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?
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.
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.
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.
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.
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.
An interstellar species will probably have "arms"
While the number of legs and a lot of other factors will vary, it is strongly beneficial to have at least one, preferably more, prehensile limb that is not used for "walking". These limbs could be anything from humanoid arms to tentacles and trunks.
Other than this basic requirement, non-humanoid forms are going to be far more likely than humanoid ones. 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, or they could be quadripeds with extra limbs on their back for using tools. 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.
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.