When we consider what alien life might look like, it seems more likely that life would be, well, very alien. There is no reason to expect bipedal, human-shaped and human-sized creatures. Or to expect these creatures to live on the same pace and time scales as we live, there is even less reason to imagine they would have similar internal cultural conflicts to our own with nations, and love stories that run in parallel to the human. The so-call "Hard Science Fiction" alien must be very alien indeed. The only real point of community between us may be that we are alive, and that we are curious about space.

I'm playing with the notion of what happens if when we meet aliens all of this preparation for extreme difference turns out to be wrong. What if the aliens are literally just blue people with antennae? What if they have not only many of the same biological features but also many similar cultural features? How would real scientists react? Disbelief?

They must be related to us. Someone took neanderthals to another planet somehow. But, let's suppose that this isn't the answer. They are totally different genetically and if we have a common ancestor at all it was in the single-celled era of life.

There must be something about our shared traits that solves the problems of survival especially well. This is possible. But, let's also suppose that there are many diverse and "alien aliens" as well.

This is just a coincidence. Again possible, but it feels ... weak as a real solution.

What does encountering human-like aliens tell the serious xenobiologist about the universe?

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    $\begingroup$ Possible reactions from persons are probably not on-topic on Worldbuilding.SE, but the slightly related question "What could scientifically explain humanoid aliens?" would be; the answers to that question serving as the theories these scientists would come up with. $\endgroup$
    – KeizerHarm
    Commented Jun 24, 2023 at 15:50
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    $\begingroup$ Not to make this question ridiculous, but isn't alien life in basically all movies an answer to your question? Because people have trouble connecting to alien things, many aliens are human like. Avatar (blue humans with antenna hair), most of the star wars or star treck or stargate aliens to name a few. Sure there are incomprehensible aliens, but they are generally more cardboard setpieces. Any alien of note is practically always humanoid. $\endgroup$
    – Trioxidane
    Commented Jun 24, 2023 at 17:59
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    $\begingroup$ @Trioxidane There aren't many "hard SF" movies. So the OP is asking about what a "hard SF" universe seeing "soft SF" aliens would react. $\endgroup$
    – Yakk
    Commented Jun 25, 2023 at 3:02
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    $\begingroup$ Another alternative could be that some unknown beings were periodically intervening in the evolution of both species, akin to the unseen monolith makers in 2001 except with more of a focus on bodily form as opposed to just cognitive abilities. It would be interesting to trace the ancestral line of the humanoid aliens (ignoring all the other side branches on their world) to see if it was fairly similar to our own ancestral line at all stages, including say lobe-finned fish like ancestors (this might suggest continual guidance), or if they had converged from more different ancestors. $\endgroup$
    – Hypnosifl
    Commented Jun 25, 2023 at 18:04
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    $\begingroup$ "Soft" aliens would be a very big clue towards something more fundamental, like simulation theory or intelligent design (God is real) $\endgroup$
    – Dragongeek
    Commented Jun 26, 2023 at 8:08

10 Answers 10


Sorry, they are likely to be crabs. Everything evolves into crabs.

But convergent evolution is a thing. And if becoming a technological space faring civilization is difficult (or equivalently, slow/unlikely), then it is somewhat reasonable that the fastest way of getting there might be somewhat similar.

We have reason to believe that interstellar civilization is hard, and (if we pull it off) we'll be among the first in the universe to do it. More accurately, it seems plausible there are zero in our past light cone, because once you can pull off interstellar civilization it should expand at a non-trivial fraction of the speed of light.

So, given that, maybe somehow our humanoid shape is a short and "easy" path to interstellar civilization, and ditto our social systems.

An intelligent life form without enough competition and arrogance might dwadle away at perfecting life for a billion years before bothering to cross the gulf between stars. One with too much violence and not enough cooperation would self destruct when before it could handle the energy scales of interstellar travel.

Similarly, while lots of species on Earth have gotten quite smart, humans are

  1. Dry land livers, which may be needed for the early stages of the technological singularity

  2. Possibly bilateral symmetry works better on dry land than alternatives do. So while non-bilateral symmetry can evolve technological intelligence, that might add many billions of years to the expected budget.

