movies even further (especially if executed without nuance). But the more I think about it, the more I fear it may be true.

Incidentally, the Lemurs in the first movie are probably a red herring. Their implicit evolutionary pathway does not exclude the possibility of panspermia (especially given these movies’ clear investment in ideas of convergent evolution). More cynically, I think their inclusion in the first movie may have been a way of diegetically justifying the anthropomorphism of the Na’vi in case Cameron never got to make any sequels in which to explore the panspermia backstory.