In almost all science-fiction stories that include alien races, said aliens are similar to humans in many ways: bipedal (and often humanoid), communicate with sounds coming out of a mouth, orient in space thanks to light, are fleshy (they look like carbon-based animals with an inner skeleton)...

Why would aliens be so similar to humans?

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    $\begingroup$ "Why would aliens be so similar to humans?" Are you looking for answers based on an assumption that they would be so similar to humans, or are you looking for answers which start out with a blank sheet and try to reason about what they would look like? $\endgroup$
    – user
    Sep 28, 2014 at 13:51
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    $\begingroup$ No I assume they look like us (it is fundamental for many stories). Say for example "Star Wars' universe is real", why do they look so much like us? $\endgroup$
    – Sheraff
    Sep 28, 2014 at 13:54
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    $\begingroup$ So you are essentially looking for an answer to "if aliens look similar to humans, then why is that?"? $\endgroup$
    – user
    Sep 28, 2014 at 14:14
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    $\begingroup$ Yes, is that unclear? Should I edit? $\endgroup$
    – Sheraff
    Sep 28, 2014 at 14:19
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    $\begingroup$ Sorry, I realized after I posted that comment that the way I phrased it really was rather close to what you were getting at. What I meant was more along the lines of "if aliens were to look similar to humans, what would cause them to evolve that way?". In movies, I imagine it's much because it's easier, both on the movie crew as well as the viewers, if aliens are largely "humans with some single odd aspect". $\endgroup$
    – user
    Sep 28, 2014 at 14:20

14 Answers 14


(This went long, sorry about that. Summation at top, details at bottom.)

Similar environments and somewhat similar evolutionary histories will produce parallel forms. On most Earth-like worlds, the same forces that shaped humans and all our ancestor species will also more often than not produce "humanoids" as a tool-using species. Not Star Trek, "just-like-humans-except-the-nose" humanoids but bipedal, feet at the bottom, arms or arm analogs towards the top and a head with a big brain at the top.

Short answer:

Since most niches on a terrestrial world require movement, linear body plans have an innate head start in filling most niches. That is physics. Despite the randomness of variation, linear body plans are easier to adapt to movement niches than radial body plans because fewer variations are needed. Once linear organisms dominate more niches, they have more innate variation and thus subsequent species are easier to generate when new niches open up. The linear plan and its generic template becomes core to a larger number of species and subsequently harder to change radically. It's easy to tweak a linear form to shoehorn it into a niche than convert a linear form to a radial or add all the needed variations to a radial.

For a linear body plan, a humanoid body plan is the easiest to evolve to free up limbs from movement to manipulation.

Therefore, rough humanoid shapes are statistically more likely than non-humanoid on Earth-like worlds.


People forget that the only random part of evolution is the production of variation. Selection, the other part of evolution is the utter opposite of random. In selection, the environment compresses a species into a specific shape using variation as "lubricant" (by analogy.)

This means that similar environments develop similar forms regardless of evolutionary history. On Earth this is called parallel evolution. The canonical example is the mouth shape of the flamingo begin a near exact small scale replica of the mouth shape of baleen whales even though they share no history of having similar mouths. Why? Because they're both filter feeders of microscopic organism in water and they both have hinged jaws. That shape is the most efficient. It's physics at that point, not biology.

If we went to another planet, found an animal with hinged jaws eating the same way, they'd have the same mouth shape.

The same applies to all evolution in similar environments. The fact that the environment is on a different planet is irrelevant.

On any Earth-like planet, physics will dictate two primary body plans, the radial and the linear. Radial body plans are better for animals that are primarily sessile, like anemone, which move slowly usually while actively attached to the surface, like starfish, or which move in line with gravity, like jellyfish. Any animal that moves perpendicular to gravity, without attachment and must do so efficiently will have a linear body plan e.g. rotifers, crabs, fish, reptiles, birds, mammals.

These body plans are laid down very early in evolution history and are encoded in the most core of our genes. Their close fit with optimal physics is what has kept them around for billions of years.

Most animals on Earth have a linear body plan. (Just like technological vehicles, which aren't based or inspired by biology.) That body plan presents the minimum forward surface area for resistance in water, earth or air. Sensing is clustered in the primary direction of movement. Control projections (limbs are at the bottom in water, earth or on land because gravity provides a control vector of its own. Aerial creatures reverse this by hanging from wings at the top, since they have no buoyancy.)

