Lets say that at least one alien is proven to exist, and it is roughly humanoid. It may have Klingon ridges on it's forehead, or blue skin, or have an extra set of limbs somewhere, but it's effectively humanoid in structure.

These alien are carbon based, can survive in our atmosphere with only limited, if any, life support systems. They reproduces sexually. Regardless of their culture their basic basic evolutionary psychology is at least roughly equivalent to our own, they like the same things, they pair bond in the same ways, they have a herd mentality and can't tolerate extend isolation. They conduct wars and institutionally nurture babies and are emotional and creative but can make logical facilities just the same ways humans do.

This species is 'relatively close' to us, on a astronomical perspective at least; close enough for us to one day theoretically have to worry about avoiding each others territory, because like us they value territory. They are also 'relatively close' on a technological level, sure they may have ray guns and ships that can travel at greater speeds, but their only a few centuries difference in tech, not millennial differences.

If more then two intelligent species exist then at least the majority of the ones we become aware of is humanoid.

Any scientist will tell you that the odds of this occurring is astronomically slim. However, humanoid aliens are more relatable then a strange methane based species that has limited sense of self or a crystal based species that never utilized sexual reproduction and has no sense of pare bonding or even traditional children.

So how does one justify the use of humanoid aliens, short of A wizard did it.

So far the only explanation I can think of is that all the species shared a common ancestor, but that requires hypothesizing an ape-like creature from earth was magically transplanted to another earth-like world it could survive on, in other words an unbelievably unlikely scenario is the closest to a believable scenario I can think of.

What other approaches may justify an alien species so similar to humans as to be relatable by them?

Edit: I should have made it clear. I'm presuming that the argument that "aliens just evolved like us because of convergent evolution" is invalid due to how astronomically unlikely I consider it is that a sapient, or even sentient, alien species would manage to follow such an exact path. I would accept convergent evolution combined with other approaches to justify why it occurred, for instance if a claim was made that many sapient species exist but we only find ones that are built like us because we only look for signs we associate with intelligence, which are all biased to presumption of species with similar evolution.

I wouldn't accept that as a sufficient explanation, but it does explain why we'd only run into humanoid aliens. Another possible explanation in that vein is that somehow, life is very common and the universe is just biased towards one kind.

The point is, I am looking for an answer that justifies humanoid creatures with presumption that evolution is not somehow per-programmed to be humanoid by default.

  • $\begingroup$ Perhaps this has some answers. $\endgroup$ – KillingTime Sep 8 '15 at 20:30
  • $\begingroup$ @KillingTime that is the question that inspired this one. But since the answer to that one generally seems to be "no" that leads the question of how to justify humanoid aliens when you can't just say "it's how everyone evolves" $\endgroup$ – dsollen Sep 8 '15 at 20:33
  • $\begingroup$ @DaaaahWhoosh I would not consider it a duplicate, but that is partially because I did not clarify my intent sufficiently in my original question (edited). I want to presume that convergent evolution is not an option, which is the majority of the answers of the above question suggest. $\endgroup$ – dsollen Sep 8 '15 at 20:59
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    $\begingroup$ @DoubleDouble yes, but the time span to find them is "how long humans are space faring before we blow ourselves up", and as I stated in the question I want them to be somewhat 'close' on an astronomical scale. Thus both time and distance is bounded, dropping those odds drastically. Infinite number of monkeys pounding on keyboards can eventually write Shakespeare, but a mere thousand probably won't manage it before the end of your lifetime.. :) $\endgroup$ – dsollen Sep 8 '15 at 21:33
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    $\begingroup$ I don't really see why you'd consider convergent evolution to be insufficient explanation for why a space-faring civilization would be similar to humans. Consider what a species would need to build such a civilization; advanced manual dexterity and ability to communicate complex ideas at the very least. Those alone rule out innumerable possible body plans and lifestyles. Also consider that many things like encephalization and sexual reproduction which might seem random are favored by evolution. I think the odds are very good that any interstellar people would be similar to humans in many ways. $\endgroup$ – EldritchWarlord Dec 22 '16 at 21:51

If you need aliens to whom human readers can identify with and relate to then the form of the mind is more important than the body.

Compared to what we understand about evolution and possible bio-chemistries, almost nothing is known about the actual functioning and evolution of human or animal sapience.

It may therefore, at this point in time, be more productive and plausible to posit convergent emergence of an architecture of mind than convergent evolution of physical form.


