Why Evolution is Important or How I Imagine Avatar II

(Photo from here.)
I've been talking about biology and evolution for a while now without much talking about how it is important in my writing or life in general. Or, at least, I'm going to take another crack at it and see if I can get it right.

One of the principles of understanding evolutionary biology is the importance of heritage.

Mammals, reptiles, birds and amphibians belong to tetrapoda, "four feet". Each of the groups has four basic limbs and a head. Some have a tail. Some don't. Some have gills. Some have lungs. Some have both. Some have eyes. Some don't. But they all have four limbs-- even those, like whales, that don't. By that I mean that though they do not have four functioning limbs at birth they do have developmental traces that confirm their tetrapod ancestors.

We are tetrapods because our root ancestor was a tetrapod. Our heritage, from that distant root ancestor, is to be tetrapods. We're pretty much stuck with it. The complexity of creating developmental patterns means that if we change any one thing we have to change a lot of other things. Consequently, whales often have vestigial limbs and have to do funny things in development to prevent the growth of hind legs. It's also why whales move their tails up and down instead of left to right like fish or icthyosaurs. Mammalian tails evolved from animals that supported themselves on land. Consequently, the spine of mammals-- of which the tail is an extension-- are oriented to operate and support weight in a dorsal to ventral pattern. Fish, on the other hand, originated in water and never had to support such weight.

Why, you might say, do ichthyosaurs appear to be structured so that they move their body from side to side rather than up and down?

Hm. Good one, that.

While ichthyosaurs are reptiles and co-existed with dinosaurs they split from the main reptile line prior to dinosaurs. Their anatomy more resembles lizards than anything else. One of the characteristic components of lizards is the shoulder and hip joints that are splayed out rather than vertically under the body. Look at a dinosaur or a rhino. The load bearing legs are vertically under the point of attachment to the body. That means the load is transferred directly to the limb rather than at an angle. I'm guessing here but I suspect that the orientation of the limbs factored in the degrees of freedom of the tail and the body. Watch how a crocodile walks. Then, note how a crocodile swims. Heritage is everything, isn't it?

My point with this is how you do world building. For example, let's say we create a world populated by intelligent dogs. They have two eyes in the front, a nose, they walk on hind legs and have tails but hands. This means they have relatives-- there are lesser canidims (from hominims) that have smaller brains, bigger bodies and may or may not walk on their hind legs-- the moral equivalent of the great apes. This also means there are more distant relatives that run on the ground but have something like hands. And even more distant relatives that still have two eyes and four legs.

Now, it's possible that you could have competing strategies in the biosphere. We see it here. Hexalimb systems as exhibited by insects. Decalimbed systems as exhibited by shrimp (decapoda.) But in the size range of land animals, say from a pound to multiples of tons, the dominate life form is tetrapoda. Perhaps this is coincidence-- we only have a sample size of one, after all. But it should be noted that once tetrapoda managed to control this size range they kept it within the family: archeosaurs gave way to dinosaurs which gave way (briefly) to birds and then to mammals and birds together. At no time did some new n-poda creep up out of the ocean and step up. There could be a lot of reasons for this and the discussion of it would be lively and fun. However, it's not germane to this discussion. Suffice to say that, barring something drastic, once a group holds onto a role they are hard to dislodge.

Consequently, or canidims would be part of a rich pool of similar animals. No biologist would have difficulty placing canidims in the general pattern of evolution.

So, if you put that canidim as a native of a world with giant bugs it would be a biological anomaly that would require explanation.

If you have ancient intelligent fire breathing dragons in your world, you also must have tiny stupid short lived smoking dragonettes. (Which, incidentally, is one of the problems I have with Harry Potter. But that's another story.)

Which brings us to Avatar.

Let's look at the intelligent natives: tall, human like, two hands, two legs. Walk on two legs. Two eyes. Tetrapods. Vertebrates. They could be human cousins.

Let's look at the animals: six limbed. Sometimes four eyed. Clearly vertebrates but not tetrapods.

Consequently, if the Na'vi evolved on their planet they separated from the main line many, many millions of years ago.

Now, there could be an explanation for this beyond the idea that James Cameron has a foot fetish. (More is better?)

It could be that the hexalimb approach is only apparent. The extra two limbs are derivative of the original two limbs. I.e., they are all tetrapods but in the dominant form two of the limbs have split.

It could be that there were many more tetrapod forms in the past and the only remaining tetrapods are the Na'vi all others (or mostly others) being outcompeted by the hexapod forms.

It could also be (and this is my theory) that the Na'vi, while indigenous, are not native to this planet. They are also invaders-- a better, more gentle invader to the world but no more native to this world than the humans. It could even be that the Na'vi form is not their original form but one more suited to Pandora.

This last idea makes the most sense to me. Consider the differences from the main vertebrate population. The Na'vi don't just look alien to us. They look alien on Pandora. Consider that strange ability they have to connect to the nervous system of the other organisms. That ability doesn't just evolve-- there would have to be intermediate forms. And this is a common trait across many vertebrates, animals that have split off from the Na'vi line millions of years before.

There's a last idea based on the film. It could be that the world of Pandora is itself a sentient creature-- this is strongly suggested by the film. In that case, the world of Pandora is created and evolution does not apply. The Na'vi look like humans, talk like humans, have sex like humans because they were created to be like humans. Analogous to Gaea in John Varley's Gaea Trilogy.

Okay. My opinions of Avatar were based solely on the biology and appearance of the movie. It looks like there's going to be a sequel or set of sequels. We'll see how it turns out.