So, on every Earth-like world, any creature that moves pretty much at all, in any environment will have a linear head-tail and a top-bottom basic layout. All but aerial forms will have their control surfaces on the bottom. On land that will mean bellies or limbs. It will be a very common pattern because of the physics.

Once you start with a linear body plan, the evolution towards a bipedal humanoid becomes easy and arguably the easiest option to create a tool-using species.

On land, the lack of bouyance for support control and the advantage of low frictions mean that limbs that lift the animal off the ground have a significant energy advantage. But the trade off is that limbs require a lot of information processing. The hardware of limbs without the software isn't very useful.

The easiest system is a vast number of small weak limbs, each supporting a small amount of weight and moving in a simple repeated motion itself triggered by a signal from adjacent limbs. That's how millipedes (evolutionarily very old) work. What passes for their brains just sends signals like start, stop, forward, reverse, to the first leg on either side and the legs automatically do the rest.

Not a very flexible system. Millipedes quite often run off edges or plow into obstacles because the back legs don't stop moving in time.

Controlling each leg individually is better, if you have the brains to do so. The next step up is few legs, better controlled. The simplest system there employs two intermeshed tripod. A tripod always sets firmly regardless of the terrain so one tripod is always set firmly, then the second lifts and positions itself firmly, then the first tripod lifts. Repeat the sequence and you have the motion of a six-limbed insect. It's likely we will find six-limbed insects or insect-scale critters to be quite common.

Arthropods, with eight or more limbs, evolved underwater from millipede-like forms. Their control systems are slightly less sophisticated than insects so they still have more than six limbs that operate in a semi-cascade. The evolutionarily younger arthropods, usually walk insect-tripod style and task the extra limbs to some other function e.g. web spinning or swimming.

Land vertebrates didn't evolve from insects but fishes. Legs evolved from fins while still underwater to provide anchoring for immobility for hiding and hunting. Fish need four control vectors on the bottom so all land animals start with four limbs and a tail. But the basic physics of the tripod still applies.

At larger scales, the biomatter gets less relatively rigid so a strict tripod doesn't work. Again, more computational power solves the problem. Reptiles that walk use shifting tripod with three planted limbs and one moving one. Each limb/foot spends 75% of its time planted, and 25% of its time moving. That's partially why reptiles wiggle side to side as they walk. They lean into the tripod while moving one limb.

(Hopping is an alternative mode but it's only useful for a certain class of motion. Frogs for example, hop for distance and speed but wiggle like a reptile when moving slowly. Hopping insects use the meshed tripod when moving slowly.)

But the more brain power you apply to the problem the more dynamic you make the whole system and the less time you have to rely on static gravity and fixed footing to provide stability. Mammals (and birds) evolved from reptiles so they start with four limbs but with more brain power, they need only one limb planted for a static load. The other three limbs can be doing other things. Animals like horses use the time in the air to get very long strides, very quickly. Animals like cheetahs use their air time for both for long, high speed strides and lateral control. In either case, at speed, only one limb is touching the ground at any one time and often, no limbs are touching at all.

If you only need one limb touching at a time and you don't actually run fast, say you live in a tree or a cluttered ground environment, then you really only need two limbs for basic support and motion. One leg supports while the other provides control. The other two can be tasked to other things like manipulating the environment.

Look at raccoons, they walk on all fours but spend a lot of time sitting or squatting while using their forelimbs for manipulation. They don't have opposable thumbs but instead, their palms fold over top to bottom and side to side to make a surprisingly efficient grasping surface, especially for grabbing wet slick things.

(Human hands evolved to grasp cylinder shaped branches. Species that don't evolve their manipulating limbs in trees will have different manipulating criteria.)

Once you spend a lot of time sitting and manipulating, and your brain grows to give more control, you eventually reach a point where it pays more to dedicate the bottom limbs to motion and the top to manipulation. Balancing the body atop the bottom two legs gives the manipulators more room to work and the sensors clustered in the head a greater range and sweep.

At this point you have a basic "humanoid" with legs at the bottom, a torso (formerly parallel to the ground - now perpendicular), manipulating limbs on the upper body above the legs and a head on top. As it grows more intelligent, the head gets bigger to contain more neurons.

That would look like a human from a distance.

This basic form was made highly likely nearly a billion years ago back with the dominance of the linear body plan.

