Avatar may have broken new ground in certain cinematic techniques. But when it comes to alien creatures, Avatar went out of its way not to be too original.

As a reader of evolutionary biology, I was at first disappointed by how recognizable the movie's bestiary was. But if I stick to my Darwinian guns, I can predict a prequel that could explain the movie's biodiversity.

In the movie, humankind has made its way to Pandora, a life-supporting moon in a nearby solar system. There, we encountered blue-skinned humanoids called Na'vi. Alongside the humanoids, there are dog-oids, horse-oids, jellyfish-oids, rhinoceros-oids, lemur-oids, and just to be original, dragons. Aside from the dragons, the film was content to attach two extra limbs to existing Earth fauna and call it good.

Apparently, audiences don't really want "original" when it comes to alien creatures. Concept artist and creature designer Neville Page told Variety that "you have to be careful you don't get too creative or too wacky, because most people can tell when it just doesn't feel right."

As for the humanoids, Cameron began with something more alien-looking. Wikipedia reports that Neytiri (the Na'vi love interest) was originally conceived with "fins, gills, and other protuberances." But again the alienness of the creature was toned down to make her more palatable to human audiences.

In fact, Cameron told Playboy "Right from the beginning I said, 'She's got to have tits,' even though that makes no sense because her race, the Na'vi, aren't placental mammals."

So the fauna of Pandora was constrained to recognizable forms for the sake of the audience. Further, the humanoids and the other creatures didn't evolve from the same primordial soup. One sprang from the libido of the director, the others from his idea that the creatures should be sleek like race cars-but nothing too creative or wacky.

Linguists may relish that the Na'vi language didn't just take English words and add an extra syllable or two, it introduced an entirely original alien language. But biologists don't get that same satisfying sense of completeness, even if Cameron can bandy terms like "placental mammal."

Or do they?

I'd like to offer an interpretation that lets Neytiri keep her tits, lets the vertebrates look like sleek race cars, and yet explains how they can all cohabitate on Pandora.

If Pandora were Earth, the lemurs would be the humans' closest cousins (at least, of the animals we see on screen). But on Pandora, the lemurs look like the horses and rhinos and nothing at all like the Na'vi. The Na'vi only have 4 limbs, not 6 like all the other large vertebrates. And the Na'vi breathe through their faces, not through holes in their chests. Even one of these adaptations would be enormously costly in evolutionary terms. Clearly the Na'vi didn't evolve from the lemurs. They must have come from a different genetic line.

They are so different that, donning my exobiologist hat, I hypothesize that the Na'vi are an alien race. Think of the irony. Avatar tells the story of a destructive invasive species, homo sapiens, laying waste to Pandora, whose strongest defenders are themselves an alien species.

The Na'vi can't be recent arrivals because their ponytails allow them to connect with the rest of the moon's creatures a la USB. That suggests co-evolution... unless those USB ponytails were brought specifically as tools to domesticate the local fauna.

I can imagine a prequel where we borrow a trope from Titanic: the narrator is a very old Na'vi woman who remembers the time when she was a girl, a time when the Na'vi had just arrived and the direhorses and mountain banshees still roamed free. Perhaps her own father was the model of pragmatic cruelty, a tamer of direhorses who gains fame as the first Na'vi to break the spirit of a mountain banshee. Like the human scientists and soldiers in Avatar, the Na'vi biologists and horse-traders could be split along ideological lines about whether to study or to exploit the thriving Pandoran biosphere.

Maybe in her old age, after the peaceful biologists had won the political fight, she could even, Cassandra-like, warn the Na'vi to be afraid of future space-farers because although her kind found a balance, she knows first-hand the sort of ecological destruction that comes when a sentient, non-native species is introduced.