In countless science fiction series, mainly the more popular ones, almost all of the aliens appear humanoid, that is bipedal with 2 arms and two legs. Is it a bad idea to include this in worldbuilding? Is it just a lazy excuse, a way of humanizing the alien or is there a good reason for an alien to be humanoid?

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    $\begingroup$ In some universes there is a reason. In others there is none. In others, aliens aren't humanoid. So the only possible answer is "it depends". $\endgroup$
    – Mołot
    Commented Nov 21, 2016 at 18:23
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    $\begingroup$ If this is asking "why would aliens be humanoid or not be humanoid" I think you'll find that information here and here and here and also in plenty of places here. Possible duplicates on the first three links $\endgroup$
    – Zxyrra
    Commented Nov 21, 2016 at 18:30
  • $\begingroup$ This is really a writing question that comes down to: how much do you want to explain the differences to your audience? If your species is completely inhuman, expect to be doing a lot of explaining. $\endgroup$
    – Kys
    Commented Nov 21, 2016 at 20:48
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    $\begingroup$ Its easier to fit the actors inside. $\endgroup$
    – Phil M
    Commented Nov 23, 2016 at 22:25
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    $\begingroup$ This is in the VTC queue, but I'm not going to vote to close. This is a good question for the worldbuilding-process tag, which is becoming unpopular as it "feels" like it's "opinion-based." This is one of those few question types that intentionally break (by community consent) SE's rules for the greater good. We should be asking general questions about how to build worlds, not just questions about solving difficulties with just one person's world. So, voting to leave this one open. $\endgroup$
    – JBH
    Commented Jan 17 at 18:26

7 Answers 7


It is not a bad thing.

The humanoid form has adaptions which are flexible, capable, and robust, enabling said organism to interact more or less successfully in a number of environments. It is not unlikely that it will prove to exist on multiple inhabited worlds in real life.

This does not mean that there will not be non-humanoid life forms either. Just look at the variety our world has managed to produce.

Conceptual difficulties

On the other hand, it is fairly difficult to truly put yourself in another's perspective, let alone a truly alien perspective. After all, if a human can conceive it, is it an alien perspective anymore? One possible explanation for the phenomena of anthropomorphic aliens, is that it is easier (and in some cases more believable to an audience) for aliens to be just a little bit alien.

They are different enough to be "alien", yet similar enough to empathize with despite the differences. This is also not necessarily a bad thing. Many stories even include the point in the meta-plot or even plot of the storyline as to why so many aliens are humanoid.

Truly Alien

As challenging as it might be to create a truly alien mindset, that has not stopped many from trying. The results range from the amusing, to the thought-provoking, to the bizarre, disturbing, and incomprehensible.

Obviously, there does exist those works where someone couldn't simply be bothered to try, or other constraints (budget) didn't allow for excessive creativity.

In the end...

... it is up to you. Pick what suits your need, make any in-plot excuses, and go for it.


Short answer: It depends.

Long answer: it depends on the purpose of these aliens. Many of the popular series (Star Trek/Wars) use bipedal aliens for purely pragmatic reasons: it's much easier to apply makeup to a human actor than to make a puppet or CG model. And even when a puppet/CG is used, it's much easier for a human to animate a humanoid model than something entirely alien. We have a very thorough knowledge/intuition of how humanoids behave, which won't carry over if we make the alien a 9-legged spider.

Additionally, a humanoid alien works as a form of narrative shorthand. The audience has a good sense of how a humanoid works, which means the writer gets to focus on the differences that he views as important. We don't need to consider how a Wookie might use tools or eat food or sense the world, since all of that is essentially the same as a human. All we need to care about is that Wookies are strong, furry, and speak in roars. So as a storytelling technique, I wouldn't necessarily call it lazy. It just means you're focusing on other things than the number of limbs the alien has.

Now, if the nitty-gritty evolutionary biology is what you care about, there's certainly no reason a bipedal humanoid can't work. While there's a lot of bias here, humans are living proof that the design can be very successful. Having limbs dedicated to manipulation means those limbs get to specialize and become very good at manipulating things. And that dexterity is one of the keys that made us so successful.

