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How plausible is an upright, humanoid creature that is inverted, with the head at the bottom and the posterior region at the top? This creature could have evolved from a typical biped that evolved into an ape-like creature that brachiated with its hind legs, which then convergently evolved into a humanoid. The creature is not a tetrapod, and has a shoulder-like system on all limbs. Are there any issues with the inverted humanoid or it evolutionary history?

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They are not plausible

There's a lot that makes this build troublesome. First is obvious. Hygiene. Things from the top go down. Some creatures are build to be strong against these things, but it does require energy and a special evolution to get this. Just having it go down your body, not even to your head, can be dangerous. Even without cuts or damages it can be bad, but anything open will receive a dose of dirty whatever.

Others aren't as obvious. Peristaltic movements (hope the translation is correct) now have to push up. Compared to an arm pushing food up, the gut isn't so effective or efficient against gravity. You'll expend a lot of energy just for digestion. As an extra bonus it's difficult to push water and such up. Not impossible, as the gut keeps gasses down, but their relative lightness and push from solids and liquids down can help a lot. In general just inefficient and requiring a lot of adaptations to function.

But your head between/in front of your legs also limits vision, possibly hearing and it's more prone to damage. You don’t see just less from a lower ooint of view and require to look both up and down to navigate, you can't look easily past your legs. A knee, branch or just falling and hitting things with your crotch is unpleasant. The same with your head is downright dangerous. Especially if you try to move sensitive organs like ears, nose and eyes in better positions, it'll most likely also be more susceptible to damage.

There's probably a lot more, but this should already dissuade such evolution. It's just inferior, bringing a lot of problems.

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  • $\begingroup$ Evolution doesn't work to perfect things. Most of these wouldn't be very big problems for the first, brachiating humanoids, and the later humanoids would more or less be stuck with their suboptimal shape $\endgroup$ Jun 1, 2021 at 17:13
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    $\begingroup$ @IchthysKing I can only say that I fundamentally disagree with that statement. Hygiene is very important to bith humanoids and brachiating humanoids. If feces and other run down the body, I'll say they don't even get to evolve before they die out from disease and infection. The suboptimal shape will also make it mighty difficult to thrive, unless there's little predation and easy food in abundance. I'm not saying it's impossible. I'm answering the question if it's plausible. It is not plausible, but not impossible. $\endgroup$
    – Trioxidane
    Jun 1, 2021 at 18:16
  • $\begingroup$ @IchthysKing except in here the hygienic conditions (or lack of), the potentially higher risk of diseases and the issues related to circulation and digestion could easily act as selective factors that favor a more "normal" posture over the inverted one. Not even sloths, which would benefit from being able to do their needs in the trees engage in such activities, even though going to the ground to relieve themselves is incredibly dangerous. Same for how flying foxes do their deed. $\endgroup$ Jun 2, 2021 at 0:07
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I asked a similar question to a biology teacher when I was in high school, and he told me this:

It is very important for a mobile creature to see where it is going. That would not be possible if your seeing eye were the eye of your [redacted].

A few years later, Spore came out. I could put what my teacher had said to test, with mixed results.

Some creatures do manage to have their rears quite close or overlapping with their heads. Crabs and cephalopods are examples of creatures who do have a discernible head and who defecate through it. However those are a far cry from a humanoid shape or even a tetrapod one (consider the humanoid crabs of Fallout, though).

For a creature that hands around by the feet as you describe, consider bats. They also "brachiate" somehow with their feet, and they spend a lot of time hanging upside down. But when they do move, their head is ahead and their hind is behind.

This also made me think of spiders, who sometimes hang from a ceiling before pouncing on prey. They too move with their heads to the front most of the time.

If you wish a butthead to be a sensible biological feature, you've got to figure out how it can be an advantage. I just can't think of one. Easier grazing? Other animals manage by having a long neck. Defense? That gets harder when you can't see much well due to eye position. Sexual selection? Maybe, but introduces more problems than it solves. So... I think this one is a no.

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