Explaining the Avatar Ecosystem

According to Tam of View from the Porch, the ETs in Avatar don't match the rest of the ecosystem:

The thing that was most jarring to me about Avatar is something that will probably stand out like a sore thumb to any other SF geek: The aliens themselves.

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Now go look at all the big critters in Avatar: They're all hexapedal, or six-limbed. They tend to multiple visual organs. Their breathing orifices, separate from their mouths, are located low and forward in their torsos...

Not the alien people, the Na'Vi, however. They're just big blue humans with tails and slightly feline features.

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Egalitarian, monogamous, recyclers who love Gaia... how alien is that? These people work at my local organic grocer's. I've met more foreign subcultures in midtown Atlanta.

There's an obvious explanation: Greenpeace refugees went to a previously uninhabited planet and genetically engineered their descendants to look like Hollywood extraterrestrials.