We’re a few parts into this series now, and we still haven’t discussed what aliens look like! It’s time to remedy that. Here, we’ll look at alien physiology and body structure. From cells to organs to “the surface”, we’ll see what goes into making something seem alien, yet plausible.
On the inside
A lot of the earlier parts go into making aliens. After all, they are organisms evolved in and adapted to a specific environment. That’s going to affect their nature. If you’ve ever watched some of those David Attenborough nature shows, then you probably know this already. Fish that live deep in the ocean tend to be flatter. Desert plants have ways of capturing and storing water. Humans lost most of their body hair, while gaining the larger brains that enable us to write (and read) posts like this.
Physiology, of course, is about more than just appearances. It’s also about how the body works. And that’s a complicated matter. We, as humans, have a bunch of organs, almost all of which have to function just right. Some have redundancy—we can live without one lung, or one kidney. Others, like the heart, brain, or liver, are alone; notice how many of those have extra protection. And then there are a few that don’t really seem to be of any use. We can get by just fine without tonsils, for instance. I do. And the appendix literally does more harm than good, having the sole purpose of occasionally becoming inflamed or worse.
Other complex organisms have organs, too. Some of them have different sets of them. Ruminants (like cows) have multiple stomachs, for example. Birds, being egg-layers, have extra equipment for that purpose. And so on.
But the systems that organs control are fairly general. Those things an organism must do will often have dedicated systems. In addition, there will be a few other “support” systems to make these work. So let’s look at what’s biologically required of us, and we’ll see how that translates to aliens.
Intake
Everything living requires some sort of energy input. For us, that comes in the form of food, water, and air. We need all of these to run the chemical reactions that create life as we know it. So does everything else. Thus, we’ve got a mouth that’s front and center, our universal access point for input. So do most other animals.
Plants are a little different. They don’t need to “eat” in the same way we do. Much of their energy input comes from photosynthesis, a different kind of reaction using sunlight as the power source. But they still require water, and they still need air. (And that includes some oxygen, not just CO2.) So you won’t see plants with mouths, except in the case of carnivorous plants—and most of those instead use a trapping mechanism.
As for aliens, the situation depends on evolutionary history and environment. If your aliens eat and drink, then they’ll have some equivalent to a mouth for that purpose. After the mouth, there’s the digestive tract, where nutrients are extracted from the food. While it won’t necessarily be human-like, it has the same function as ours, so it might be somewhat similar to something on Earth.
Output
Not everything is digestible. The leftovers are mostly useless to us, so there’s no point keeping them around. Thus, our bodies get rid of them, in the form of waste. Animal waste, including human, does have its uses (e.g., as fertilizer or fuel), but it’s mostly just that: waste. Because we don’t want it inside us, we’ve got a system to get it out.
For solid waste, of course, we’ve got a dedicated part of the body. Liquid waste (urine) gets mixed in with other parts, however, in a bit of evolutionary parsimony: we’re not going to use both functions at the same time, so it wouldn’t hurt to let one organ do two unrelated jobs. Other organisms, including aliens, might not do this, which is okay unless you’re really into that sort of thing. Excretory systems, in an advanced, sapient species, may develop cultural taboos, too, but that’s a subject for a later post.
Reproductive
The sole reason to live, from an evolutionary perspective, is to reproduce. At its core, that’s why sex is enjoyable—if we didn’t like it, we wouldn’t do it as much. And so it stands to reason that reproductive organs have a lot of cultural significance attached to them. But they’re also interesting from a biological standpoint.
As stated above, human reproduction overlaps with excretion, but that’s not necessarily a given in aliens. What is, though, is that they’ll reproduce. And it’s likely to be sexual reproduction, not the asexual style used by, say, bacteria. Sexual reproduction requires at least two parents (possibly more, as in the recent news about three-parent IVF), but that gives it the benefit of better genetic mixing. By taking genes from two sources, organisms have a better chance to resist a bad mutation. That’s not the only upside, but it’s one of the biggest.
The internal part of reproduction has its own intricacies. For mammals and many other animals, only one parent actually contains the reproductive machinery. Males can impregnate, but females give birth. It doesn’t have to be this way. Other species on our planet show hermaphroditism (some or all members have both sets of reproductive organs). Another possibility, though harder to make work, is more than two sexes. And then there are odder methods. An alien race could be all females, but some can temporarily express “maleness”. Or the males could carry eggs for a period of time.
