Why do we have big gluteal muscles?

Why do we have big gluteal muscles? Daniel Kolitz. Gizmodo, September 21, 2020. https://www.gizmodo.com.au/2020/09/why-do-we-have-butts/

Some experts gave their opinion in 2018. Among them, Jason Bourke, Paleontologist and Assistant Professor of Anatomy, New York Institute of Technology College of Osteopathic Medicine at Arkansas State:

The structures we refer to as butt cheeks in humans are comprised of the gluteal muscles: gluteus minimus, gluteus medius, and gluteus maximus. Of the three gluteal muscles, gluteus maximus is responsible for the signature shape of the human derriere. It originates from a line that runs from our upper ilium (the pelvic bone that people often call their hips) down towards our coccyx (tailbone). The muscle attaches close to the top of our femur (thigh bone). The gluteus maximus functions as our major leg extensor and is the main driver of our legs when running, climbing stairs, getting up from a seated position etc. It’s the powerhouse muscle we call upon whenever we need to perform major postural changes, or when we need to move our legs fast. That’s why sprinters, and weightlifters that do heavy-weighted squats tend to have very round and firm buttocks (i.e., “squat butt”).

Because the muscle originates and inserts across a fairly small distance, it needs to produce a lot of force to get our legs to move. This makes the muscle very large, but since there is not a lot of space for the muscle to sit, the muscle fibres expand outwards and – thanks to gravity – downwards resulting in our hallmark hind ends.

The shape of our rear end is practically unique to our species. As we transitioned from quadrupedal apes to bipedal ones, our pelvis underwent radical changes to handle the weight of our entire torso resting on top of it. This required substantial reorientation of many of our hip muscles and it put our major leg extensor (gluteus maximus) in this weird position where it seems to almost hang off of our pelvis. There are a few other species of mammals that have what we might term “butt cheeks”. Namely horses, which show substantial developments of their rear ends as well. As with humans, well-developed gluteal muscles are responsible for the roundness of horse butts. However, unlike humans, horses achieved this shape via expansion of a different gluteal muscle, their gluteus medius. In fact, a large gluteus medius is pretty standard for most mammals. Humans are unique in expanding our gluteus maximus instead, which is no doubt a response to the unique physical demands of our strange way of walking.