Vertebrates on land are limited in size by various factors. However, in the water, they can get much larger. How large could an aquatic humanoid creature (like cthulhu) realistically get, and what adaptations would be necessary for it to grow as large as possible? How would it sustain itself?

The only requirements are: 1. It is completely or partially humanoid in shape 2. It is as large as possible 3. It could realistically evolve on Earth

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    $\begingroup$ Suggested reading. pnas.org/content/pnas/115/16/3995.full.pdf Physiological constraints on marine mammal body size $\endgroup$ May 21, 2020 at 20:09
  • $\begingroup$ It could have evolved from a seahorse for all I care. Then, it's not humanoid at all. I think you might want to reconsider the foundation of your question, if seahorses are acceptable. $\endgroup$
    – elemtilas
    May 21, 2020 at 21:04
  • $\begingroup$ @GaryWalker that's a great link! I hadn't seen that study before. $\endgroup$ May 21, 2020 at 21:14
  • $\begingroup$ @elemtilas I meant that as long as the resulting animal is humanoid I do not care whether the animal it evolved from is humanoid itself. $\endgroup$ May 21, 2020 at 21:20
  • $\begingroup$ Okay, then, no seahorses... $\endgroup$
    – elemtilas
    May 21, 2020 at 21:22

1 Answer 1


About 98 feet, give or take

Obviously, there would need to be a few changes to survive in the water, though I will keep to your requirement of 'partially human in shape'. Completely human is fully impractical, and I will stick to the requirement of 'realistically evolve on Earth'. Alright, let's start.

The first thing we do is make a few adjustments to the human. Independent legs are tossed aside right away - we're combining them to turn them into a single tail, and as long as we're doing that, we're going to adjust the feet so we turn this into a proper fluke tail. Might as well. The next thing we do is subtly adjust the method of getting oxygen to the lungs - we moved the nose to the creature's back and play around with it so that it becomes more of a 'blowhole'. Then we make adjustments to the creature's arms and make them as finlike as we possibly can. Obviously, since we're going for a realistic evolution, we're then going to want to streamline the whole creature to make sure it can cut through water as much as possible. Lastly, when enlarging it, we can't do it uniformly, thanks to the square cube law - the legs and arms will have to appear small to the enlarged torso, but if you want realistic, it's the path you're going to have to take.

And what you're left with looks remarkably like a Balaenoptera Musculus, except slightly more humanoid. I mean, it's not that humanoid at this point, mostly thanks to the streamlining we did in order to make it realistic in the water, but the skeletal structure closely resembles that of a human, and that's what counts.

  • $\begingroup$ I think you misunderstood. It does not have to have evolved from a human, it doesn't even have to be a mammal, just humanoid in shape. It could have evolved from a seahorse for all I care. $\endgroup$ May 21, 2020 at 19:13
  • $\begingroup$ Ngl clicking the link and seeing it was just the blue whale made me chuckle, though yeah, the main difference would be that it'd be a bit closer to a giant filter feeding leopard seal $\endgroup$ May 21, 2020 at 19:17
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    $\begingroup$ @Praearcturus a marine environment offers no pressures towards a humanoid form. For it to look humanoid would mean it came from a humanoid creature. We could be talking about a chimpanzee or other primate here. $\endgroup$ May 21, 2020 at 19:20
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    $\begingroup$ @ProjectApex OTTERS $\endgroup$
    – user75689
    May 21, 2020 at 19:39
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    $\begingroup$ @Kyu fair enough $\endgroup$ May 21, 2020 at 20:14

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