But one can see many different avenues to get there. Humans evolved from tree-dwelling proconsul and then moved to the ground. We evolved then primarily for tool use and long distance running.

But a species might start out bipedal and fast, like a kangaroo, optimized for fast running and then slowing down as it evolved manipulation. A wallaby is a slow kangaroo that occupies a niche much like that of raccoons. It squats and manipulate and hops only slowly.

Ancient sloths were apparently almost entirely bipedal and used their forelimbs to pull down foliage to eat, if they had to manipulate the trees to a higher degree they might develop "hands" of some kind.

But it's far form determinate. Even minor changes in environment, say, higher gravity, could change things. On a higher gravity world, animals might need more limbs even if they had big brains. A tool-using species there might look more like a very squat centaur.

Chance could play a role. While selection is not random, it can only work with the variations that occur at any given time. While physics prefers certain forms, if variation does not provide the raw material for the optimal form at the optimal moment, then selection will take what variations it has and use those to compress a species into the best shape for the circumstances.

If, early on, a radial genus got a significant edge in an ability/function other than linear motion e.g. sight, oxygen breathing, bones, faster neurons, etc., then it could out-compete linear body plans even if they had the edge in motion.

Once radials occupied a vast number of niches, they would block the evolution of the linear organisms into those niches. At that point, selection would find it easier to modify a radial body plan to be more linear than to parallel evolve all the radial specific systems all over again in the linear branch.

(It very much like the problem of creating entirely new computer platforms and operating systems from scratch. The dream is to get rid of all cruft and barnacles that the systems accumulate but in the time it takes to create a new clean system, the old system has evolved further. The new systems never quite catch up. Sigh.)

For example, consider a species that starts out like a radial star fish but begins to swim. A quick optimization for streamlining would be to fold the radial limbs all back into a tear drop shape with the sensory cluster, formerly at the top and center of the radial form, not at the front. It would now look superficially like a fish.

With streamlining being so important to fast moving aquatic forms, the radial limbs could simply fuse externally to making what looks like a unitary fish-shaped body from the outside but the basic radial template would always be there. Turn off just a couple of genes and the fish shape might pop out to a radial shape. It would be like fish that had a tendency to to give birth to octopi. (But, an octopus with an internal skeleton with tentacles more like snake bodies i.e. flexible but rigid and load bearing.)

On land or shallow fresh water, a radial genetic base might be an advantage. It might shape some of its radial modules to a fish shape but leave a few free flexible like tentacles for anchoring. When it found itself beached, the tentacles could move it around. Eventually, it would breath air and move to land. How it would "walk" would vary, but from very early on in its evolutionary history, it would have manipulating limbs, perhaps a lot of them.

(It's easy to make another radial limb just by altering a single gene. The species will have a master gene, a hox gene, that basically says, "take the basic starfish "limb" starting point and repeat it "X" units. Altering that single gene would produce an arbitrary number of limb units which could then each be customized to function as needed.)

Perhaps virtually all animals on land or sea would have a limb or two capable of manipulation. In that case, "technology" might evolve from genetic behaviors long before brains got big enough to generate the behaviors from "software." A real world example of a genetic technology is a termite mound, a very sophisticated structure we would instantly recognize as technology if termites were the size of dogs and their mounds the size of skyscrapers. Another example would be beaver dams, ponds and canals.

(We just have a bias for "tools" things held in our hands, but shaping the external environment to a specific form for a specific function is clearly technology. Burrows, nests, and other structures qualify.)

That would be a very interesting world even before sentience evolved. And not a humanoid to be seen.

But such a world is statistically less likely than a humanoid one because it requires a less efficient movement form, the radial body plan, to get a significant, non-movement advantage at just the right time while at the same time, the linear body plan competitors don't.

In summation, when we go out and actually discover a large number of Earth-like worlds sentient, we would find the majority of worlds have the physics-preferred forms of fish-shaped swimming organisms, bird/bat-shaped flying creatures, four limbed, big brained land animals and bipedal, largely vertical standing, two-armed, big-headed, "humanoid" tool-using sentients.

The worlds that didn't would be less common but much more interesting.

