I am interested on writing an anatomically correct Cthulhu, made by HP Lovecraft.

Some information we know about him:

  • Can make people go insane just by the sight of him.

  • "Sleeps" for extended periods of time under the sea.

  • Very old

  • Resembles an octopus with a anthropoid outline

  • Is described as "tenebrous", "slobberingly", "gelatinous", and "greasily"

Given these characteristics, what species could Cthulhu evolved from, what would lead them to evolve that way, and what would they look like?

  • It can fly or propel itself into the air.
  • Is worshipped as a god.
  • 2
    $\begingroup$ Welcome to worldbuilding. We don't deal with 3rd parties' fictional worlds, and some of the characters you have listed are not really selected by evolutionary pressure. Please carefully read the help center and rework your question. $\endgroup$
    – L.Dutch
    Commented Oct 26, 2020 at 17:22
  • $\begingroup$ Pretty much anything could be worshipped as a god. That bit is not really pertinent to anatomy $\endgroup$
    – Atog
    Commented Oct 27, 2020 at 0:55
  • $\begingroup$ I would say the most likely ancestor would be octopi because they are very gelatinous and can change their form or texture. The flying bit seems challenging, but there are "flying fish". Octopi have pretty short lifespans though. Maybe it drives people insane by emitting some hallucinogenic substance? That seems semi-plausible $\endgroup$
    – Atog
    Commented Oct 27, 2020 at 0:58
  • $\begingroup$ Dont forget: you can drive a boat through Cthulhu's head and he is not much bothered by it. He is insubstantial. $\endgroup$
    – Willk
    Commented Oct 27, 2020 at 1:45
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    $\begingroup$ Anatomically correct lovecraftian humans: Under what evolutionary pressure can human like creatures form that are reliably driven mad by looking at a tentacle monster or weird architecture? $\endgroup$
    – mart
    Commented Oct 28, 2020 at 17:25

3 Answers 3


Most of the descriptions of Cthulu really come down to "indescribable", and "different, even from itself".. This implies Cthulu's physicality is not 3-dimensional, but something more? different? And it is the attempt to perceive that which is more than mere physical existence that causes the insanity.

Note that most religions ascribe this same non-describability, non-perceivability to their Gods and their greater Servants. Most agree that to look upon the face of god, or sometimes even to hear his/her/its voice directly, is to lose your mind.

Cthulu is just one of the more icky descriptions in this vein.


First off I am not a biologist or archaeologist so I won't go into high detail about what evolution path is plausible but I think I can give you some very usable pointers. I will leave out "worshipped". As it is formulated now there can be no clear right answer it will be very subjective. You might get a better answer in creating a new question asking for what characteristics are common in gods or animal deities or something.

Real creatures that can inspire Cthulhu's anatomy

1. Sleeps/Hibernate for extended periods of time and has a long lifespan

There are several animals that become extremely old. If we want to create a believable monster that has existed for eons we can take some inspiration from these creatures. But why do some aimals live for such along time and we don't?

One of the more well known(but still not fully understood) reasons why humans lifespan is limited is because of something called telomeres. As you probably know almost all of the cells of the human body(with some exceptions) are replaced as we age. However an issue with this (very simply explained) is at the end of our chromosomes there is a sequence called telomeres. These shorten as cells duplicate and after awhile they reach a point where the cells cannot properly copy themselves anymore.This can be fixed with an enzyme called telomerase that can repair the telomeres however we no longer produce that enzyme after we have left the womb.

There are however many sea creatures that solve this problem. Which lends further credibility to an ocean dwelling Cthulhu having solved this as well. A notable example of a long lived sea creatures is the lobster.

Two-in one inspiration for the long sleep and the long lifespan: A type of jellyfish called "Turritopsis dohrnii" or immortal jellyfish. It is essentially immortal because if it is "exposed to environmental stress, physical assault, or is sick or old, it can revert to the polyp stage".[1] So the jellyfish attaches itself to the seabed and reverts into to an earlier stage of development where it rebuilds until it is reborn again. More so than the lobsters high age resistance due to telomerase this more of an all out fix that gives more of an actual immortality.

2. Can fly/propell itself through the air Evolution happens because of outside factors, where a creature evolves to better and better handle the situation they live in. If the environment is stable with few changes creatures will likely be extremely specialized. For this reason I have a hard time drawing on any real life creatures that would be capable of flying while simultaneously spending the majority of time under water. The opposite exists but it also makes sense for i.e birds since prey(fish) is in the water. There need to be a reason for the creature to leave the water.

Perhaps this Cthulhu evolved in an extremely hazardous environment where high mobility and migration between land/air and sea is a key for survival? This however creates a lot more issues with how the anatomy would work than it solves.

I think your most realistic bet is something that can propel itself a short distance out of the water. Since you want an octopi appearance why not go with a water jet stream? This is already the propulsion used by squids2. It could theoretically propel a creature out of the water if strong enough.