While there are some patterns that "seem natural" for space faring species, it is worth noting that evolution is not seeking a goal. Whatever it finds it has to work with, it works with "happily." Accordingly, the shape of a space faring species can be markedly dependent on what was most useful at previous layers of evolution.

As an example, our children exit through a remarkably small channel in the pelvis, and we pay tremendous prices for that. If we had evolved from marsupials, we may have a very different physical and social structure. Now, this small shift may still leave us humanoid, but its pointing at just how much of evolution occurs because "it's what we had at the time, so we make it work."

So whatever species found the "secret" of intelligence first will be the one which shapes the planet's "dominant species." Consider if intelligence was first needed by a segmented arthropod like a centipede which found it evolutionarily selected for to have two brains, one in each end. That way, at that dumb level, if one head was damaged, the other half continues furthering its genetic material (assuming "genetic material" is universal, which could also be debated). Soon it may find that a language between the brains, done along the nervous system, is effective for capturing prey by moving in unexpected ways. This could lead to inter-animal language very rapidly. It is not unreasonable to guess that such a creature, with language skills, could evolve towards the dominance of their planet.

And they would be looking at this blue green ball pondering, "There's no way there's intelligent life down there. Certainly not space faring life. There's no way they could have managed the complicated piloting skills required to leave the atmosphere without two brains connected together to dance across the controls. Only with two brains can you possibly get into space."

Never mind the monkey with a hammer... evolving into a slightly taller monkey with a liquid fueled rocket engine.

  • $\begingroup$ I down voted this because the claim that evolution does not have a goal is orthodox, but not entirely accepted and certainly not proved and should not be presented as such. Quite a number of physicists and biologists believe it does have a goal and the one normally cited is that life seeks to maximise long-run entropy production, otherwise known as the 'Maximum Entropy Production Principle'; MEPP. This view is entirely consistent with what we know about the arrow of time. Its therefore plausible to posit convergent evolution of intelligence so as to align with this goal. $\endgroup$ – rumguff Sep 8 '15 at 22:39
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    $\begingroup$ I believe the "no goal" referred to not having a physical blueprint that evolution was working towards. For example, humans are not "more evolved" than chimps; simply differently evolved. $\endgroup$ – Michael Richardson Aug 30 '16 at 15:53

Well, it's happened once. It has proven a successful design, leading credence to it happening again. Most vertebrates on currently on earth have 4 main limbs for locomotion. whether they are flippers or wings, legs arms. So it is actually fairly reasonable that humanoid is just as reasonable as any other design. The torso is designed to protect sensitive organs, the limbs to move the body around...

So while we like to believe that aliens will truly be alien in nature, physically they could easily be similar to us. Socially could be a completely different matter. However, most of our leaps in tech and knowledge, have come about from our 'willingness' to share from our social nature. So being social is likely to be a factor in speeding up a species evolution. Any that are not will, we will adapt much faster than them and prevent them from being a threat.

  • $\begingroup$ You're arguing that aliens could be similar to us, but you're not making an argument as why it would be likely, if at all. $\endgroup$ – durandal Dec 9 '17 at 1:32

The way I understand your question is (taking your edit into account) : "How to justify that all aliens evolved independently into human-like creatures, but without justifying it by the fact that evolution of sentient beings must result in human-like creatures ?"

Restated that way, the question is contradictory. Therefore there is a false assumption in the question. There is two possibilities :

  1. The evolution of the different sentient beings was not independent. It can be that there is a common ancestor or that the first sentient capable of inter galactic travel created all the others based on itself or wiped out all species which were different from it.
  2. The evolution as we picture it is not correct. It can be, once again, that the first sentient creature created all others, or something deeper about evolution or the universe. For example if the universe is a computer simulation due to people lacking imagination, or things like that.

You have of course more possibilities if you accept divine interventions or magic.


Let's start with the question of why all those life forms are carbon based and have similar biochemistry. This can be explained by Panspermia. Yes, it means that life forms were transferred through space, but that's not ape-like creatures, but simple bacteria. And as you can read on the linked Wikipedia page, it's a concept actually considered by scientists.

OK, so we have basic biochemistry right. Obviously, such bacteria will thrive on planets whose conditions are sufficiently close to ours. So let's tackle the next problem: Why would the aliens reproduce sexually?

Well, evolution is driven by a remixing of genetic material. Bacteria are doing it by direct exchange of genetic material, but that's a way that cannot work for multi-cellular organisms. So the evolution of multi-cellular organisms is slowed down if there's no way to mix genomes. Now sexual reproduction is exactly the mechanism how multi-cellular organisms mix their genes.