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    $\begingroup$ "Therefore, rough humanoid shapes are statistically more likely than non-humanoid on earth like worlds." While I appreciate your rationalisation, it's probably a stretch to be doing statistics on things we don't even know exist, let alone have any raw data on. $\endgroup$
    – NPSF3000
    Nov 22, 2014 at 13:41
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    $\begingroup$ @Theik, but the amount of intelligent, higher order animals almost always have bilateral symmetry, have two eyes, ears, nose, mouth, two limbs/wings/etc., two legs, and so on. Whether they walk on all fours, or stand upright, a LOT of the creatures that we would call "intelligent" that don't swim look surprisingly alike, all things considered. It isn't much a stretch to conclude that higher intelligence equals the need for grasping limbs, much like a human. $\endgroup$
    – m t
    Jun 19, 2015 at 16:17
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    $\begingroup$ @NPSF3000 The pedantically correct statement would be "Therefore, our priors should assign a rough humanoid shape a higher likelyhood." $\endgroup$
    – Taemyr
    Jul 13, 2015 at 9:10
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    $\begingroup$ @NPSF3000 - I'm not basing my statistical claim on biology but on physics, which is universal. Just as in chemistry where reactions with a lower energy of initiation are overwhelmingly favored but higher energy reactions still occur, the rules of biophysics dictate that natural selection will favor the lowest energy forms making them more common than high energy forms. To make my statistical claim, I just need to know the which forms physic favors. $\endgroup$
    – TechZen
    Jul 14, 2015 at 18:28
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    $\begingroup$ I think this is an amazing explanation, but it's also horribly flawed. The expectation that bilateral symmetry would be preferred, or that Terran evolution is likely to be mimicked on other planets with high-order species is... well, it's a gross assumption based on many things that we don't really understand about life outside of our planet. We're assuming DNA-based, carbon-based, oxygen-breathing lifeforms on a world constructed much like our own. But think of the guesswork there - if we were any other sort of creature, we could probably not imagine our makeup either. $\endgroup$ Sep 12, 2016 at 17:22

While there are reasons why aliens could look like us (anything is possible), there are far more reasons why they would not look like us.


If we have an alien species that has human-like traits or is even "mostly" human, how could this occur?

One logical explanation is that some other species (that probably doesn't look human) has been abducting humans (possibly for millennia), relocating them onto other worlds, and modifying them to fit their new environment and/or to meet whatever other requirements they may have had. It would probably become evident that there had been interference if said world was examined thoroughly by scientists.

If there is just one species out of many (aside from humans) in a setting that looks humanoid, this would also be reasonably plausible.

However, to have a setting where almost all the aliens are humanoid is probably the result of either a limited imagination on the part of the author, or is a calculated strategy - the author may want to make a social commentary that would offend a real-world group, but by saying that "aliens" are doing it, the commentary becomes socially acceptable. The setting may have been designed for visual media such as a movie or television, and aside from reasons of budget and practicality, audiences relate better to more human-like characters. There is also the factor that if a story contains very alien aliens, it is more difficult to keep the story about what the aliens are doing rather than what they are.

Why Not?

When we are talking about the biology of different species, even species with no common ancestry at all, there are universal traits - meaning that there are problems that all species will face with a common solution, e.g. the Square-Cube law means that larger alien species will have thicker-looking limbs and a heavier skeleton than smaller ones.

There are also parochial traits - meaning that while there may be a common problem, the approach to solving it can vary, e.g. many terrestrial species have eyes, but insects have compound eyes, while vertebrates have eyes with a retina and lens.

Given that the majority of features of an alien will be parochial traits, the odds of an alien with no common ancestry looking human is quite low.

However, there are a number of reasons why a sentient alien would have similar traits. A sentient tool-using species must have a brain (or brains) big enough to do the processing necessary to support sentience (a universal), but they need not be in a head (parochial). A tool-using species must have appendages able to manipulate the objects that they use (universal), but they need not look like human hands (a parochial). They need not be mobile, though they probably would be, and they need not be warm-blooded, though there is a fair chance that they would be.

So, we have a mobile, tool-using alien. This means it must have some means of being mobile, but this could be almost anything, a slug-like foot, hundreds of cilia, wings - or legs, amongst even more possibilities. If it had legs, there could be as few as one, or there could be more. The odds of there being two is fairly low, as it is not the most stable configuration.

Our tool-using alien must have some way of manipulating the environment, but evolution (which is probably another universal) need not adapt a locomotory appendage to that task - what if an alien's manipulator evolved from, say, it's tongue? Elephants' manipulators evolved from their noses/lips after all.