  3. While climbing trees might seem strange, the existence of trees on a Earth is what gave is Coal, which was a huge part of bootstrapping technological civilization. Trees are trees because they evolved a super-strong material that lets them grow tall and crowd out competition; that super-strong material doesn't decay easily, leaving to huge deposits of dead trees that form into coal. Climbing trees gives you a pre-intelligence reason to evolve grasping hands with opposable thumbs, which in turn make even early artifact-based technology (shaping rocks etc) plausible, which in turn places evolutionary pressure on more abstract intelligence.

So plausibly we have bilateral, land based, climbing species with hand-like graspers having a huge advantage over other species at forming technological civilizations, and a narrow band of competition and cooperation needs to exist in their society to both make them do the stupid thing and go to the stars, but not so stupid as to destroy themselves with their technology.

Our forward facing binocular vision is a hunting adaptation; hunting was one of the first yields of intelligence, as it took a tiny primate and let it consume the planet's megafauna as calories, spreading it over the entire planet. Hunting led to herding and biotech breeding of both prey species and photosynthesis crops -- both of them where key in pulling us into the stone age of technology.

Alternative paths to interstellar civilizations are possible. All it requires is that this be a short cut compared to others. Like, biofilm intelligences living in a holographic computational model on an ice planet might develop physics and tool use and the like eventually, but maybe it takes an average of 30 billion years for that line to play out. And maybe liquid methane based life is viable, but their slower metabolism means that the evolution speed is also slower, so they are billions of years slower again.

Meanwhile, our pattern of interstellar technological civilization takes an average of 10 billion years, and the outlier fast ones take a handful of billion (like we did) to rise up from non-life.

So in this hard SF situation, humanity has colonized a few dozen galaxies when it starts seeing evidence of another civilization a few giga light years away. We continue to expand, and knowledge of that civilization crosses human space. A few more doublings of human space later, the two civilizations finally get into contact, call it 2-50 billion years from now.

Everyone is shocked at the fact that the origins of these two civilizations, as far removed from the current beings in them as we are from non-celled abiogenesis in a tidal pool somewhere, looked similar. Funny that.

Oh, and we know we are fast for the simple reason we have a fallow planet, and not the interstellar equivalent of a parking lot.

Look at what humans did to Earth -- almost the entire planet is turned into a giant tool to advance human wants. The biomass of land mammals has massively increased, fed by our converting huge amounts of plant mass into food for us and our livestock, while wild mammal biomass has plummeted. (Non-mammals, like ants, are at the wrong scale, so care less about what we do at our scale)

A successful "hard SF" interstellar civilization based off of humans is likely to do the same to the entire solar system, then galaxy, then multiple galaxies in a wave of expansion that moves at a non-trivial fraction of c (say, 0.001 c?) after it passes its initial stages.

They'll arrive in solar system with "pond scum" as the highest form of life and disassemble planets, build custom biospheres, etc. They won't be intentionally destroying the possibility of the system evolving independent intelligent life, but it will be a byproduct.

When they find "higher" life, they'll still meddle over evolutionary time scales, because entire solar systems are places that could support millions of billions of intelligent beings for billions of years before any intelligent life would naturally develop and reach to the stars; a lot of potential to sacrifice for the equivalent of a trilobite. Unless that life is extremely rare (which this hypothesis doesn't require), at best a few reserves may develop. But those reserves need to last for astronomical periods of time for life to evolve technology independently there, which is also ethically fraught. Why not give that life a leg up?

In this "hard" SF universe, millions of different biologies can be possible, billions of different intelligent patterns could exist; but whichever one is faster at reaching interstellar civilization swallows the local universe. And as we haven't been swallowed, it must happen that our pattern has to be pretty fast.

Naturally in the billions of years it takes to spread our interstellar civilization, we'll continue to evolve.

So, by the time we reach the other species, billions of years in the future, the two crab like beings will marvel at how their form when they left their home planet looked similar. But neither side will be surprised the other looks like a crab, because it will be well known that everything evolves into crabs.

