With science discovering new exoplanets nearly every day now, proving that our solar system is not all that special (and most definitely not alone), it's a super fun time to write sci-fi!
...but wait. Before you grab that pen and start creating your alien planet, now may be a good time to step back and learn something about evolution.
I think most of us grasp the basic concepts of evolution: species change over time, and it usually has something to do with out-competing each other and/or fitting into their environment. But if you're going to sit down and construct an entire alien world, you need more than the basics.
Example: James Cameron's Avatar attempted to create an alien world with fully fleshed out ecosystems. I say "attempted" because, in my opinion, they failed.
Setting aside whatever positive or negative feelings I have about the story itself, the glaring issues with the created creatures made it hard for me to concentrate on the movie. The sad thing was, I could see that a great deal of thought had gone into the invention of these animals. Clearly, though, not many thoughts from functional morphologists.
Instead of a detailed rant covering each and every one of the flaws I noticed, I'll focus on the biggest one. And no, it's not the tail-mind-meld thing, which I just have to chalk up to movie magic so I don't hurt my brain. No, my personal biggest issue is the number of limbs Pandoran animals have:
Six, for the most part.
Frankly, that's alright with me. It makes me curious as to their skeletal structure, but it doesn't raise any improbability flags. At least, not until we examine where the exceptions are:
The Na'vi only have four limbs.
The movie attempts a cutesy way of explaining this. Fairly early on in the movie, this guy shows up:
Some sort of primate-like critter, hanging out in the trees. Instead of six distinct limbs, this guy has two hind limbs, and its two pairs of front limbs appear to be fused.
I see what you did there, Avatar. Giving a nod to our own evolution: humans are primates, and the Na'vi must be from whatever family this guy is from. So this guy has fused forelimbs as sort of a evolutionary step towards only four limbs.
Too bad this is just plain silly.
Tree-dwelling animals would not fuse limbs together. Tree-dwelling animals evolve to hold on well to branches, and the more limbs for that, the better. Reducing the number of limbs is absolutely backwards from what happens in reality. Six limbs would be an amazing advantage in the trees. Heck, just look at spider monkeys. They use their tail as a fifth "limb" because extra limbs are just that awesome in trees.
Unless those lemur-like fusing limb aliens evolved from a creature who had been a ground-dweller, it just makes no sense for selection to favor fused limbs in the trees. The Na'vi themselves, as a primarily tree-dwelling species, should've retained six limbs as well. But they didn't.
The Na'vi must have left the trees sometime in their evolutionary past for the limb proportions they have in the movie (human-like limb proportions) to make any sense. They obviously switched over to bipedalism as an adaptation to living on the ground rather than up trees, and lost their extra set of limbs because...
Okay, I still can't explain why they'd completely lose their extra set of limbs. I mean...EXTRA PAIR OF HANDS. SO USEFUL. WHY DID THAT DISAPPEAR?
Ahem. Anyway. Continuing with my confusion, while the tree-dwelling animals are, for who knows why, reducing their flexibility and number of limbs, it appears the ground-dwelling animals hold onto their extra limbs.
Excuse me while I go flail about in frustration.
Animals that specialize in running along the ground are known as cursorial animals, and evolutionarily-speaking, cursorial animals evolve to reduce how much their limbs come in contact with the ground (think horses as an example: they reduced even the number of toes that touch the ground down to one toe per leg). Therefore, if any animal on Pandora should fuse limbs...it would be these ground-dwelling ones. But in every instance of a ground-dwelling critter on Pandora, it of course has six limbs.
So maybe none of them are actually cursorial? They don't run?
Except for all those scenes with stampeding animals in the movie. Oh dear.
Frankly, the Na'vi are the only potential ground-dwelling animal for whom it would make sense to keep all six limbs, since they only use two limbs to touch the ground anyway. But they're the only ones who lost a set!
Perhaps now you begin to understand my issue with this movie. This is just one example of where the body structure of Pandoran animals fails to make sense. As mentioned, I won't go into all the other instances here. It would take far too long.
If you're going to create an alien world, do yourself a favor and read up on evolution, and in particular, functional morphology. Functional morphology is the study of how the structure of a body part relates to its function. It answers the question, "Why does that thing look like that?".
Whatever you do, though, don't fall into the trap that advanced species must resemble us. The Na'vi could've had extra hands! So cool! But as biased human creators, they were designed instead to match our own bodies. Perhaps to make it easier on the human characters who had to take on the alien forms. Perhaps to make it easier on animators. Or, most likely, because we associate our own form with higher intelligence.
Just some food for thought as we begin to imagine what could be alive on all these new planets science keeps finding. Aliens are awesome in stories, but they're even cooler when they're biologically plausible.