So You Want To/Create Believable Aliens
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(This article still needs Wiki Magic.)
Ah, space aliens. They're exotic, they're mesmerizing, they let the Science Fiction writer explore all sorts of themes and plots that just wouldn't be possible with plain old human beings.
But if you are attempting to write a work of hard science fiction, your aliens have to be realistic. The sophisticated reader, who is knowledgeable in physics, chemistry, and biology, must believe that these otherworldly creatures could actually evolve on real planets in the real universe.
This is not an easy task. George R. R. Martin said as much in his 1976 essay "First, Sew On a Tentacle (Recipes for Believable Aliens)". Your alien species will come from a world with its own evolutionary history, its own flora and fauna -- if distinctions like "flora" and "fauna" even make sense in that world's biosphere -- and must occupy some evolutionary niche on that world, or by all rights it shouldn't exist at all.
The only case study we know of
Thus far, we know of only one planet where life actually arose. But just in our one biosphere alone, we've seen an amazing diversity, an astonishing spectrum of bizarre and wondrous organisms that all fall under the umbrella of "life." Much has been made of how alien the humble starfish is when compared with a human being, but even a starfish still has cell nuclei and mitochondria and shares the same genetic code with us. There are life forms on, and in, the Earth whose very chemistry is different from ours, and that show us unequivocably that the route from microbes to man didn't have to play out anything like it actually did.
The first organisms on Earth that could reasonably be called "alive" were probably short, self-replicating nucleic acid chains. It's now known that some RNA strands can act like catalysts, helping other chemical reactions take place simply by virtue of the strand's shape; a few of these "ribozimes" can even catalyze the creation of other RNA strands, including copies of themselves. They may have "lived" inside phospholipid membranes that became the ancestors of modern cell membranes, and they would have even been subject to a very primitive kind of natural selection. This video shows one possible chain of events that could have occurred on ancient, pre-biotic Earth and led to the first living organisms.
But it didn't have to go that way. There could have been other catalytic, self-replicating molecules that made their way onto the planetary stage instead. Or, at least, we have no reason to think that something else could not have worked. Our cells have nucleic acid chains and phospholipid membranes because the life forms that eventually made their foothold on ancient Earth had nucleic acid chains and phospholipid membranes. But on another planet, with conditions similar to but not identical to the primordial Earth, it might very well be the case that some other, similar molecule would have "won the race" instead, and the biosphere on that world might rest upon genetic material that isn't made up of chains of nucleic acids.
Ribozymes, like modern protein enzymes, sometimes need "cofactor" molecules to function properly. Some of these cofactors are individual amino acid molecules, or very short molecular chains (2-3 long) of amino acids. Amino acids occur naturally in some comets and asteroids, and were doubtlessly present on the ancient Earth. This was likely how proteins first started becoming bound up in living organisms: Those Ribozymes that had the necessary chemistry to capture free-floating amino acids had access to more cofactors than those ribozymes that didn't, and could thus out-compete them. The ability for two amino-acid-carrying ribozymes to joing their amino acids together in a chain would also have been useful; indeed, such behavior can be (and has been) "evolved" in a laboratory. Eventually, one specific short RNA strand can become associated with one specific amino acid throughout a given protocell, forming what we in the modern world would call Transfer RNA. From there it's a short step to associating a specific string of RNA nucleotides with a specific piece of tRNA, and thus with a specific amino acid. This is the origin of the genetic code. This video explains this process in greater detail.
But the genetic code we have today -- a mapping of 64 different nucleotide combinations to 20 different amino acids -- is not universal to all life forms on Earth. The mitochondria inside your cells, for example, have their own DNA and replicate themselves according to their own drummer, but their genetic code is slightly different from the genetic code in your cell nuclei. It's mostly the same, but not entirely the same. An RNA/DNA using organism that evolved on another planet could --and, indeed, almost certainly would -- have an entirely different genetic code. Maybe they only make use of 16 amino acids, not 20, and get away with having codons that are only 2 nucleotides long instead of 3. Maybe they don't use amino acids to build their bodies but something else, and that something else has 10,000 variants instead of 20; if they use 4-nucleodite RNA/DNA like we do, each codon would have to be at least 14 nucleotides long.
For the first billion or so years of life on Earth, the only life forms were bacteria, bacteria-like archaea, and viruses. Bacteria are small, surounded by a peptidoglycan-based cell wall, and keep their main DNA in a single loose strand anchored on one end to the cell membrane. Then, some 2.7 billion years ago, a new player entered the stage: the eukaryotes. Eukaryotic cells are considerably larger than bacteria, have a motile surface (lacking a peptidoglycan-based cell wall), and most importantly carry their DNA in a separate interior bubble called a nucleus. The large-scale structure of eukaryotic DNA is different from bacterial DNA, allowing the cell to carry its genetic material in several different strands, or chromosomes. All multicellular life on Earth descended from these early eukaryotes.