Just as important as the organs at work is the way reproduction happens. Are babies born live, as in mammals? Do they hatch from eggs? How many are born at once? For this one, humans are mostly one-at-a-time, but multiple births aren’t exactly rare. Other species, especially those that lay eggs, have larger litters or clutches.
(Oh, and before you ask, it’s astronomically unlikely that we’d be compatible enough with an alien race to reproduce with them. Half-human hybrids, though great for storytelling purposes, are not the hardest of science.)
Senses
Organisms must experience the world around them, if for no other reason than to obtain food and find a mate. For that purpose, we have our senses: sight, smell, taste, touch, hearing. Each one has a use, and each has developed over the ages.
Evolution determines which senses are present. The ecological niche of a species is a good indicator of what its sensory capabilities will be. Cave-dwellers often have poor eyesight—if they even have eyes at all. Carnivores might have heightened senses of smell to track prey, while scavengers may have weakened taste. Here, the best guideline is reasoning: think of what your aliens would need where they live—or, if they’re highly advanced, where they originally came from.
Exotic senses can exist, within reason. Echolocation is popular in Earth animals, especially those without a good sense of sight. An herbivore living in a dangerous area might develop better peripheral vision. Hearing in the infrasonic and ultrasonic is perfectly valid for aliens, as are infrared and ultraviolet sight. Some birds seem to have sensitivity to magnetic fields. About the only things that aren’t possible are those that, well, aren’t possible. Like psionics, or seeing X-rays.
Each sense is going to have at least one organ behind it. We’ve got eyes, ears, the nose, the tongue, and the skin. Others are possible, though. Whiskers, antennae, suckers, tails, and anything you can think of can go towards the sensory system, if you can give it a good reason for being there.
Body plans
The outside of the body—what a species looks like—is probably more important from a storytelling point of view.
We humans have a distinct body symmetry. Left side looks like right, and we’ve got a lot of double organs, like lungs, eyes, and ears. And most animals are the same way. There might be small differences, such as fish with both eyes on the same side of the head, but those tend to be exceptions that prove the rule. This bilateral symmetry isn’t the only option. Starfish, for example, show a radial body plan: arms sticking out from around a center. Many plants aren’t really symmetrical at all, instead opting for a kind of “fractal” body plan.
Another thing we, as animals, have is segmentation. You might recall from science classes long ago that our bodies are divided into three segments: head, thorax, and abdomen. The head is where the brain lies, while the other two make up the body proper. And each of those segments has a pair of limbs (it’s a pair because of that bilateral symmetry). If we had another segment, say between the thorax and abdomen, we might have a second pair of legs (like a centaur) or arms (like Goro from Mortal Kombat). And if we kept growing new end segments as we aged, we’d be literal human centipedes!
Interesting minutiae
That covers the most important parts of an organism’s physical body. There are plenty of other systems (circulatory, nervous, endocrine, etc.) that will likely have counterparts in an alien race, but they follow much the same logic as those mentioned above.
Some other bodily things to think about include:
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Defenses: Some organisms have evolved methods to defend against predators…or each other. Some plants are poisonous. Many snakes are venomous. Cacti and porcupines are both covered in sharp and pointy armor, while turtles and snails opt instead for hard shells. If your aliens were very recently not near the top of the food chain, then they’ll likely have their own ways of protecting themselves.
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Pheromones: Human pheromones are mostly mythical, but a lot of animals do have them. They’re used to attract mates, mark territory, or as a further defense mechanism. Aliens could have them, as well, although they likely wouldn’t have any effect on us.
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Vestigial organs: Evolution rarely discards that which is no longer needed. If it’s not actively harming the reproductive process, it’ll likely stick around far past its expiration date. That’s what happened with the human appendix, and it’s possible for an alien species, too.
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Blood: It’s a staple of sci-fi that aliens don’t always have red blood. Sometimes, theirs is even toxic to us. Both cases result from using a different chemical mix than our iron-based hemoglobin. So if you’re looking for a hard-science excuse for green blood, that’s where you’ll want to start.
Conclusion
I could go on for hours, but this post is long enough already. Again, the best way to create aliens is to think about them. Environment affects appearance. Ecology matters. Inside and out, an alien race is beholden to its environment. If it isn’t, then it’s only a matter of time before it becomes so. But we, as worldbuilders, can work backwards: make what we want, then create the world to justify it.