As for aliens being fleshy, that again is a parochial solution. Depending on the gravity levels in the alien's place of origin, they may be incredibly fragile with soft, easily torn flesh to incredibly tough with flesh that is more like wood. There are ways that beings with a body structure composed almost entirely of woody material could move, for example by expanding and contracting cells asymmetrically within an appendage. Plants move (usually very slowly) this way.

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    $\begingroup$ I really like how this answer looks at the question from both ends. Good one. $\endgroup$
    – user
    Sep 29, 2014 at 7:28
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    $\begingroup$ Yes but it does not focus on answering the question. Maybe the "why" part should be the first one. $\endgroup$
    – Sheraff
    Sep 29, 2014 at 10:02
  • $\begingroup$ Good point. Edited. $\endgroup$
    – Monty Wild
    Sep 29, 2014 at 13:52

Anything is possible.

Common DNA

Ok, before you just pass this off, this is actually possible. Humans could have common DNA with other aliens. While unlikely, it is possible. If human DNA did not evolve on earth but was actually planted their by aliens (a reasonable conclusion if aliens exist in a world) or if it was carried there on a habitable asteroid, then it would be possible. This process is known as panspermia.

Evolutional Factors

Humans have many of the best qualities for a sentient species. We are warm-blooded, which means we can have large brains. Generally animals evolve in such a way that they have four limbs, because it is best for covering ground. Humans walk upright, giving us two limbs to use for tools and things. This means that we developed sentience faster because we could learn from our hands.

The biggest thing that would make aliens look like us is if they had bilateral symmetry. This means that an organisms body is roughly symmetrical (two arms, two legs, two lungs, etc.) and that equal and opposite parts generally work the same. If the above paragraph is true, then generally an alien would look humanoid if it had bilateral symmetry. Bilateral symmetry would be likely to evolve, because it has evolved on Earth in multiple places. One you have bilateral symmetry, things like eyes would generally evolve like they do on us, forward facing to be able to see the arms using tools.

Some things wouldn't look the same, they may not be fleshy, though it is likely that aliens would be vertebrates. Or maybe the communicate differently, but again that is only likely if they had a different breathing system than lungs.

Overall it is hard for us to imagine non-humanoid aliens because we are used to a world like this one. Humans only like so much otherness, after a while we prefer that it is the same. But there are a few tangible reasons why aliens would look like us.

Main reference article.

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    $\begingroup$ Just a clarification: Panspermia does not imply a habitable body carrying microorganisms or genetic material, nor does it imply artificial implantation. In fact, a very plausible version I've read uses the existing discovery of extremophiles on meteorite rock and nebular conditions as a means of panspermia - the meteorites aren't really what you'd call "habitable" nor are they placed there. $\endgroup$
    – mechalynx
    Sep 30, 2014 at 22:04

Why would aliens be so similar to humans?

Because they're 1) alive and 2) successful.

The first part of this is essentially an extension of the rare Earth hypothesis concept. "Why are we on Earth?" - "Because if it wasn't Earth, we wouldn't be here." Likewise, based on everything we know about chemistry and (even theoretical) biology, aliens would need to develop on a world with liquid water and they'd very likely still have carbon as the core of their chemistry since science isn't different for them. Since the science isn't different, their biology is unlikely to be different and their world is unlikely to be very different.

The second part is a bit dicier. The general argument comes that the sort of things that make us successful and space-faring (opposable thumbs, intelligence, drive to explore/expand) would make any organism in a similar environment successful and space-faring. Further, it's not just one or two of these sorts of traits that will evolve. After all, chimps have opposable thumbs, dolphins are smart, and norwegian rats explore but none are exactly travelling the stars. It's not entirely unreasonable to extrapolate that in a similar environment (which is pretty scientifically likely) the same set of biological traits are required to achieve a similar result.

  • $\begingroup$ We might be so succesful that we wipe ourselves out in a few years. Even if not, as the same question in a million years, which is not much on the cosmological scale. In both cases point (1) fails. Also, go 100,000 years back in time and point (2) fails. Time is a factor and most civilisations in the universe (if any) don't exist at the same instance. $\endgroup$
    – durandal
    Dec 9, 2017 at 1:46

Well, of course the REAL reason is because aliens on TV and movies are played by human actors, and it's just too difficult and expensive to have an alien that cannot be "constructed" by applying make-up and prosthetics to a human being. Yes, it's possible to make a puppet alien manipulated with sticks and strings, or to build an alien with hydraulics or servos, but it's difficult to operate such a create day after day as you try to put on your show. And actors generally pride themselves on their ability to display emotion. That's tough to do through a mask and pretty much impossible through servomotors. Actors would hate it.

