Spock examining the horta from TOS' The Devil in the Dark.

A bipedal alien says hello to a non-bipedal alien.

Fictional animals come in all shapes and sizes, as it should be when the only limiting factor on biodiversity is the imagination of the artist. So why is it that, out of all possible forms, sapient creatures are overwhelmingly depicted as bipedal with an upright stature? What a coincidence that they should all so strongly resemble humans!

Actually, there are some good reasons for this resemblance and valid arguments to support the belief that, intelligent or not, real-life aliens would look, well, alien.

Why Are So Many Fictional Aliens Bipedal?

Aliens from the Mos Eisley cantina.
Only two legs in a galaxy far, far away.

First, because of constraints. In movies, television, and, to some degree, video games, there are often technical and budgetary limitations. Before CGI, aliens had to be either puppets, animatronics, or actors in makeup, prosthetics, and costumes. Often it was the latter because it’s the cheapest and simplest option. Even after the invention of CGI, this is still the norm. Since actors are bipedal, so too are the aliens they portray. In the case of video games, when the models for humans and aliens are similar enough that the same animations can be applied to both, it saves time and effort.

Second, there’s pesky bias to consider: human bipedalism is structurally unique, and humans like to think they’re hot stuff. Thus, bipedalism becomes part of the visual shorthand for sapience, the truly defining human trait. Making aliens bipedal is one way to make them relatable. But while the human brain might be wired to appreciate human-like shapes, this underestimates our capacity for empathy. We care very much about our pets and often attribute intellectual and emotional depth to them, a phenomenon known as anthropomorphism. The same and more can be afforded to aliens.

Now, I don’t think I need to explain how budgetary restraints and lack of creativity don’t reflect scientific reality. However, there are also strong, scientifically backed reasons for bipedal design. So let’s dive right in to the good stuff: theories of evolution.

How Convergent Evolution Could Create Bipedal Aliens

The xenomorph evolved limbs to grasp you and hold you close!

Bipedal aliens could be the result of convergent evolution – when organisms independently evolve similar traits to adapt to similar environments or niches. Examples include sharks and dolphins, which evolved similar streamlined bodies and prominent dorsal fins. Some would say that a species that is intelligent enough to develop language, technology, culture, and civilization on par with our own would have likely taken a similar evolutionary path.

What is that evolutionary path? Well, early human ancestors evolved the inclination to walk on their hind legs before many changes in their skulls, and therefore brains, ever took place. Let’s look at Australopithecus, a genus related to and preceding Homo, that disappeared around two million years ago. The skull of the Australopithecus is apelike, with a relatively small brain case and a relatively robust jaw. But the skull aside, they share traits with us that indicate bipedality.

  • The opening where the spine attaches to the skull is anteriorly positioned, allowing the head to be held aloft.
  • The lower spine is S-shaped, which aligns the head and torso well above the center of gravity.
  • The pelvis is broad and bowl-shaped, improving stability.

Because these features arrived before changes to the skull, it is possible and even likely that the development of erect posture facilitated later changes in the brain and behavior by consequence.

How would that happen? Let’s take the use of tools as a given for any creature capable of forming a civilization. To make complex tools, one needs the ability to manipulate the environment. Hands are certainly good for that, but bipedalism isn’t required for hands. Many tools can be constructed while sitting down. Recall our close relative the chimpanzee, a renowned tool-maker, who has hands quite similar to our own but only walks upright on occasion.

The only advantage bipedalism brings to the table is the ability to carry tools, which doesn’t necessarily influence the ability to craft tools. Or does it? Consider this: if you can’t conveniently carry your tools around, you have to abandon them and start fresh every time you need that tool again, which doesn’t make much sense unless the tools are relatively simple, like the sticks chimpanzees use for termite fishing. If you can conveniently bring your tools with you, refining them isn’t a waste of your time. Think about how long it would take to chisel a spearhead, and imagine having to repeat the process every time you needed one. Not a very effective survival strategy.

A number of animals that make tools can learn to do so by observing another member of the same species. Tool use and even gestures can spread through social transmission. But observing another using a tool and improving upon the design is uniquely human.

What’s more, the oldest stone tools ever found predate the oldest known fossils of genus Homo by half a million years. Remember the ape-brained Australopithecus? They could have crafted these tools, and their ability to carry them might have made that practical when it simply wasn’t before. Reliance on tools might have led to natural selection for larger brains in humans.

How We Became Bipedal to Begin With

Coquerel’s sifaka lemur’s terrestrial locomotion by Charles J Sharp used under CC BY-SA 3.0

If we want to create well-thought-out bipedal aliens, we should consider how they became bipedal, like us. Convergent evolution between two organisms requires a similar environment or niche. The niche of humans can be hard to define, because we have used technology and culture to construct and expand upon our natural niches. The same can be assumed for any intelligent life. So, let’s forget niche and focus on environment.

The transition from quadrupedal to bipedal movement occurred around the same time as certain changes in the African savannah: the forests were thinning, creating vast grasslands. Our arboreal ancestors, adjusted to living in the canopies, had to adapt, driving our quadrupedal ancestors to split into different species.

Ancestors of orangutans would branch off first, becoming extremely specialized for life in the canopies. The ancestors of gorillas and chimps became more versatile. Gorillas and chimps are mostly quadrupedal knuckle-walkers, but they also developed the ability to walk upright on occasion, a locomotive strategy known as facultative bipedalism.

What early humans would eventually do, by contrast, was take advantage of the newly opened grassland niches their relatives had yet to exploit. This is where bipedalism comes in handy, according to the savannah hypothesis. It is more efficient for traversing the flat expanse, and it elevates the head for increased scope of vision.