Via https://xkcd.com/2314/ :

everything evolves into crabs

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    $\begingroup$ It's called carcinogenesis. $\endgroup$
    – Daron
    Commented Jun 25, 2023 at 6:53
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    $\begingroup$ @Daron I googled it, carcinogenesis is about cancer, the term you're thinking of is carcinisation. Only applies to different crustacean groups tho, it's an interesting aspect of convergent evolution that when it comes to specific body shapes it only seems to work for groups that start out "close enough" in body types (like ichthyosaurs, dolphins & some sharks all having a similar shape and swim style, non-vertebrates never evolving this). Some things do re-evolve in very diff branches, like camera type eyes or eusociality (ants, naked mole rats). $\endgroup$
    – Hypnosifl
    Commented Jun 25, 2023 at 19:38
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    $\begingroup$ Love this. Some notes: some scientists believe theropods (the basic T-Rex/velociraptor shape) also evolved independently multiple times, and trees themselves are another example of convergent evolution. "As we haven't been swallowed, it must happen that our pattern has to be pretty fast." is somewhat circular. For example, it could be we haven't noticed relatively-close advanced life because we're looking in the wrong place, and the advanced high-pressure beings living in Jupiter and other gas giants likewise don't find earth-like planets interesting. $\endgroup$ Commented Jun 26, 2023 at 15:29
  • $\begingroup$ @JoelCoehoorn The problem is a K3 civilization eats stars. Like, they swallow the entire output of entire galaxies. Interstellar civilizations in hard SF are going to be somewhere between K1 and K2 at the least - interstellar expansion is energy expensive. "Planets" are akin to "great caves"; they are reshaping solar systems, and what their native planet looks like isn't all that important. $\endgroup$
    – Yakk
    Commented Jun 26, 2023 at 15:31
  • $\begingroup$ The phenomena is called "Convergent Evolution" and explains how animals with very distinct and distant relations on the evolutionary tree evolve similar designs (For example, Sharks, Whales, and Dolphins have very similar body designs or features, despite billions of years of evolution separating the three species (with the latter two believed to have evolved from land dwelling mammals that returned to the ocean while the former never left).+ $\endgroup$
    – hszmv
    Commented Jun 26, 2023 at 19:28

Actually, one might explain that successful, technological lifeforms ...

  • have a small numbers of limbs with a small number of fingers/toes on each, for a combination of strength and fine control which is superior to either a big, undifferentiated trunk or many small cilia;
  • have four limbs, two arms and two legs, because that number can make a transition from a horizontal, legs-only body posture more easily than either trilateral symmetry or higher even numbers,
  • have their main sensory organs close to the brain, and the optical sensors high up from the ground, so the brain is in a head,
  • have the olfactory sensor close to the food intake and have the olfactory sensor linked to the breathing system (for continuous airflow), thus putting the breathing system close to the food intake,
  • have the whole olfactory-breathing-food-intake jumble between the eyes-and-brain and the digestive system.

Of course I'm writing this as a human, a centaur might disagree. They would point to the stability and ground speed of four legs on the ground.

  • $\begingroup$ Good points about the head and brain, after all that's what we see in every higher animal and a lot of lower ones. I think there's a lot more room for variation when it comes to legs and manipulators. In terms of technological advance, strength would be needed primarily to pound metal into shape, but a water-powered repeating hammer made of sticks could achieve the same if the animal isn't strong enough on its own. Two-handed dexterity is helpful to hold an item steady and work on it at the same time, but if the animal has a single trunk you could always just have two of them cooperating. $\endgroup$
    – causative
    Commented Jun 24, 2023 at 19:50
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    $\begingroup$ No, they are probably crabs. $\endgroup$
    – Yakk
    Commented Jun 25, 2023 at 3:02
  • $\begingroup$ @causative, strength is needed for all sorts of things. Chopping firewood, hurling rocks at predators (because their evolution went for brains rather than teeth), cracking nuts to get at the edible stuff inside. Even if a workaround exists, on average needing that workaround would make them less likely to succeed. $\endgroup$
    – o.m.
    Commented Jun 25, 2023 at 4:17
  • $\begingroup$ @o.m. Water-powered hammer, or several weak animals working together to lift a hammer, can chop firewood. Or they may have other methods depending on their biology, for example they may have beaver-like teeth that can chew branches into pieces. Or they may get their firewood from small woody plants that don't need splitting. They don't need to hurl rocks (or spears) at predators - they can dig holes and traps instead. Even a very small animal can dig a big hole over time for trapping, especially with teamwork. A squirrel or bird can crack nuts, that's no great feat of strength. $\endgroup$
    – causative
    Commented Jun 25, 2023 at 5:37
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    $\begingroup$ @o.m. It is the fact that humans did need workarounds for what other animals did naturally, that created the evolutionary pressure for us to grow big brains. Needing workarounds is not a problem. $\endgroup$
    – causative
    Commented Jun 25, 2023 at 5:41

The degree to which evolution is predictable and reproducible is a debated topic in Biology. The extreme of one camp holds that the end point of evolution is strongly determined by random events, so if you wind back the clock and re-run evolution from just after the last mass extinction and you'll end up with a completely different flora and fauna. The extreme of the other camp holds that random events have little impact so if you re-ran evolution you'd end up with basically the same flora and fauna as today.

