Take a look around the animal kingdom. Even our closest living relatives among the great apes (chimpanzees, bonobos, and gorillas), don’t have proportionally as big butts as humans do. The main reason for this probably comes down to our unique style of locomotion.
The pelvis is made up of three parts: two innominates (or “hip bones”) and the sacrum. Each innominate is also made up of three bones (the ilium, ischium, and pubis) that fuse together during growth and development. And it’s the ilium that’s the real difference-maker between us and our ape relatives. A chimpanzee’s ilium is relatively tall and flat, with the flat sides facing forwards and backwards. Our ilia, on the other hand, are short and curved around more to the sides, making our pelvis bowl-shaped. These size and shape differences are linked to the evolution of bipedalism and the reorganization of our gluteal muscles that make upright walking possible.
The last thing that helps make human butts unique is the fat — which might also have something to do with us becoming bipeds. Humans have relatively large brains that use a lot of energy. Our bodies store energy as fat, and we have a relatively high percentage of it for a non-aquatic mammal. This has led anthropologists to suggest that our body fat helps buffer our metabolically-expensive brains against lean times.