It’s important to point out that the savannah hypothesis has come under scrutiny in recent years. Bipedalism might have begun in the trees before moving out onto the open plains. The vertical climbing and knuckle-walking of gorillas and chimps uses similar muscles to human bipedalism. The common ancestor of all surviving great apes may have used a sort of hand-assisted bipedalism by grasping overhead branches while climbing.

Regardless, it is fair to say that the advent of human bipedalism was a unique occurrence that happened under extremely specific circumstances. The specific environment had to change when it did. Timing is just as important as place. Without certain pre-adaptations unique to apes existing at the same time as the shift in environment, bipedalism might not have been possible at that time.

While it’s possible for all these factors to align somewhere else, requiring all your alien civilizations to have arboreal origins seems pretty limiting. Let’s explore some alternatives, shall we?

Not All Bipeds Are Humanoid

Kangaroo Boxing by Scott Calleja used under CC BY 2.0

There are certainly other bipedal animals on Earth we can look to for examples. Aside from leg number, these bipeds share little in common with humans, varying wildly in appearance and lifestyles.

Let’s look at the kangaroo and velociraptor. These are two very different beasts, both bipedal, though the similarities don’t quite end there. Note that both their torsos lean forward and must be counterbalanced by a tail. Contrast this with the tailless human, standing fully upright. These differences have functional consequences, changing the act of walking.

Whereas humans balance smooth strides with alternating rotation of torso and pelvis, the kangaroo hops. Its legs are locked together in parallel motion, unable to move independently, and for this reason it can’t even walk backwards. Despite this, kangaroo hopping is among the most energy-efficient forms of locomotion in the animal kingdom. When there’s no rush, kangaroos will forego hopping in favor of crawling, using their forelimbs and tail in addition to hindlimbs, making for a technically pentapedal gait, though this actually consumes more energy than hopping does.

Velociraptors might have lurched forward or waddled. Unlike the kangaroo, their tails were inflexible and would have been capable of limited side-to-side movement at most. Still, the tail is speculated to have provided stability during speed bursts and sharp turns.

Another alternative body plan that includes bipedalism comes in the form of our close relatives the chimpanzees. They are primarily knuckle-walkers, but they are capable of walking bipedally temporarily. Walking this way appears uncomfortable and just plain awkward for them. They must shift their weight side to side, bowlegged, but they will do so to carry food for short distances. Surprisingly, there appears to be little difference in energetic cost between two- and four-legged movement for them.

While these creatures are different enough from us for their civilization to prove challenging to imagine, there will also be familiar touchstones that will ease the way. Maybe, like us, your creatures put their pants on one leg at a time, but an additional sleeve is required to accommodate their tails. Would a hopping creature use stairs like we would, or would they require a different design? It’s enticing questions like these that make worldbuilding so much fun.

Hands and Carrying Don’t Require Bipedalism

African elephant (Loxodonta africana) reaching up by Charles J Sharp used under CC BY-SA 3.0

So we’ve established that freeing up the hands gives humans an advantage that any sapient creature would need if they were to get to the point of forming a civilization. Let’s consider ways to achieve that without bipedalism.

Perhaps your alien species has a different strategy for transporting items, maybe in a pouch like how a marsupial carries offspring. Additionally, many rodents and some primates make use of cheek pouches for transporting food, which is another strategy that does not require free hands.

There are also many potential prehensile appendages besides hands. Some animals have prehensile tails, but tails being posteriorly positioned makes them unlikely to be as good for manipulation, as they are out of the field of vision. A muscular hydrostat is a possible option; examples of these used for grasping include octopus tentacles and elephant trunks. Another type of muscular hydrostat is the tongue, but it would require a lot of modification to be useful in carrying or manipulation, and it would need to be dry. The penises of some animals, such as elephants and tapirs, are also prehensile.*

Muscular hydrostats are less firm and more flexible than hands are, which could be a downside or an advantage, depending on the situation. Keep in mind that pre-existing structures can be, and often are, modified to suit new purposes. If an animal starts carrying certain objects with their penis and it improves their success, their penises might become even better at grasping over time.* Think about it—a trunk is a modified nose. You could probably make any structure prehensile, if you tried hard enough.

Anyway, you might notice that any lone appendage is not going to be particularly dexterous. Carrying doesn’t require fine motor control, but crafting does. The solution to this is to try combining multiple grasping appendages on the same organism. For example, corvids might be bipeds, but they do not gain the “freeing of the hands” advantage because of their wings. They have been observed making and using tools, however. They might use their beaks in combination with their feet to make tools, like grasping a piece of wire in one foot while bending it with the beak into a hook shape. While the beak and the feet are not particularly dexterous in isolation, they work well together.

Try combining any of the appendages that have been mentioned above. Your species could also be quadrupedal with grasping hands, like a chimpanzee, but with an additional appendage of some kind, like a trunk or a tail, to aid in carrying while the hands are occupied in locomotion.

While there is so much variety just on Earth, you don’t have to limit yourself to using its creatures for inspiration. For example, you could have an animal with six or more limbs who uses at least four of the hindlimbs for locomotion while reserving those in front for grasping. Think of something like a centaur. While not strictly scientific, psychic abilities could also be used to manipulate the environment and carry objects.

A species identical to humans except for a pair of pointy ears is lazy, but so too is a species with wildly alien anatomy that still lives and acts exactly as we do. It all starts with the small differences and builds up from there. Everything is connected; nothing is arbitrary. If we still had feet for climbing, would we shoe them? Would we clasp toes with our lovers? How would we dance, if at all? A different body has different capabilities and limitations, resulting in different psychology, culture, technology, and society.

This article is an abridged version of Questioning the Bipedal Default from Volume 2, Issue 5 of Worldbuilding Magazine. Read the full article.

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