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As far as I'm aware, there's no way to be certain what any extraterrestrial entity might look like, seeing as how we can't verifiably prove such entities even exist. But I've heard speculations as to what they might be like should they exist. Said speculations have ranged from the hypothetical extraterrestrials being everything from indistinguishable from humans (perhaps because the development of life and life-sustaining planets in the Universe follows a specific pattern) to being utterly beyond anything us humans can imagine (because aliens would be, simply put, alien).

In short, I'm asking if there are any arguments that are particularly compelling and might sway me one way or the other. Again, I'm personally of the opinion that we simply have no way of knowing at this time what alien physiology would be like (if they even have physiology), but if there's an argument out there that might clarify or change my perspective, I'd appreciate hearing it.

Edit: I am aware that similar questions have already been asked (hence the flag), but these questions primarily attempt to address why aliens should look like humans, not whether they would look like humans in the first place. The other questions also address what aliens might look like during a benevolent first contact, which is not a part of my question. I am specifically asking whether it is more likely that aliens would be humanoid, or something entirely different, regardless of whether or not they are benevolent or ever make contact with us.

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The reasoning goes something like this:

  1. Assume an intelligent, tool-using alien species.
  2. They will have some sort of limbs and hands. Bilateral symmetric or radial? Radial ones are at a disadvantage during the evolution from a scurrying scavenger to an upright body plan. So bilateral is it.
  3. Four, six, or more limbs? Too many limbs are unnecessary, and hinder going upright. Call it four limbs.

So now we have a four-limbed being that evolved to walk on two legs, and to use the two arms/hands to manipulate tools.

  1. At least two eyes. spaced for binocular vision to allow range estimation.
  2. The olfactory system needs to be close to the food ingestion opening.
  3. The sensors like eyes and ears will be close to the brain.
  4. The sensors should also be relatively high on the body and able to traverse. Call it something like a head, on top of the body.

There are many things that could be different, of course. Let the special effects guys have fun ...

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    $\begingroup$ So you're mostly suggesting that a number of the physiological traits that make humans unique are likely to have developed with "intelligent" extraterrestrials as well, given their usefulness? $\endgroup$ – C. S. Wright Jun 27 '17 at 21:06
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    $\begingroup$ @C.S.Wright, yes. Of course an intelligent slime blob would say the same about flexible tentacles. $\endgroup$ – o.m. Jun 28 '17 at 4:08
  • $\begingroup$ yeah, good point. $\endgroup$ – C. S. Wright Jun 28 '17 at 23:00
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There is absolutely no way to know what aliens might look like, that is true. But I'd say that the chances of any particular alien species even remotely resembling humans are very low.

My argument is based on the only reference points we have: the species currently living on planet Earth, and those that show up in the fossil record. Of all the species that have ever lived on Earth, how many of them are are indistinguishable from humans? One: humans. How many could be considered very closely humanoid? A few genera (Homo, Australopithecus) and maybe a dozen species.

If you're looking for more loosely humanoid aliens, you might have better luck. Bipedal aliens are probably fairly common; after all, on Earth, there are plenty of bipedal birds. Quadrupeds are probably more common; judging by Earth's biosphere, it looks like they're easier to evolve.

If you're only interested in sapient, spacefaring aliens: There's simply no way to know. We really only have one data point here (ourselves), so we really can't make any predictions at all. However, some other species are more intelligent than average and may even be sentient (elephants, dolphins, crows, octopi). Before a species can make spaceships, though, they need to master the use of tools and probably also fire. I'd say that spacefaring aliens would most likely be a land-dwelling species with some kind of dextrous limbs that are good at grasping things, such as human hands, elephant trunks, or bird beaks and claws.

As for why most sci-fi movies ant TV shows have aliens that are almost exactly human: It's hard to find actors that aren't.