But assuming real aliens and not actors ...

Presumably some body forms would not be conducive to an intelligent, tool-making creature. It's hard to see how a creature with no hands or reasonable substitute could use tools and build machines. Such a creature might be intelligent, but it is very difficult to see how it could be technological. A race of dog- or turtle-like creatures who build starships is rather implausible. Maybe a sufficiently clever sci-fi writer could come up with a way to make it plausible.

If you assume an evolutionary scenario. the probability that creatures would evolve on another planet to look at all similar to humans seems quite remote. Even allowing for what I said in the previous paragraph, why would you expect aliens to evolve with five fingers instead of, say, six? Or with arms coming out the sides of their bodies and hanging down instead of, say, coming out the front of their bodies, or coming out the sides and sticking upwards, or thousands of other possible arrangements. Is it inevitable that sight, sound, and smell organs must all be on a "head" of some sort, and that this head must stick out the top of the body? Why couldn't ears be on the stomach? Etc.

Suppose you found someone who had never seen any creature other than humans, and you showed him a human being, a Star Trek alien, a horse, an eagle, and an octopus, and you asked him which one he thought came from a different planet? Which do you think he would pick?

A creationist might be somewhat more likely to believe that aliens could look so much like humans. A creator God could create very similar creatures on two different planets. It would at least be plausible to the creationist. But I'd think it would still be unlikely. God created many different kinds of creatures here on Earth. If he had that much imagination and variation here, why would he then recycle terran designs on another planet?

If the environment on another planet is different -- significantly hotter or colder, or driver or wetter, or different gasses in the atmosphere -- than any creature there would have to be different from humans to thrive, maybe even to survive if the differences are great enough. And to suppose that a creature would have different body chemistry and significantly different internal organs, but still look like a human -- that seems very implausible from either an evolutionary or creationist viewpoint. To the evolutionist it would have to be a coincide of mind-boggling proportions, surely trillions to one odds against. To the creationist it would be "why would he do that"? God didn't make dolphins look just like people or some other land creature but with different internal organs so they can live in the sea, he made them look completely different. Why would he break this pattern on another planet?


We only talk to things that have faces

Suppose that the universe has a diversity of alien life forms, but that we consistently fail to recognise any of them as life forms. Those that live outside of the goldilocks zone, or have drastically slower or faster metabolisms, etc..

We ignore (or hunt, farm, mine, crush, poison, or burn) them because we can't communicate with them and deem them inherently unintelligent.

In a universe rich with diverse life, or with technology that allows us to search many different worlds quickly, there's a lot we can afford to overlook before we happen upon something similar to ourselves which we recognise as "obviously intelligent". The more numerous the life forms, the more likely it is we can happen upon something very much like ourselves before being driven to making a serious effort at trying to comprehend the genuinely alien intelligence of creatures we didn't really "get" when we first saw them.

So it may not be that all the aliens are unusually similar to us, but instead that we can only have meaningful interactions with aliens that are sufficiently similar to us.

The rest, we eat.

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    $\begingroup$ That's been the plot of a few SF stories. Rather, recognizing that the other life form is alive and/or intelligent, when it is so alien. That's sort of the contradiction of the OP's question though. $\endgroup$
    – JDługosz
    Feb 3, 2016 at 6:48
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    $\begingroup$ @JDługosz, things like Solaris and Roadside Picnic? Seen the films (loved the films!), but not read the books. The challenge, there, is that alien aliens come across as kind of supernatural -- although that's likely a limitation of film -- and it's easier for me to imagine humans stumbling forward without ever really coming to terms with what they're facing. $\endgroup$
    – sh1
    Feb 3, 2016 at 7:07
  • $\begingroup$ Look for Robert L. Forward and Stephen Baxter (in particular, the story collection Vacuum Diagrams $\endgroup$
    – JDługosz
    Feb 3, 2016 at 7:14
  • $\begingroup$ If they reach the agriculture phase, we'll recognize them as intelligent no matter how alien. $\endgroup$
    – Joshua
    Nov 2, 2016 at 18:25
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    $\begingroup$ @sh1: Ants don't weed the garden. $\endgroup$
    – Joshua
    Nov 2, 2016 at 19:28

A group of ancient aliens, evolved from mammal-like, ape-like animals, traverses the universe stopping here and there, going down to the surface without any care of how they might affect the ecosystem. In some planets they see lizard-like creatures predating on mammal-like furry critters, and since they find them cuter that the reptiles they scare away the predators, giving mammals an evolutionary leg up via butterfly effect.
In other planets they find ape-man like beings and they interact with them, again without a second thought to how the affect them, which is by leaving the impression of incredibly powerful beings that would affect how they chose mate in the following generations, with the individuals that resemble them, with their walking on two legs and wide foreheads, being considered the best mates.
So... Irresponsable aliens.