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    $\begingroup$ I like this answer. In most stories with really ALIEN aliens the focus is on their alieness; how they work, how their society is different, etc. When the aliens are wrinkled forehead aliens the focus is usually on some human social issue cloaked in sci-fi trappings. The intent of the story is just different in the latter, to have wacky aliens would distract the reader. $\endgroup$
    – Jason K
    Commented Nov 21, 2016 at 20:20
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    $\begingroup$ Star Trek specifically has an in-plot rational as to why so many aliens are humanoid. Not to say that was the intent from the beginning, just mentioning that it has a rational explanation. $\endgroup$
    – nijineko
    Commented Nov 21, 2016 at 21:09
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    $\begingroup$ @nijineko I think I would call it a rationalizing explanation, rather than a rational one. ;) $\endgroup$
    – Ghotir
    Commented Nov 21, 2016 at 22:05
  • $\begingroup$ @Ghotir I would have to agree with that, considering when the 'explanation' in question was offered in the publication timeline of the entirety of the storyline universe. (ie: very much after the fact.) $\endgroup$
    – nijineko
    Commented Nov 28, 2016 at 0:30

For all we know, the only way to have human-level intelligence is to be humanoid.

Notably bipedalism seems to have been pretty important in our being intelligent:

  • Staying on two feet requires more brain-power than on four
  • Delivery needed smaller heads, leading to a longer brain development (and therefore an increased ability to learn)
  • Hands are pretty useful if you want a technological species

You can totally imagine a technological alien civilization, but you would need it to have bodies as good as ours for that purpose and find a good reason to compensate those three key points.

So, it is not bad world building. It's either lazy or realistic world building, depending on how you look at it.


If you make a humanoid alien, they might be easier to relate to. However, if you want your aliens to be truly different, make them different. I do not think creating bipedal aliens is being lazy. I think if you have a six-eyed, four armed, eight-legged alien, you have to figure out how they move and what they eat or breathe or how think, and that it is more difficult to make them real. If you go with the humanoid, the reader will make certain assumptions.

I think it only matters what your story is about. If is about a totally different way of being -- make them non-humanoid.

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    $\begingroup$ OTOH, I recall a book (some years ago) about a race of trilaterally symmetric, trisexual aliens descended from something similar to land crabs, who dwell in a desert environment and regard water as corrosive - yet they were psychologically quite human. In fact, the protagonists were an alien Holmes/Watson duo. $\endgroup$
    – jamesqf
    Commented Nov 22, 2016 at 5:06
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    $\begingroup$ it's a good point -- and remember good old ET? He was only vaguely human-like, but we related to him as we might a foreign child. I think we as humans prefer to have an understanding of our protagonists -- at least the main ones. The spider people who invaded Earth in Starship Troopers were all the more frightening because we could not relate to them. $\endgroup$
    – WRX
    Commented Nov 23, 2016 at 17:28
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    $\begingroup$ FTM, dogs aren't particularly human-like, but I'm sure I'm not the only person who likes the average dog better than the average human :-) $\endgroup$
    – jamesqf
    Commented Nov 24, 2016 at 4:26
  • $\begingroup$ fair point, but we would expect a dog-like creature to be sort of like a dog. Mine talk to me all the time. $\endgroup$
    – WRX
    Commented Nov 24, 2016 at 16:27

I would call it realistic, whether we are here or on another planet, all species would go through evolution. We ourselves are a product of billions of years of evolution, and we exist because we are a good combination of successful traits. So we already have a very tight limit of how aliens could look like.

If we want them to be more advanced than cave men, the limits become even more drastic. They need the ability to use more complex communications, they need limbs that allow advanced interaction with their environment, they need ways to travel at least a certain distance with tolerable speeds and carry at least some weight. They need the ability to embrace their environment in detail and they need the brainpower to crate logical connections.