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  • $\begingroup$ I very much agree with everything to you say except for one thing: "However, some other species are more intelligent than average and may even be sentient", there is no reason to believe sentience is directly correlated with intelligence and thus that sentience would be limited to the species you list. The only thing we've been able to establish is that there is a (strong) connection between sentience and a nervous system. Because of this it may be that anything with even a basic nervous system is sentient. $\endgroup$ – AngelPray Jun 27 '17 at 4:41
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    $\begingroup$ @Somebody Else 37, So it's probably fair to say that aliens may look like us, but at the very least would have distinctly noticeable differences. If they didn't look like us, it shouldn't be any more surprising than finding an animal on Earth that is utterly inhuman. $\endgroup$ – C. S. Wright Jun 28 '17 at 23:11
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Humanoid lifeforms would have to evolve from something like the tetrapod morphology found here on planet Earth. "Tetrapod" simply means four limbs. Four-limbed organisms developed from free-swimming fishes as their fins adapted to become the legs of four-legged land animals (this radically simplified a long and complex evolutionary history). Our species was derived from tetrapods who had become more or less upright with a tendency towards bipedalism.

If there are other planets where tetrapodal lifeforms have evolved it is possible once they reach a tool-making phase that they will be upright, bipedal organisms. However, they may be humanoid or resemble humans in the sense that the science-fiction author Poul Anderson suggested, and quoting Gilbert and Sullivan, "in the dusk with the light behind them". This would be due to their arising from completely ancestors as the human species.

While humans are the descendants of free-swimming bony fishes, other planets may follow other evolutionary pathways any sapient life that emerges may be completely. Had the land been colonized by benthic or bottom-dwelling organisms instead of tetrapod body plans the major animal forms might be hexapods. Though not six-limbed creatures like insects, but creatures similar to mammals or reptiles but with six legs.

If this is so, the sapients might be quadripedal hexapods (four-legged six-limbs) or bipedal hexapods (two-legs six limbs, so possibly four arms).

Also, if arthropods had developed proper lungs for their respiratory systems instead of tracheoles restricting their intake of air to diffusion through pores in their carapaces, then they could developed into large animals and not remained tiny.

These examples are based on known lifeforms on our planet. We simply don't have examples of what another biosphere might produce. Also, this discussion has been restricted to considering the progeny of Earthlike planets. Hot jupiters if they were inhabited would have radically different organism and an utterly different evolutionary history. This could produce almost anything, but it would be definitely unlike anything on Earth and most assuredly not humanoid.

The biggest unknown in this whole matter of whether extraterrestrial aliens would be humanoid or not is the way the organisms in the total ensemble of an alien biosphere would develop. There is no absolute reason that aliens would be humanoid. Certainly consideration of convergent evolution suggests that humanoid aliens could exist. But they might be only one of many body plans that are suitable for sapient organisms.

One aspect of humanoid aliens, which science-fiction often gets wrong, is if they existed it is quite likely any two humanoid aliens from different planets would be mistaken as coming from the same planet or are of the same species. Mid-twentieth century science-fiction had alien humanoids walking among us without any need for a major disguise. Many features of even humanoid aliens could be quite different. Their faces could be unlike ours. Noses and mouths might not need to be part of their breathing apparatus.

In summary, aliens are more probably different. This will be due to their different biologies and evolutionary histories. Some aliens may be humanoid, as G&S said, "in the dusk with the light behind them".

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  • $\begingroup$ good answers, and nice G + S reference. It seems like aliens might (though I stress "might") look like humans, but even if they did, they would almost certainly be distinguishable from us nonetheless (i.e., facial structure, body type, etc.). $\endgroup$ – C. S. Wright Jun 28 '17 at 23:08
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    $\begingroup$ @C.S.Wright Good to have my answer appreciated. This was about general principles and didn't go into too much depth. Even if convergent evolution produced humanoid aliens would be very different from us. Think about alternative evolution on Earth where it was other primates that became sapient. They would be humanoid but definitely different. Also, if raccoons, kangaroos or mongooses had become sapient. Humanoid, yes, but like us. Devising non-humanoid aliens is always an interesting challenge. No wonder humanoids are the preferred option. Easier too. $\endgroup$ – a4android Jun 29 '17 at 4:10

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