3. Causes severe mental issues "insanity" in humans on sight What you are describing is essentially a sight that would make someone immediately enter a psychosis state merely from looking at something. People can enter a catatonic-state (non-responsive) after severe mental trauma but from what I've heard not from an instant reaction. I think you would do better in achieving this by having your Cthulhu leaking some dangerous hallucinogenic or other substance as was mentioned by @JamesFaix. There are definitely substances that can achieve this but I don't have the knowledge nor do think it might be proper discussing that in detail in an open forum.

EXTRA: "Non-Euclidean geometry" is a word used several times by H.P.Lovecraft in his stories. While a glance a wikipedia suggest he perhaps uses this slightly wrong he essentially means when something isn't mirrored from side to side. Everything in nature adheres to this rule and when something doesn't it looks just wrong. Cause by faulty DNA it usually makes plants funny and animals horrible. Making something non-euclidean would certainly help with making it look just plain wrong and a "freak of nature" at least helping you create something that is repulsive to look at.

[1] https://en.wikipedia.org/wiki/Turritopsis_dohrnii

[2] https://en.wikipedia.org/wiki/Aquatic_locomotion#Jet_propulsion

  • $\begingroup$ If the term is used properly, most lifeforms are non-euclidian in a sense. Euclidian geometry deals with flat surfaces, where for any given line and a given point, there is exactly one line you can draw through that point that will not intersect the other line. Non-euclidian geometry comes in two main flavors: elliptic geometry means surfaces "curve in" and there are no parallel lines, like with longitude lines on a globe; hyperbolic geometry where surfaces "curve out" and there are multiple non-intersecting lines you could draw... $\endgroup$
    – Atog
    Commented Oct 28, 2020 at 14:00
  • $\begingroup$ Human faces have examples of both types. Your head is round and elliptic geometry can be applied to its surface. The sides of your nose curve out and hyperbolic geometry could be applied. However with Einstein's relativity you can also consider the shape of space itself to be hyperbolic in regions of high gravity, slightly curved near a planet and massively curved in near a black hole. This is how gravitational lensing happens. Maybe cthulu does this to spacetime around him and that helps with the madness somehow? $\endgroup$
    – Atog
    Commented Oct 28, 2020 at 14:04

#Colossal Cuddlefish

Cephalopods have a variety of tricks to obfuscate their appearances. Cuddlefish have the addvantage of using these to hypnotize fish, which sounds rather Cthulhuan. The real challenge is retaining the fancier abilities with gigantism, since Evolution tends to result in abilities being lost if they are not useful enough to be worth the cost (see brains in Echinoderms, or sightless cave-dwellers, or humans' ongoing relationship with wisdom teeth). A Cthulhu-esque giant cephalopod doesn't really need the shapeshifting and hypnosis, unless it's neighbors are similarly enormous.

Perhaps there is a giant cuddlefish population in an environment with other large marine life, but not Cthulhu-sized. A rare mutation causes this creature to get exceedingly large, and one or more of these individuals is particularly long-lived.

Which brings us to the one big problem with cephalopods: lifespan. I'm not sure if this applies to cuddlefish, but when an octopus reproduces, its fate is sealed. This might well be the reason there aren't legions of intelligent octop[pi|podes|pusses] dominating the oceans, since they can't transmit information to their offspring. However, this shortened lifespan has a straightforward solution: sterility. A long-lived colossal cuddlefish is most likely not going to reproduce, even if its smaller brethren do so at the cost of their lives.

While you could replace the hypnotic color effects with chemicals or infrasound, I do find the related cephalopod abilities for stealth relevant. If a colossal cuddlefish has much more dynamic uses of its stealth abilities, well, it won't be very stealthy (what with being gigantic and all), but it will allow it to look far more Eldritch than your typical, already out-there cephalopod.

The hardest issue is the flight. Cephalopods can survive out of water for short bursts, but giant creatures suffer a penalty from their own weight when leaving the water. So right off the bat, a colossal cuddlefish is less likely to fling itself over a barrier. It most likely cannot reshape its flesh into something aerodynamic, and it would not have an obvious means of filling itself with a gas that could help it stay aloft for any significant time in air. If it can inflate itself by accumulating gas in its body, that can support some of the other features - giant but not too giant (the inflation makes it look bigger), the ability to produce terrifying noises you'd not generally expect from cephalopods, and much more support in air than you'd expect for such a large and squishy creature.

So Cthulhu is a giant, celebate, mutant cuddlefish that inflates itself like a balloon, and probably "flies" by expelling the gas, doubtless mixed with water or some other fluid to make it look less like a giant deflating balloon and more like alien geometry as its image is refracted in the mist. It retains enough shapeshifting to make pinning down its true appearance difficult while it yet lives, and its ability to alter its color and generate sound can mess with the minds of many a creature. An evolutionary reason for the gas is the primary mystery. Perhaps the gigantism made it vulnerable to gas-producing gut microbes over the centuries? This would help it to support its weight and get around more efficiently, but would be unlikely to be a default feature of this species.

  • 1
    $\begingroup$ I've never heard Cthulu describes as "cuddly" before. I believe you meant "cuttlefish", which is sort of a squid. Cuddlefish is either an australian band, or a squid plushie, or an adorable critter in the Subnautica game. $\endgroup$
    – user79911
    Commented Oct 29, 2020 at 6:40

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