In my post about aliens, I promised I would give biology its own post. Since we haven’t met any aliens, there’s no way to be entirely certain what is possible and what isn’t. So it’s always kind of an open question: can there be a being that’s also a rock, biochemically? To which I say, if you can imagine it, you can make us believe it. After all, there’s no proof it isn’t possible!

That said, the more you invent about an alien species and the biological justifications for their features, the more believable we will find them. For instance, you could give them black and white stripes because they look super cool—but if you also tell us this is excellent camouflage among the tall grasses of their homeworld, given the colorblindness of their chief predator, it’ll also seem perfectly plausible.

Inspiration from the animal kingdom

The number one place to get ideas for how aliens work is by looking at other life that we have discovered. Most things you can imagine have been done. An electricity sense that can detect the electrical impulses in the beat of its prey’s heart? The platypus. A creature that eats iron and survives boiling temperatures? Mariprofundus ferrooxydans bacteria. Echolocation? Bats and dolphins.

That there is what we call proof of concept. It has happened, therefore it could happen to your aliens.

Now you can go full-on furry if you want to. You can invent cat aliens that have a great sense of smell and reflective eyeballs and sleep 18 hours a day. But I tend to prefer making a nice mishmash. If it reproduces like earthworms, then I also want it to camouflage itself like an octopus, while looking like a llama. This is how you get weird things, things that start to twig the reader’s brain as eldritch and unimaginable, or at least that you’d have to look at several times to really understand.

Along with this, consider the habitat. Once you know the biome a creature lives in, you’ll know what features it has to have. If it’s from a very dry planet, it probably lives off very little water. If the planet is cold, it’s probably compact and blubbery. And what if the planet has constant volcanoes or lightning storms? What would it need to evolve for those?

Similarities to humans

Now, it’s not against belief that most species in the galaxy would look something like us. More than a knobbly forehead, probably, but they aren’t necessarily shapeless blobs, gaseous clouds, or giant clams.

If we assume that the species we meet are intelligent, use tools, and have built spaceships, then we can extrapolate a few things. First, they have hands, or some kind of equivalent like adept tentacles. Second, they must be social—they will have a means of communication and form groups. Space travel isn’t a solo venture. Third, most of the beings we find out there will likely be land-dwelling: the ability to create fire is a really important step to more complex tools. Beings that don’t use fire may be highly intelligent and create wonderful and rich cultures, but it’s just a lot less likely that they’d build spaceships.

It’s likely that they’d walk upright in some way, so they can look forward at what they’re doing while leaving some limbs free to use tools. This might be something they do only sometimes, like bears, or whenever they’re getting around. The human stance is kind of suboptimal, though, so it wouldn’t surprise me if something more like the bird or kangaroo stance was more common—keeping the body weight balanced with a tail.

Being either a predator or a scavenger seems most likely, because an intelligent brain has very high energy needs. However, looking at other intelligent animals like whales and elephants, you can see it’s not impossible for them to be mellow grazing types. On Earth, predators usually have forward-facing eyes, so that might make the faces of other aliens look familiar to us—provided they have the same number of eyes.

Aliens will definitely have senses, some of which might overlap with ours. Sight (sensing the electromagnetic spectrum) is very likely to be useful, though they might not see the same part of the spectrum as we do. What if they see radio waves or gamma radiation? In places that are dark or very murky, hearing might be more useful. Smell and taste are both types of chemoreception: where might an alien’s chemoreceptors be? Could they, like many animals on Earth, create scents of their own to communicate with? And touch is the great leveler: it is hard to imagine a creature that would not develop a sense of touch.

It’s possible that the options are even more limited than we think. Biochemically, there are some things that molecules will and won’t do. We don’t yet know if it’s possible to have a being that isn’t based on water and carbon. We do know it’s possible to have a biosphere just like ours—carbohydrates, proteins, fats—and nothing on the whole planet is edible to us, because the molecules are backwards compared to the ones we can use. My point is: if you want a thing to happen, construct the rules such that it had to be that way. The limits are what you say they are.

The intersection of biology and culture

Any single biological fact about a life form can be extrapolated to lots of cultural concerns. If, for instance, they create pheromones for communication, perhaps their whole language relies on smell overtones or even smell alone. If they reproduce asexually, our whole system of romantic partnerships wouldn’t exist for them—so what bonds do they form, and how do they reinforce them?

Think of an infant of your imagined species. What teaching has to go into that child to help them grow from a being of pure instinct into someone who can cooperate with others? Do they have to be taught language, how to control their telepathy, how to project colors on their skin, or how to resist the temptation to sting people with their venomous tail? If that has to be taught, there will be a whole cultural apparatus for teaching it, possibly even a religion.

What kind of houses would they build? Tunneling creatures would be most comfortable underground, while flying creatures might construct cliffside aeries. How much space does one of them need to be comfortable? Creatures with a strong sense of smell and scent communication might need more than ones that rely on touch.

The weird edge

Right in the space between “impossible” and “not a sapient being at all,” things get interesting. I did say that most spacegoing beings would be like us in some key ways. But it’s fascinating to imagine the exceptions to that rule.

Can you imagine…

• sentient plants?
• colonial organisms—hundreds of bodies linked into a single hivemind
• beings that survive in such different environments (like, say, a gas giant) that they can’t share atmosphere with us at all?
• tiny beings the size of your hand who live on your space station and fix your stuff when you’re not looking?
• lifeforms that metamorphose into something completely different?

If your imagination likes to stretch that far, the sky isn’t the limit—it’s only the beginning. Just ask why your creature has those traits, and what the effects are on their thinking and culture.