Whether life is made from DNA, some sort of odd molecule, or binary data, it will be subject to evolution. That’s inherent in the definition of life. Everything living reproduces, and reproduction is the reason why evolution takes place. Knowing the how and the why of evolution can help you delve deeper into the creation of alien life.
How it happens
For life as we know it, evolution is the result of, basically, copying errors. DNA doesn’t replicate perfectly; there are always some bits that get flipped, or segments that are omitted or repeated. In that, our cells are a bit like an old record or CD player, skipping at the slightest bump. Sometimes, it knocks playback ahead, and you don’t get to hear a few seconds of your favorite song. Other times, it goes back, replaying the same snippet again. It’s the same for a strand of DNA.
Mutations, as these genetic alterations are called, happen for a variety of reasons. Maybe there was a glitch in the chemical reaction that produces the DNA replication. Perhaps a stray bit of radiation hit a base molecule at just the right time. (Digital organisms would not be immune to that one. Programs can crash due to bad memory, but also from cosmic rays—interstellar radiation—hitting the components. And as our processors and memory chips get ever smaller, the risk only increases.) Anything that can interrupt the reproduction process can be at fault, and there’s almost no way to predict what will happen on the base level.
Most of the time, these errors are harmless. A single base being swapped usually doesn’t do much by itself, although there are cases where they do. Our genetic code has builtin redundancy and error correction mechanisms to prevent this “drift” from causing too much harm. Single-celled organisms have a little more trouble, as they don’t have billions of copies of their genes lying around. They tend to bear the brunt of evolution, but it can be in their best interest, as anyone who knows about MRSA can attest.
A few larger errors (or a compounding of many smaller ones) can cause a greater change in an organism. That’s where natural selection comes in. Species adapt to their environments. All else being equal, those that are better adapted tend to reproduce more, thus ensuring their genes have a higher likelihood of passing on to further generations. Thus, evolution acts as a sort of feedback loop: beneficial mutations ensure their own survival, while harmful ones are stopped before they can get a foothold. Neutral mutations, however, can linger on, as they have little outward effect; its these that can give a species its variety, such as human hair and eye color.
How you can use it
Assuming current theories are anywhere close to correct, all life on Earth derives from some microbial organism that lived three or four billion years ago. Through evolution, everything from dogs to sharks to apple trees to, well, us came to be. There are a few open questions (What was that primordial organism? Is there a “shadow” biosphere? Etc.), but that’s the gist of it. And that tells us something important about alien life. If it exists, it’s probably going to work the same way. The Grays of Planet X, for example, would be related to everything native to their homeworld, but not to the aquatic beings of Planet Y. (Unless you count panspermia, but that’s another story.)
That does not mean that all life on a planet will look the same. How could it? A quick glance out your window should show you anywhere from ten to a thousand species, none of which are visibly alike, and that’s not counting the untold millions that we can’t see. Gut bacteria are necessary for life, and their also our ten-billionth cousins. Nobody would mistake a dog for a dogwood, but they both ultimately come from the same stock. So try to avoid the tired trope of “everything on this planet looks that same”.
On the other hand, the vagaries of evolution also mean that life on one planet probably won’t look like life on another. Sure, there may be broad similarities (physiology will be the subject of the next part of this series), but it’s highly unlikely that an alien world will have, say, lions or bears. (However, this doesn’t necessarily apply at microscopic scales, as there are fewer permutations.)
Classification
For worldbuilding, you’ll likely be most interested in the species level. That’s how we define humans, as well as many of the “higher” animals. We’re Homo sapiens, our faithful pets are Canis familiaris or Felis catus, and that nasty bug we picked up is Escherichia coli.
But closely related species share a genus, and this might be something to keep in mind, especially if you’re creating a…less-realistic race. Unfortunately for us, genus Homo doesn’t have any other (surviving) members; the Neanderthals, Homo erectus, and the “hobbits” of Flores Island were all wiped out millennia ago. But that doesn’t mean your world can’t have multiple intelligent species that are closely related. They can even interbreed.
Higher levels of classification (family, order, etc.) are less useful to the builder of worlds. The traits that members of these share are more broad, like mammals’ method of live birth or the social patterns of the hominids. Really, everything above the genus is an implementation detail, as far as we’re concerned.
Adaptation
Now, back to natural selection. Species, as I’ve already said, adapt to their environments over time. We can see that in animals, plants, and any other organism you care to name. Fur changes color to provide camouflage, beaks alter their shape to better fit in nooks and crannies. Blood cells change to protect against malaria—but that leaves them more susceptible to sickle-cell anemia.
If an organism’s environment shifts, then that can render the adaptations useless. The most dramatic instances of this are impact events such as the one that killed the dinosaurs, but ice ages, “super” El Niños, and other climate change can destroy those species that find themselves no longer suited to their surroundings. And species are interconnected, so the loss of population in one can trigger the same in another that depends on it, and so on.
Apex
Much of this is background material for most aliens. The ones that are most interesting to the public at large are those that are intelligent, civilized. Like us, in other words.
We are not immune to natural selection. Far from it. But we have managed to short-circuit it to a degree. People with debilitating disorders can live long lives, potentially even reproducing and thus furthering their genetic lines. Adding to this is artificial selection, as we have performed on hundreds of plant and animal species. That’s how domestication works, as much for a wolf as for a grapevine. We take those individuals with the most desirable qualities and work things out so those are the ones that get to reproduce. It works, as attested by the vast array of dog breeds.
So aliens like us—in the sense of having civilization and technology—won’t be as beholden to their environment as their “lesser” relations. They won’t be bound to a specific climate, and they’ll be largely immune to the small shifts. Does that mean evolution stops?
Nope. We’re still evolving. It’s just that the effects haven’t really shown themselves that much. We’re taller than our ancestors, for example, because taller men and women are generally seen as more attractive. (A personal data point: I’m 6 feet tall, a full 12 inches taller than my mother, and my father was 5’8″. Not that that seems to make me any more attractive.) We live longer, but that’s more a function of medicine, hygiene, and diet, not so much genetics. Parts of us that have evolved relatively recently include Caucasian skin and adult lactose tolerance.
If our species continues to thrive, it will continue to evolve. One sci-fi favorite is space colonization, and that’s a case where evolution will make a difference. It won’t take too many generations before denizens of Mars have adapted to lower gravity, for instance. People living on rotating stations might learn to cope with the Coriolis forces they would constantly feel. It’s possible that there may come a time when there are living humans that cannot survive on their original homeworld.
And the same may be true for aliens. As an example, take Mass Effect‘s quarians. In the third installment of the series, they can (if you play things right) return to their homeworld of Rannoch. But centuries of living as space nomads spoil the homecoming, as they find themselves poorly adapted to their species’ original environment. A race of many worlds will discover the same truth: evolution is unceasing.