  • $\begingroup$ cool original answer $\endgroup$
    – Sheraff
    Dec 3, 2016 at 11:16
  • $\begingroup$ That presupposes that ape-like animals evolved in the first place. $\endgroup$
    – JDługosz
    Dec 4, 2016 at 0:43
  • $\begingroup$ But not ape-like apes. Sometimes they would be ape-like birds, or ape-like cats. They just have to look enough like those alien ancestors so they find them likeable. $\endgroup$
    – ThreeLifes
    Dec 4, 2016 at 13:47

Small answer: from a evolutionary point of view most mammals have 5 fingers/toes. This is either because all mammals have a common ancestor with 5 fingers/toes OR because 5 fingers and toes is the ideal number.

Apply this idea to aliens and they might be similar.

If the environment they evolved in is very similar, they'd probably evolve to function/look similar.

  • $\begingroup$ It's the common ancestor. Also, have you counted the toes of your cat lately? $\endgroup$
    – durandal
    Dec 9, 2017 at 1:49

The theories that say aliens should look similar to us assume the evolutional analogy. Whale is somewhat similar to fish, and bat is somewhat similar to bird, just because it seems the most optimal form for the way of living. There are still a number of important differences remaining because of the different evolution path.

So two legs, two hands and most important receptors (sound, vision) at the highest point, double vision and sound receptors for stereo perception, and the brain as close to the receptors as possible - seems making sense if we assume evolution leave no redundant organs like a third eye.


Considering the huge variation in lifestyles and body plans of creatures on Earth, even when they share large amounts of DNA (A banana plant shares about 1/2 of the same genes as a human being; think about that next time you eat a banana....), the idea that an alien species from an entirely different planet, with a totally different evolutionary history, different DNA (or maybe using a totally different system for reproduction and evolution) is quite unlikely at best.

There may be some convergent evolution in similar environments and ecological niches (Sharks, Ichthyosaurs and Dolphins, for example), but humans are actually generalists who are not specialized for a particular niche. A generalist creature from a different evolutionary history will probably be very different from humanoid.

  • 1
    $\begingroup$ But maybe we're well optimized at being generalists, and that intelligent beings will almost always be generalists and then there'd be convergent evolution towards a somewhat humanoïd shape too ? My question was also meant to encompass every tiny aspect that may be similar to us (eg. maybe aliens would be made of radiowaves). $\endgroup$
    – Sheraff
    Apr 5, 2015 at 7:35
  • $\begingroup$ Actually, physically we are pretty specifically adapted to a single environment (tropical plains peripheral to light forests), it's our exceptional brains/intelligence that allows us to adapt so generally. $\endgroup$ Jun 22, 2015 at 17:57

It's not far-fetched to assume than if aliens existed, they, along with humans, could've come from a similar source. Also, they may have developed a society similar to our since there are just certain things in our world that work best. Outside condition and there origin both would explain similar looking creatures.

  • 1
    $\begingroup$ What do you mean from "A similar source"? I doubt you're suggesting that both came from Earth. A point I dispute: That society has achieved the optimal way to do anything. There are inefficiencies in everything we do. It's just a matter of finding them. $\endgroup$
    – HDE 226868
    Nov 21, 2015 at 1:44
  • $\begingroup$ I am not at all suggesting both came from earth. It's plausible that both came from some other source. Also, your point that there are inefficiencies in everything we do is valid. There are always going to be some inefficiencies in everything any society does. I'm just saying maybe in something we have found the best way to do it, even with some inefficiencies. $\endgroup$ Nov 21, 2015 at 1:51

I doubt that there is much which could top TechZen's answer, but a relevant theory which hasn't been touched on is Panspermia. EDIT: Missed post by DonyorM

Panspermia: the theory that life on the earth originated from microorganisms or chemical precursors of life present in outer space and able to initiate life on reaching a suitable environment.