Most Biologists, naturally, fall somewhere between those two extreme camps but I think the prevailing view is that evolution is mostly non-deterministic. But, of course, that's impossible to test on the large scale: we have one Earth and we can never know how different it could have been.

That would be dramatically overturned by soft SF aliens. It would make clear that the intelligent humanoid form, and various biological features, are effectively inevitable given the right environment and time.

And that's about it. There's no fundamental shake-up of evolutionary theory required, just a surprising result about how convergent evolution of intelligent life is.


Intelligence on earth has evolved in whales, corvids, and octopi. Intelligence throughout the universe takes on wildly exotic body plans. But every time a UFO shows up on our door, some bipedal humanoid comes strolling out with its buddies.

Why don't the intelligent trees, crabs, and oozey things come visit us? Our drive for exploration (not just migration) turns out to be essential for spacefaring. Why else would we invest so much effort to leave our perfectly beautiful planet? It turns out that the humanoid body plan is perfect for exploration. The other intelligent body plans are content to just send out probes to do their exploring. Some of them really like guests to visit, though. The sapient trees throw real ragers whenever a new species touches down for the first time.

What about the social aspects? Well, developing your science, engineering, and infrastructure to the point of spacefaring requires you to cooperate reasonably well with your own kind. You probably also need to dominate your planet well enough to pillage its resources without some other creature trying to eat you. So your humanoids cooperated well enough to wipe out their planetary competition, and then they learned to cooperate well enough to build spaceships.

Physics is also our friend when it comes to convergent evolution of spacefaring species. The habitat requirements of an aquatic species would make the initial development of rocket science practically impossible. The same goes if our planet's gravity were too much higher. If we were much smaller, building an orbit-capable rocket would become a herculean feat in the face of the rocket equation. So those who can travel the stars come from planets not too different from our own.

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    $\begingroup$ What evidence is there that whales evolved intelligence? At best, they're as smart as chimps, maybe only as smart as dogs. Corvids are interesting (they casually fashion tools out of scavenged metal), but octopodes are again, probably only as clever as dogs. If UFOs do show up, why would you expect the whale people? How would whales even do metallurgy? Same problem with octopodes. $\endgroup$
    – John O
    Commented Jun 25, 2023 at 5:47
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    $\begingroup$ It's a huge equivocation to consider any of these animals "intelligent". They're certainly smarter than the average beast, but they're nothing like human level intelligence. $\endgroup$ Commented Jun 25, 2023 at 9:15
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    $\begingroup$ The plot for my next sci-fi novel: a fleet of UFOs arrive to Earth... But instead of landing on ground, they appear to sink in the depths of oceans. The aliens are actually fish-like beings who live underwater. They create colonies deep in our oceans, and coexist with us with barely any interaction at all. $\endgroup$
    – Stef
    Commented Jun 25, 2023 at 16:54
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    $\begingroup$ @Jack Aidley, or maybe humans have nothing like octopus level intelligence. By default we compare other creatures' intelligence to humans because it is easiest for us to relate to, but really their intelligence is as alien to us as our intelligence is to them. We tend to consider animals intelligent when they seem to understand some human language directed towards them or we see them using tools, but don't consider ourselves as lacking in intelligence for being unable to understand the communication methods used by those animals; Not much removed from how colonizers judged indigenous tribes $\endgroup$
    – RisingZan
    Commented Jun 25, 2023 at 20:02
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    $\begingroup$ @Stef That is, until the king's daughter starts venturing to the surface to gather human artifacts. Blinded by her curiosity, she strikes a rash deal with a power-hungry witch... $\endgroup$
    – skeep
    Commented Jun 26, 2023 at 2:30

In a scenario where it's a fact that humanoids can travel to distant stars and influence selective pressures, I think the most plausible selective pressure for convergent evolution that settles on humanoids is artificial selection by prior humanoids (or mechanical agents thereof) that traveled to distant stars.

Of course, evidence doesn't always bear out the most plausible explanation.