Then, some 2.4 billion years ago, Earth experienced the worst case of air pollution in its history. A very successful bacterium called cyanobacteria hit upon a way to synthesize the glucose it needed to survive, by using plain old carbon dioxide and water in the presence of sunlight. Unfortunately, this process gave off a deadly byproduct: oxygen gas. Oxygen is highly reactive, and spells instant death for any organism that isn't aerotolerant. It's thought that most extant species of bacteria simply died out in this Oxygen Holocaust. Many others retreated to places where oxygen couldn't reach, such as stinking mud pits. A few, though, managed to evolve aerotolerance and survive. Eukaryotes were among the lucky few who evolved aerotolerance.
And once all this oxygen was lying around, it didn't take long for another organism called purple bacteria to hit upon a way to use it. With the right metabolic pathway, a purple bacterium could get a lot more useful energy out of a single glucose molecule by combusting it with oxygen than it could by simply fermenting the glucose. They now produced more energy than they could possibly use, and eventually, they and the eukaryotes hit upon a wonderul mutually-beneficial deal: The purple bacteria could live inside the eukaryote, protected from small predators and given access to all the nutrients at the eukaryote's disposal, in exchange for which the purple bacteria would turn glucose and oxygen into energy that the eukaryote could use. The descendants of these original, symbiotic purple bacteria evolved into modern mitochondria, which exist as organelles inside every eukaryotic organism on the Earth today.
(Insert further discussion of terrestrial biology here, with an eye for how it might have gone differently somewhere else.)
Your world's biodiversity
An excellent source for thoughts on the different forms that aliens might take, from their metabolism to their art, is Robert Freitas' Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
Unless they've embarked on a systematic process of mass extermination on their homeworld, your aliens will not be the only species on their home planet. They will be part of an enormously diverse biosphere, containing things as simple as the first organisms to have evolved there (like Earth still has bacteria) to things as complex as themselves, and every level of complexity in between.
Evolution is your best friend (and your worst enemy)
Evolution -- random variation coupled with natural selection -- will be the shaping force of any biosphere, whether the life in that bisophere is based around proteins and nucleic acids, or silicon crystals, or self-organizing superheated plasma. The basic selection criterion of evolution is reproductive success: that is, for any given organism, how many copies of its genes can it create in a given time period which are themselves capable of reproduction, before that organism dies?
If something kills the organism before it gets a chance to reproduce, no copies of its genes will get made. If it reproduces but its offspring are sterile, the copies of its genes that got created in those offspring will die with them. If, on the other hand, the organism dies while saving the lives of its siblings, those siblings likely contain copies or near-copies of its own genes, and will thus be preserved. To be successful on the evolutionary stage, an organism must be able to survive to its reproductive age and then actually produce fertile offspring.
Thus, every feature of an organism -- its senses (if any), its means of locomotion (if any), its robustness in the face of environmental adversity, and even its psychology -- must serve a purpose (or have served a purpose in its evolutionary past) that either increases its odds of survival, or increases the number of offspring it creates, or both.
Many lifeforms have a lot of biological quirks that don't provide much of an evolutionary advantage, but also don't provide enough of a disadvantage that it's been weeded out by natural selection. However, most of these are so widespread because they used to be advantageous. For instance, goosebumps; they come from your body trying to fluff up fur it doesn't have, either to keep in heat better or to look bigger and scarier. Handy for lots of mammals, including our distant ancestors, not so handy for modern humans. Unless your aliens have genetically engineered themselves to remove these sorts of things, they'll probably have a few strange little traits like this.
The bad news is, this can creates an enormous burden for you as the writer. The good news is, if you pull it off, your readers will appreciate the effort.
You may have heard of the term "convergent evolution": that when something works very well in a certain environment, it may develop independently in multiple, mostly-unrelated species. For instance, this is the reason dolphins and sharks have similar body shapes and coloring, despite the fact that one is a mammal and one is a fish. This can be a handy concept to draw from: for instance, perhaps your aliens have two ears for the same reason many Earth animals do- it allows one to judge directions and distances much more accurately. However, don't overdo it. There's a difference between "similar" and "identical," and random chance has a significant enough role in evolution that even if you had a planet completely identical to Earth and let life evolve there, it would likely take a very different form from ours.
