It’s an old trope. The aliens have two arms, two legs, eyes, and walk like us. If an author or filmmaker is feeling really frisky, they make the alien bugs to make them “not human.” Which only makes them even more Earth-like if you think about it. If alien worlds almost guarantee a different shape to alien intelligences with different methods of perception and communication, why, then, do most content creators, including your humble narrator, go with some variant on the great ape? Or arthopods if we don’t want not-humans?
To answer the question at the beginning, if the planet is a desert or a forest, it saves time world-building. Sand and lush vegetation are two logical outcomes for a wet, temperate world, and those of us telling you stories don’t have time to spin up an entire ecosystem for a setting that might not even last one or two chapters. In terms for filmmaking, you have to shoot somewhere. And even for Gen Z, which grew up with CGI already a thing, another green-screened movie has all the appeal of knitting needles in the eye. And really, I’m undervaluing the needles.
Speaking of filmmaking, the hard fast answer is makeup and costuming. It’s easier to make Chewbacca a yeti with a gun than it is to create a badass cephalopod with seven eyes and his genitalia in the back of his “head” (if it even has a head.) Also, Chewie sprang into being in 1975 or 76. Back then, it was just easier to bury Peter Mayhew in a man-shaped carpet tailored for someone over seven feet tall.
But then we have Spock. And have the non-Muppets on Farscape. And even CGI-heavy Babylon 5. Leonard Nimoy, Zachary Quinto, and Ethan Peck shaved their eyebrows (Well, Ethan hadn’t until Strange New Worlds was greenlit), popped on some pointed ears, and spent an hour getting a slightly green tinge to their skin. Why not just put a Halloween mask on them?
There’s another factor in filmed science fiction. The viewer has to relate to the alien if they’re significant characters. You could probably make a spider or a fish or a blob relatable, but it would take a lot of work. Even in literature, aliens who are not at least vaguely human or shaped like some kind of mammal or reptile we know are usually shorthand for “not human,” whereas the Spocks, the Cylons, and even the orcs and uruk hai from Lord of the Rings have at least something in common with humans, even looking like just another type of human. The writers of Deep Space Nine even pointed it out when Odo lamented he had to take on humanoid shape to interact with others. Quark rather inelegantly says, “Our tolerance to other life forms doesn’t extend beyond the two-arm, two-leg variety. I hate to break this to you; but when you’re in your natural state, you’re more than our poor old genes can handle.” Quark, both rightly and wrongly, suggests that humanoid shape is a common function of evolution, that we are what we are, whether we be human or gray or some goofy looking thing George Lucas and his successors have cooked up, because that form worked best for containing a big brain while being able to flee predators. Intelligent goo goes against instinct.
And then we come to Andy Weir, he of The Martian fame. After helping Mark Watney science the shit out of this and going on to a crime novel set on the moon, he turned his attention to this very topic. How does life evolve in space? Project Hail Mary begins with a major disappointment: The microbes interfering with the sun’s ability to heat the Earth have DNA. Wait. That’s an Earth chemical! Sure, it’s possible life on Mars or Europa or elsewhere in our solar system has DNA driving its cellular reproduction. Most of what makes up the solar system, including us, came from the sun. But why would a life form that clearly originated somewhere else have DNA? But then Weir turns around and gives us Rocky, an alien engineer who is truly alien. Yet truly relatable. Rocky has three fingers at the end of each of his five appendages. He, or rather they as he has no true gender, lives in a world defined entirely by sound, and is utterly astounded to discover the concept of light (which, being a spider-like version of Scotty, he quickly devises a means to perceive), of radiation (which killed his entire crew), and is floored by the idea of relativity. There is nothing human about Rocky except that they are a smartass, which the human Grace finds easily relatable.
And yet there is some scientific basis for life forms being somewhat familiar right here on Earth. I direct you to the octopus and its cephalopodic cousins, the squid and the cuttlefish. The latter have exoskeletons. the former is completely boneless save for a beak. And they all have eyes like humans only… The gelatinous orbs that let both octopus and human use a narrow band of electromagnetic radiation to see are very similar but evolved independently. Moreover, cephalopods have multiple appendages, a trait they share with insects, arachnids, and crustaceans. Humans have four limbs, as do most mammals and their close cousins, reptiles. (Snakes don’t. Snakes are just weird. Like God and Darwin got high one night and said, “Hey, what about having a reptile with no limbs that will scare the hell out of everything else?” “Dude, that is sooooo coooool. Let’s do it.” Yeah, Darwin talked like Tommy Chong, though I believe God is really Willie Nelson.) Some scientists believe squids and octopuses will eventually replace humans as the dominant intelligence on Earth. In a billion years, so chill. You can still have calamari, and your great grandchildren need only worry about humans trashing the planet. These two forms – boneless with tentacles and the four-limbed bone mech wrapped in meat armor – are responses to gravity and environment, conditions which we will likely find on other Earth-like worlds. Intelligent life is likely to be big-brained and able to make tools. These are the most likely form factors such life would take. (With Andy Weir’s caveat that maybe they won’t. We have to meet aliens to find out.)
There’s a theory that, if we find humans on another world or something human-like, people would be terrified. I don’t agree. Humans independently evolved on another world would be a function of environment. Naked apes with the capacity for abstract thought would be the best species to dominate a world. Transplanted humans would imply even more advanced aliens somewhere in the universe. What it comes down to is that atheists would have to admit they don’t know nearly as much as they think. (That’s called being human. We don’t know everything and never will.) It also addresses one of organized religions’ conceits, that humans are the center of the universe. However, one has to consider astronomy has pretty much imprinted the idea that God, the universe, and everything is a lot smarter than you are.
I play on this in my own work. Most humanoids in the Compact Universe are similar to humans, but with obvious differences. Zarans are essentially what happens if chimps or another hominid rose to dominance, Qorori have six fingers and are nocturnal. Another species is a mish-mash of welded-together genes, which is a neat trick. In my work, only the Orags, original transplanted Neanderthals, have DNA as their genetic material. The Gelt do not, and yet they are roughly the same size as humans with five fingers, four limbs, and a similar set of languages. This similarity sometimes unsettles both humans and Gelt as they interact. An alien is supposed to have fur or eyes on stalks or something. Be a reptile. Have extra digits. This thing looks too much like me for it to have different skin and eyes and the wrong teeth. The theory, which is brought up in a couple of stories, is that all the species now interacting got their stellar chemistry from the same star nursery. Stars that formed from it formed alongside our sun, so they would have similar chemical makeups in their solar systems. Goldilocks worlds would end up being very Earth-like because the right chemical soup existed early on. So the genetics would evolve to make bipedal, sound-modulating, light-sensing beings. And then, of course, I debunk that by having a Gelt woman with a human lover lament they can’t reproduce because the nucleic acids in either species’s cells don’t match. Again, humans have DNA because almost every living thing on Earth has DNA. The woman says, “We don’t even have a double helix to plug into.”
Of course, everything is speculation. We could discover that Lovecraft was right, that Elder Gods exist out there and would drive us mad if we gazed upon them. Personally, I think Lovecraft needed some Xanax and a more active social life, but I’m not a fan. And Star Trek could also have been right. Some life is energy-based or silicon-based. It’ll be a long time before we find out.
At least I hope it’s not Independence Day in the process.