All that would make intelligent life look very similar, even on different planets.

  • $\begingroup$ Isn't that a bit like saying the universe should be populated by intelligent 2-tentacled, 8-armed mollusks, 140-ton leviathons and tiny 6-legged five-eyed insectoids because that's what works here? $\endgroup$ Commented Nov 24, 2016 at 21:51
  • $\begingroup$ I don't understand what you are trying to tell me. $\endgroup$
    – Etaila
    Commented Nov 25, 2016 at 7:41

Richard Dawkins was once asked what alien life would look like, and he responded with something like "well, it would probably be Darwinian"

A lot of things are possible via evolution. Earth only represents one outcome of evolution, and there could be many others that are hard to even wrap your mind around.

An interstellar species will probably have "arms"

While the number of legs and a lot of other factors may vary, it is strongly beneficial to have at least one prehensile limb that is not used for "walking". These limbs could be anything from humanoid arms to tentacles and trunks. Intelligent life largely revolves around the use of tools; intelligence is largely a dead-weight trait if the animal doesn't have a way to make use of it, and therefore it goes hand-in-hand with a prehensile limb or two.

Other than this basic soft requirement, non-humanoid forms are going to be far more likely than humanoid ones just by sheer probability. But at the same time, something bipedal with two arms is reasonably likely to emerge from a linear body plan because evolution tends to select against redundancy. However, there is no reason to assume that they would have all the same organs and body parts in the same place. For instance, the "face" could be below the arms, eyes and ears could be in wacky places, they might have multiple hearts or stomachs, they might be avian or amphibious, could have feathers, scales, fur, etc...

A quadriped with a prehensile tail or extra limbs on its back could also form. You may even get an invertebrate with a radial body plan, like an octopus.

Other aspects of physiology will vary based on the home planet's atmosphere and gravity, but generally will be based on the same or similar cellular structures and thus breathe oxygen and use hydrocarbons for energy because it is advantageous to be built from the most common elements in the universe. I'm no chemistry expert, but I suspect animals that inhale CO2 and exhale O2 would be unlikely or impossible because breaking down hydrocarbons and oxygen into carbon dioxide and water releases the energy they need to move and live.

Humanoid aliens are lazy and unrealistic, but practical and relatable

Most of your audience won't care if your aliens are humanoid. It's a lot easier to relate to humanoids and it's easier to dress up human actors than do CGI.

Non-humanoid aliens are going to be much friendlier to animation and books than movies and TV shows.


A species developing technologically is going to require some features. There are a couple traits essential to gaining a technological civilization. One is the ability to manipulate tools, You also need to be able to communicate complex and abstract concepts. Take Dolphins for instance, researchers are beginning to suspect that dolphins are far smarter than primates and theorize that they may even posses an actual language that may even be far more complex than anything we have.

So why did we evolve from primates instead of cetaceans? Even if it is true that they can talk to each other they don't have hands. Also They live underwater. Living underwater means that you cant discover fire. Not having hands means you can't invent tools, or begin building complex structures. I guess my point is that if they aren't humanoid that doesn't mean they cant be sentient or have developed technology, but a sentient puddle of immobile slime or a gas cloud, or some kind of silicon based living crystal pillar isn't going to be able to build anything or go anywhere, and probably wouldn't have anything very interesting to say either. If your alien race is going to be technologically advanced you need to give them a way to communicate, and a way to manipulate objects.

  • $\begingroup$ This does not answer the question, states assumptions (some verifiably false) as universal truths, and seems to confuse humanoid with "has hands and feet". $\endgroup$
    – rek
    Commented Feb 10, 2018 at 7:46
  • $\begingroup$ "not having hands means you can't invent tools" Crows [especially the New Caledonian Crow], parrots of all kinds, alligators, octopus, certain species of fish, ants, Hunting wasps and my favorite the Bottlenose Dolphin [because it exactly proves you wrong]. $\endgroup$
    – TrEs-2b
    Commented Feb 10, 2018 at 8:26

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