How did human butts evolve to look that way?

Birds are small, that means you don't need much muscle.

Random uninformed idea… this is surely wrong but… birds ‘knee’ joints are the opposite of humans, have their ‘butt’ muscles have evolved alongside their wing/breast muscles?

Birds are not mammals.

Are birds mammals?

Birds are generaly balanced with their legs in the middle of thier body. Those with legs to the back don't walk as well (but that leg position gives other advantages that the bird might need)

Birds are bipedal, but their trunks/spine are closer to perpendicular to their legs (while ours are parallel and stacked). Birds also have a non trivial amount of mass hanging down from their trunk between their legs, and their hips tend to extend significantly past their legs.

So while birds actually need significant posterior chain muscles to keep the femur extended, those muscles get to attach to an extended hip and so form more of a triangular/trapezoid shape compared to human butts. If you ever noticed that chicken thighs from the store seem a little triangular, and the bones are 'off center', then the the larger triangularish half of the chicken thigh is the chicken analogy to the glut complex.

EDIT: I think people would generally define the depth of the human intergluteal cleft ("butt crack") as one of the defining features of the human butt. In that context, I think humans have this feature at least partly because the human hip is... suboptimal in many ways. When considering the gluts as a hip/femur extensor, you have some pretty bad leverage, and so you're already biased towards having a very bulgey muscle there. As opposed to something like our hamstrings which has a much better leverage scenario, and so you see much less of an obvious bulge. By analogy, since birds tend to have an extended hip to attach to, their glut max analogs are no where as bulgey.

When a bird is perching or walking on the ground, it is almost never standing with its legs under the entirety of its body. A bird is like a rectangular box whose long axis is parallel to the ground with two sticks just slightly aft. In contrast, a standing human has a rectangular torso whose long axis is at 90 degrees to the ground with the legs coming out of one end.

To see this, do an image search for an Ostrich, which shows the 'box' effect perfectly. Smaller birds have a body box that is often at more of an angle to the ground, and with legs that are tiny compared with a human's. The bird's hips are near the back of the box but there are usually bones behind them, which is where the tail is mounted. The equivalent of the femur is tilted forward and short compared with a human's, and a bird's sternum may actually be between and below the equivalent of our knees.

Very different anatomy.

I had the misfortune of seeing this hippo behavior in real life at a zoo and the reactions it got from the crowd of parents and kids was one of the funniest things I've ever seen.

Speaking of butts and massivesci, when I'm searching YT, I prefer both kinds of callipygian: https://www.youtube.com/watch?v=gwti7-cEHh8#t=39s

For manure spreading, I prefer automation, in particular "New Idea"s: https://www.youtube.com/watch?v=oTuK2Xm3q5I#t=18s

I don't think you can run away from a lion or hyena.

It’s more for running and carrying. Humans are at least in some instances exhaustion hunters. We will chase prey over long distances. The way we run makes extensive use of that muscle to push us forward.

We also need to carry stuff over long distances: our helpless babies, food, and later in our evolution packs full of tools and basically camping gear.

Lastly, we are the only species with Sir Mixalot.

I'm not sure that really counts as kangaroos use their tails as a third leg for balance.

They were put there "recently" in that condition to keep the faithless science types busy and out of the way of ye faithful worshippers.

It's like adding NPCs, side quests, and easter eggs to games.

There is a verse in the Old Testament where Moses asks to see God's true form, but God tells him that it would kill him, so God turns away and lets Moses see his ass instead.

Which I guess is a very literal interpretation of God making man "in His image."

You bear a terrible burden, then.

That's what she said.

Well, I can see that my opinion is not a popular one, but I stand by it. As a skinny-arsed bloke, I find sitting down for long periods very uncomfortable. The literature seems to agree that women are more able to sit upright for long periods, though there is no consensus as to why.

> Pretty much all differences in this regard can be attributed to the fact that the female pelvis needs to accommodate giving birth

I don't doubt that this is true, but a thing can evolve to serve two or more purposes. Feathers likely initially evolved for the purposes of thermoregulation, as well as to serve mating behavior.