OK, but why are there sexes? Well, ultimately the combined genome will have to be in a single cell. Merging two complete cells is probably much more complicated than having one cell that's little more than a gene carrier (the sperm) to deposit its genome into another that is a complete cell (the egg).

But why are there exactly two sexes? If more than two individuals would have to meet, things would get quite complicated (having two individuals come together is already complicated enough). Now there could be more sexes where any two can produce offspring (and indeed, that exists here on earth), but certainly, the simplest solution is to have exactly two sexes, which is why this is the dominant situation here on earth, and surely also on alien planets.

Next, intelligence as we know it will only evolve among social species, as it is exactly the social interaction that enables intelligence, and ultimately culture. There may be other forms of intelligence out there, but we probably wouldn't recognize it as intelligence.

Being social species with opposite sexes, there will automatically be emotions binding two individuals together (love, loyalty), as well as competition between individuals (greed, desire for power), and the desire to punish individuals that violate social norms (revenge, hate, contempt). Such individuals will also have a conscience in order to be able to function in the group. So the emotional basis will necessarily have some similarity to our own.

Things like wars or city anonymity are then the result of larger populations than what the emotional programming can handle.

  • $\begingroup$ I'm more then willing to accept panspermia as a starting point. While I could argue any organism that survived the transfer to another planet is likely durable enough to survive on many planets quite different from earth I'm willing to give creative license to the claim that only the ones that found earth-like worlds managed to thrive for story telling. Sex is pretty useful, so while it may be our own human bias that prevents us from seeing alternatives I am willing to consider convergent evolution of sex. However, none of that requires anything human like being the end result... $\endgroup$ – dsollen Sep 8 '15 at 21:16
  • $\begingroup$ There is also asexual multiplication (cloning) both in single celled and multicellular organisms. And then there are higher organisms that change their sex when needed. Furthermore we only know one intelligent species (us) and that's hardly a decent base for statistics. If you include the animal kingdom, well squids are reasonably intelligent and not very social. $\endgroup$ – durandal Dec 9 '17 at 1:30

Yeah, I've often observed that TV aliens always look like "people except". They look just like people except that they have pointy ears, or they look just like people except that they have blue skin, or whatever.

Suppose someone who had never seen any creature other than a human being was shown pictures of a human being, a chameleon, an octopus, a pterodactyl, and an alien from Star Trek, and asked to guess which one came from another planet. Which do you think they would pick?

An evolutionist would say that a creature would evolve to fit its environment. Unless the alien's planet is very similar to Earth, the probability that it would evolve to look similar is very small. A creationist would say that God would create a creature to fit its environment, with similar conclusions.

Are we supposing that the aliens' home planet is exactly like Earth? The same gravity, atmospheric chemicals, weather patterns, etc? If not, the idea that a humanoid creature would be well-suited for that environment gets very small.

Even creatures that live in similar environments on Earth don't look anywhere near as similar as human beings do to TV aliens.

One could argue that any intelligent, technological being would just have to have certain physical characteristics. Like it would have to have a brain of a certain minimum size. It would have to have some sort of appendage capable of manipulating objects or it wouldn't be able to build technological things. Like it's hard to imagine a race of technological dog-like creatures because how could a dog build complex devices with its clumsy paws? They're just not designed for that. Maybe one could make a case that the humanoid form is the only possible shape for an intelligent, technological creature, or at least some sort of optimum shape. But this strikes me as really unlikely. I can easily imagine a creature with thumbs that bend in the opposite direction, or eyestalks on its head, or arms that come out the front of its chest instead of the sides, etc etc. Science fiction writers have imagined many very unusual-looking aliens. Of course many body shapes and designs that a writer can describe on paper would be impractical in the real universe because of biological realities. But are only the most trivial variations from the humanoid shape possible? I doubt it.

Of course the real reason why TV aliens are almost always humanoid is because in real life they must be played by human actors, and it is very difficult to fit an actor into a costume that isn't basically human-shaped. And actors want to display their talents by expressing emotion with facial expressions, using mannerisms that represent the character they are playing, etc. That gets very difficult the more non-human the costume gets. Yes, you can have a weird-looking alien that is operated by electronics and hydraulics, but it's a lot of work and expense to make such a thing convincing. A lot more than just painting someone's face blue and gluing antennae on his head.


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