Believable space-faring aliens
If you want your alien species to be space-farers -- that is, tool-makers who can build vehicles capable of crossing interstellar distances -- they will need to have some traits in common with humans. Such as:
Living in air:
To build spacecraft -- or a technological civilization anywhere past the stone age -- your aliens will need to be able to smelt metal. This means that they need access to a heat source powerful enough to raise metals to their melting point. Ancient humans accomplished this by building fires, that is, by combusting plant matter and/or coal (which used to be plant matter) in air. If humans lived under water, like porpoises or fiddler crabs, we wouldn't be able to build fires.
Hypothetically, other metal-smelting heat sources could be available to an underwater species. There are volcanic vents on the ocean floor, for example, but these only exist where you happen to find them, are surrounded by boiling hot water and toxic (to us) sulfur compounds, and eventually shut themselves down.
A species on a planet that lacked an oxidizing atmosphere would face a similar problem. If you want to build a fire in, say, a methane atmosphere, you'd have to bring your own oxidizer. Without high-tech equipment to make oxygen (or chlorine, or some other substance that will combust with methane), there must be some organisms in your planet's ecosystem that produce, and store, oxidizing chemicals so that your aliens can build smelting fires with them.
Size:
To build spacecraft, the aliens will need to be intelligent. This means that whatever they have that passes for a "brain" will need to house billions of neuron-like switching elements for abstract thought. There is probably a certain minimum size that any biological neuron-analog will need to be, so the aliens' brain itself will need to be at least, oh, several cubic centimers in size. This puts a lower limit to how small the aliens can be. Little green men may be feasible, but microscopic green men are impossible so long as we limit ourselves to life based around organic molecules.
Conversely, there are probably upper limits on such an organism's size. Any organism that moves and thinks as an organized whole (as opposed to an agglomeration of semi-autonomous cells like a slime mold or a sponge) is going to need something akin to a central nervous system to relay messages quickly from one part of its body to another. It's also going to need a means of distributing whatever nutrients its cells require and carting away the cellular waste products, akin to blood. The bigger these systems become, the more problems they incur. The apatosaurus, for example, had a very tall neck, so the blood pressure its heart had to produce in order to drive blood all the way up to its head was enormous. It adapted to this problem by having a very small brain, so that its head didn't need a lot of blood. If your space aliens have their brains in their heads, and come from a world with a surface gravity on par with Earth's, they cannot be much taller than humans unless they evolved some solution to this problem, such as multiple hearts (one down near the feet and one up near the head), or vein-like anti-backflow valves in their upper arteries. The Square-Cube Law also plays havoc with an organism's developmental needs, since growing enormous leg bones requires you to gather more food from your environment just to build the leg bones out of.
Manipulatory ability:
You can't make tools without something you can use for "hands". An elephant's trunk, an octopus's tentacles, a monkey's prehensile tail, a dogs's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.
Communication:
Smarts, and the ability to make tools, isn't enough to get you into space. You need to be able to create a cultural knowledge base that knows how to deal with its existing technology, and allows an individual to innovate new technology by building on what the culture already knows. As our own space programs have demonstrated, leaving our home planet requires both a highly advanced technological infrastructure and a remarkable degree of cooperation, and that requires the ability to communicate knowledge -- sometimes very complex knowledge -- from one individual to another.
This communication of knowledge must encompass both long-term learning, such as "Here is how you build a transistor", and short-term coordination, such as "Okay, Bob, when I lift my end of this heavy object, you lift yours. Ready? One, two, three, lift!"
Motivation:
Just because a tool-making, social, linguistic species has the ability to make spacecraft doesn't mean they'll have the desire to make them. There must be some quality about their basic psychology that drives them to explore, and to cooperate in their exploration, and this psychological quality must make sense from an evolutionary standpoint. In the case of humans, our desire to explore space probably came out of a desire to explore the horizons of our surroundings, to see if there were any wildebeasts to hunt or any other human tribes to trade or fight or mate with.
Perhaps a species of herbivores, such as Larry Niven's Puppeteers, would be motivated to see what's over the horizon by a simple desire to ensure the safety of the herd -- if they discovered a leopard, they could prepare for it and thus decrease their odds of getting eaten. Or perhaps the grass that the Puppeteers graze on (or whatever ground-covering organism passes for grass on their planet) only grows in random patches that last a few weeks, so they have to find the next grass patch or starve to death.
If your aliens are cold-blooded carnivores that only need one meal a month, they won't be motivated by the manic desire to acquire ever more resources that we humans are so intimately familiar with. Would such creatures even be tool-makers in the first place? Perhaps they need the tools to defend themselves against other predators that are warm blooded, which would lend them a certain degree of paranoia (even xenophobia). These creatures would be motivated by fears similar to those that drive the herbivores, and if us warm-blooded predatory humans ever stumbled across them they'd almost certainly see us as a threat and try to eliminate us. If they are space explorers, they are space explorer-exterminators.
Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are terrestrial ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth -- it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be something in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.