Microscopic fungus has been shown to be able to survive and hibernate in the harsh conditions of space. Over a viable (extremely large) period of time, these fungal spores could traverse the enormous distance separating habitable/soon to be habitable worlds and enter before/after atmosphere formation to enter the primordial gene-pool of the planet. All life on earth has a large similar genetic base which could be transported with these spores and be implanted in the evolution of life on the other planet. This life would inherently be similar to life existing on earth.

  • $\begingroup$ While this would give them similarities at the basic cellular/DNA level there is no reason they would evolve to look like humans. $\endgroup$
    – Tim B
    Feb 4, 2016 at 9:44
  • $\begingroup$ True, that part is well explained by TechZen. Panspermia would set the base for the physics to do the rest. If the life (arguable definition) form was silicon based or geological in nature, it's structural density would change the way in which Physics shapes the evolution of the life form, and it could make sense (going extremely off tangent) to have a polygonal shape and roll as a means of movement. $\endgroup$ Feb 4, 2016 at 10:07

There's another issue, which is the most important. Portraying aliens that are very different from us is hard.

In written fiction, the special-effects budget is unlimited, you can describe an alien that looks like anything. But for the story to be interesting, people have to be able to talk to the aliens, and make some sense of their motivations and desires. An early example of this is Hal Clement's novel Mission of Gravity, set on a very high-gravity planet, where the aliens are centipede-like creatures with a profound fear of having anything above them, because if it falls, it will crush them. But mentally, they're enough like humans that communication is no harder than learning their language. Iain M Banks' novel The Algebraist is a more recent book, with some serious attempts to portray aliens with real psychological differences. But really, they're caricatures of the English (Banks was Scottish), or humans with obsessions.

In moving picture media, we get a lot of aliens with lumpy foreheads, because that's cheap, because it allows human actors to play the aliens and actually use their acting skills, and because many people in Hollywood are scared that if they make things too strange, they'll put the audience off.


A minor qualification: most "intelligent" alien life portrayed in science fiction is often industrialized. So to even get to that point you need to have the right environment that allows for industrialization, followed by the ability to craft and manipulate very complex tools. For non-humanoid forms, this can be quite difficult.

If a space-travelling alien were to come to an Earth without humans, they might conclude that dolphins are the most intelligent life-form on the planet. The aliens might even have Star Trek-style universal translators that allow them to communicate with the dolphins. Oh, the fun they would have. However, even with the combined intelligence of a dozen humans, dolphins could never industrialize to our point.

To reach the stage of industrialization, you need to check a few boxes first:

  1. Agriculture: You need to be able to domesticate crops in order to have a stable supply of food that allows for:
  2. A surplus: Only when there is more than enough food to feed everybody you can have some people do things that are not making food, such as crafting tools which make food easier to make, or medicine, or education, or child-rearing. These kinds of jobs will be necessary to create:
  3. Cities: Even if all of the beings of this species are completely peaceful you will still need cities to maximize productivity, ensure efficient distribution of scarce resources, and so on. Once you get to this point it wouldn't be that much harder to:
  4. Mechanize: This is where you create tools and machines to perform tasks otherwise done by people or domesticated animals, using other forms of energy instead of food and muscle. By the time you've industrialized, only science holds you back from exploring the stars. But before you have the tools, you need to create the tools. Hard to do with little more than flippers and a blowhole.

I deliberately left out a lot of human cultural paradigms, such as trade, capitalism, war, religion, and so on, because they are not necessarily relevant to the development from subsistence foraging to industrialized society. However, given that dolphins 1: live underwater and 2: lack the capability to create and use complex tools, then it is clear they would never reach stage 1, and therefore miss out on stage 2, let alone the inability of the ocean to act as an environment for stage 3 (water currents are much stronger at ruining things than air currents - just think of your last sand castle- also good luck building a computer underwater), and then because of the aforementioned need to create and manipulate tools, dolphins have absolutely no chance of reaching stage 4.

So, in conclusion, while there may be plenty of intelligent life out there, it is highly unlikely that any industrialized (as in, necessary for space travel) life would form without resembling humanoids. Perhaps a planet-wide bacteria-ish intelligence with telepathic and telekinetic powers like in Sid Meier's Alpha Centauri would serve as an exception, if it exists and we discover it, but until then, it is almost impossible to even imagine another species reaching our level of development without resembling humanoids.


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