Actually I think your premise may be wrong, and hard science is expecting aliens to be a fair bit like us. I know I've read some SETI research articles about this but here's a reasonable article that sprang to hand quickly and has the gist of it: https://www.quantamagazine.org/arik-kershenbaum-on-why-alien-life-may-be-like-life-on-earth-20210318/

In short, there's good reason to think that if we find alien life, it may well be similar to life forms found on earth.

Andy Weir's ("The Martian") Project Hail Mary had a bit I found interesting, basically a discussion of why humans and this hard sci-fi alien species had similar audio ranges. We are tuned to hear "the sounds of nature". Rocks slipping, branches breaking, water splashing, all tends to happen within a certain frequency range. If you're a reasonably large land bound animal, these are the sounds you need to hear. Similarly, the bits of the electromagnetic spectrum that our eyes see is probably pretty close to what any aliens would see (assuming they have eyes, which is likely), either because it's difficult to evolve to see other frequencies (there are size limits that come into play) or because they just aren't as useful for survival.

So while exotic aliens that have acid for blood, see x-ray spectrum, and have 9 arms may not be impossible, we have a lot of reason to suppose that evolution faces similar pressures wherever it occurs, and produces similar results (barring something truly exotic, like non-carbon based life, in which case all the pressures may be different and all bets are off).

(Although we may also float the idea that after a million years of civilization, all bets are off anyway, because genetic manipulation and bio-engineering may throw evolution out the window and the aliens are whatever they decided to be.)


They Evolved on an Earth-Like Planet

enter image description here

The blue antennae guys evolved under conditions very similar to Earth. They came out very similar to humans. It turns out it's the natural course of things.

The diverse aliens evolved at the bottom of the ocean. Or inside a gas giant. Or in a desert planet. You get the idea.


Life is not random, it is shape by its environment. Two races evolving in near identical environments will probably have a lot in common, but there is still enough randomness to ensure they will not be identical. Two races from very different environments will be very different.

There are many examples in science fiction of truly alien races. Look at Pournelle's Moties or Nivens Puppeteers.


I'm going to award the check to the "everything evolves into crabs" answer, because it's almost what I've ended up with, although all of the comments and answers here have been very helpful.

Basically, our Earthlings who have been anticipating "alien aliens" meet a humanoid alien and it is very alarming as they consider all of the implications. They come to see the ways of solving the problem of spreading between planets with carbon based life are limited to a few paths that tend to repeat, and within this repetition are a few common patterns that keep arising:

  • binocular vision
  • manipulative appendages
  • not too big or too small
  • living on land
  • using fire, chemistry
  • intelligence and a desire to explore and expand

But the most important is

  • cooperative civilization of millions of individuals who can work together

As the Earth people discover more living worlds they start to realize that it's not so much that the first aliens they met are "like humans" but rather that those aliens and humans are both similar to a third thing, an ideal pattern for the space-faring carbon-based creature with DNA.

They also notice that while many planets have similar animals and ecosystem to earth, the one thing every single planet always has are ants.

And the ants of earth are the least advanced among ants. Humans (and the blue aliens) are just an awkward round about attempt at being what ants are much more naturally. Because ants are perfect.

(and the most perfect ants of all live on a planet called "Myrmecos" in English ... but that's going beyond the scope of the question.)

  • $\begingroup$ ... and actually it's the ants who made sure to get humanoids, in order to spread more easily to other biotopes (including other planets). $\endgroup$ Commented Jun 26, 2023 at 23:52

This really depends on how close the aliens are to humans. Two-armed bipeds with heads that have mouths and sense organs do seem likely to arise by convergent evolution. If we met ten really alien alien species and one that fit that broad description, we would probably just recognize that convergent evolution isn't an inevitable force, but a phenomenon of local minima in optimization space.

But if they looked like Vulcans, or even Navi, there would be a huge scientific mystery. Bilateral symmetry, dexterous forelimbs, eyes near the top - those are all easy to explain. But the specific limb proportions, facial layout, and musculature must come from millions of contingent changes. Knees could bend the other way. Heads could have all kinds of shapes. Internally, organs could be arranged in all kinds of ways.

Consider that it is well established that some traits in animals are driven by sexual selection, where one trait spreads because potential mates like it. There is an element of randomness to those traits. It is suspected that human evolution has been subject to that process, and it probably would not go the same way elsewhere.

I would say that, if they looked similar enough that they could be played by human actors, and at the same time fossil history showed a completely different evolutionary course on a different planet going back to before the human body plan evolved here, then the idea of some kind of manipulation by some other advanced species would have to be taken seriously.


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