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.
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.
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)
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 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.
That, and running away from lions and hyenas and those worst killers of Africa: the hippopotamuses.
Speaking of butts, as avid watchers of nature documentaries, the all-time funniest thing we saw in one was a hippo standing with its rear towards the camera in thigh-deep water, pooping while furiously swishing its tail back and forth just spraying it as far and wide as it could, for 5-10 seconds IIRC.
And, yeah, just search YT for "hippo spraying poop with tail", if you want to see that sh_t.
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.
> When God made the arse, he didn't say, 'Hey, it's just your basic hinge, let's knock off early. ' He said, 'Behold ye angels, I have created the arse. Throughout the ages to come, men and women shall grab hold of these, and shout my name!
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."
It turns out that he's seen god's face and at the same time hasn't seen it. The bible describes this event in a completely different way whenever it's mentioned
Let's bring human female breast size into the discussion as well, another unique feature among mammals (not counting cows..?)
Here is just the first search hit[0] I found on the subject, linking the two.
"Finally, Morris (1967) believes that breasts mimic fleshy buttocks, thereby providing a clear face-to-face signal in upright bipeds, but he does not explain the evolution of buttocks in the first place."
I vaguely recall some assigned reading in junior high school science, which described how mounting from the rear (where buttocks were an attractive sexual signal to males) was transformed to sex from the front, with breasts becoming larger as a substitute for the buttock as a sex attractant.
> Let's bring human female breast size into the discussion as well, another unique feature among mammals (not counting cows..?)
I read a comment on HN once, someone linked to some random blog, maybe a biologist or anthropologist that had an amazing theory for why breasts developed the way they did, discounting the aquatic ape theory and others.
I've never been able to find it with only the obvious keywords, maybe someone will repost it in this thread.
Of course, I also don't remember what the theory was at all, which also makes it hard.
Nice article, but let's not just skid past the subtle nominative determinism of the first peer commenter, evolutionary biologist and animal behaviorist, Mallory Wiper, who just had to take a swipe at the first comment. Of course, she may have originally been the rearmost comment, and was just moved to the front of the queue, naturally.
I believe that the fact that the female human butt is larger than the males is that it evolved to make the act of sitting down for long periods more comfortable. Effectively, they carry around a built-in cushion which makes squatting and sitting a markedly different experience for males.
Pretty much all differences in this regard can be attributed to the fact that the female pelvis needs to accommodate giving birth. On evolutionary time scales there's no proof of anything even remotely close to what you describe.
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.
