Our world is full of aliens. Ones that we’ve imagined, I mean. Large-headed, large-eyed, silver- or green-skinned visitors on UFOs. E.T. the Extra-Terrestrial’s wide-set eyes, wrinkled brow, and glowing finger. Prosthetic-foreheaded Klingons and prosthetic-eared Vulcans and prosthetic-eared Romulans. Martians who look like humans — lots of aliens who look like humans, actually: four-limbed and upright, except for the ones who are nonorganic beings of pure energy or light. (And the space whales.) They’re psychic or telekinetic or eyeless or faceless, sometimes, but usually not. Especially on screen, they’ll look like us — for the sake of empathy or the simple constraining factor of needing to use human actors. Hence the variety of prosthetics, especially in the decades before computer-generated aliens could join us on screen. (And even then, we missed the prosthetics, in the face of floppy, boneless Jar Jar Binks.)
Whether it’s done by scientists, writers, or visual effects specialists, all our imagining of what aliens look like is just that: imagining. And the bigger, more highly evolved the aliens are that you want to imagine, the bigger the leaps you need to take. Researchers who explore the possibility of microbial life beyond Earth, the different chemistries and microscopic structures that could give rise to life, do so with a practical eye. Their work can offer search parameters to astronomers pointing increasingly sensitive telescopes at planets beyond our solar system, with the chance to detect signs of life — not to mention for researchers designing probes to visit Mars and the possibly habitable moons of the outer solar system. Far fewer scientists spend time thinking about the possible forms of complex life, not just cells that use energy and hold information in DNA or its analogue, but the analogue of animals, the analogue of us.
Aside from the lack of applications for such thought experiments, there just aren’t enough constraints to call that speculation science. Astrophysicist Adam Frank told me, “If you want to use the constraints that science gives you to try and say something about life, then you’re limited to things that are super simple and things that are as complicated as we are.” Frank’s research includes modeling alien civilizations — he prefers the term “exocivilizations,” to match “exoplants” — as agents of climate change. Any technological civilization will necessarily have an impact on the energy balance of its home planet. So we can model that, and we can model exotic cell chemistries, but anywhere in between those two extremes is impossibly murky. At least for science.
The aliens of pop culture are rarely scientific projections. Sometimes a linguist comes in to devise a new language — still spoken, though, usually, like from a Klingon mouth with sharp prosthetic teeth or a Na’vi face CGIed — but most depictions of fictional extraterrestrials are driven by narrative and aesthetics. They’re also constrained by our understanding of life on Earth. The more-alien aliens are usually just extrapolations from nonhumanoid life on Earth: “lizard men and insectoids,” as Frank put it.
The simple path of thinking in that translation is: What if a different branch of the evolutionary tree attained sentience? Sometimes it’s bugs or reptiles, the B-movie classics. In Sue Burke’s novel Semiosis, it’s plant life on a planet a billion years older than Earth, with a billion years more evolutionary time. In Vernor Vinge’s A Fire Upon the Deep, it’s wolf-like creatures that compensate for their lack of humanoid dexterity by functioning as multiorganism beings, psychically linked individuals made up of three or four wolves. (Vinge’s world also has sentient plants and spiders.)
The supposition that life on another planet would follow similar forms as on Earth — plants and animals, humanoids and lupines — isn’t just an easy fictional gloss. It’s an extrapolation from the phenomenon of convergent evolution, when disconnected species evolve the same features independently, because those features are evolutionarily useful. Not every Earth animal with wings or a camera-like eye evolved from a common ancestor. Whales evolved fish-like features but are no more closely related to fish than cows are. So, on another planet with light or air, similar traits might evolve independently again. Same, the thinking goes, for the usefulness of chlorophyll in stationary plants, the usefulness of being a wolf or a spider, and the usefulness of hands. And of intelligence. They’ve all evolved on Earth because they’re useful, so they could very well evolve in parallel somewhere beyond.
In Avatar, James Cameron depicted a sentient, humanoid alien race — just taller and thinner and blue — and went so far as to sketch out the whole world around them, an ecological and evolutionary context. Most animals we see on Avatar’s Pandora have six limbs — a perfectly fine number of limbs; Earth’s insects do just fine. But whether as a nod to anthropomorphic familiarity or for motion-capture ease, the Na’vi have only four limbs.
Paleontologist and science educator Katie Slivensky wrote about the movie’s “cutesy way of explaining this,” a brief glimpse of a tree-dwelling primate analogue whose middle and front limbs seem to be partly fused, an evolutionary stepping-stone between six limbs and four. But to Slivensky’s dismay, a tree-climbing monkey-type is the very last animal that would find four limbs evolutionarily advantageous over six. Making matters worse, Pandora’s ground-dwelling animals keep their extra limbs, but, Slivensky points out, “Animals that specialize in running along the ground…evolve to reduce how much their limbs come in contact with the ground. Therefore, if any animal on Pandora should fuse limbs, it would be these ground-dwelling ones. But in every instance of a ground-dwelling critter on Pandora, it of course has six limbs.” So close, James Cameron, and yet so far.
This isn’t about policing pop culture for scientific accuracy, of course. (That would be a very bad way to do science outreach.) (That last parenthetical was a direct address to Neil deGrasse Tyson.) Slivensky told me, “A lot of pop culture goes more for the imagination than the accuracy.” But, she added, if you’re going to go ahead and not just invent an alien creature but also try to depict its evolutionary context, “It’s actually really easy to just talk to someone for five minutes and get an accurate scientific take on something. It gives you that extra layer of realism.”