Well developed buttocks are a peculiar trait of man. None of the other primates has anything so well developed. The projection called the buttocks or rump, in man, is largely due to the massive development of the gluteus maximus as a consequence of the adoption of the erect posture. While other muscles of the pelvis and lower extremity have been significantly involved in the evolution of the erect posture, it is the gluteus maximus which has been most affected. It is the muscle that contracts to extend the hip joint, in running, climbing, ascending steps, raising the trunk from the stooping or sitting position, and relaxes under tension as these positions are assumed. It is critical in maintaining balance of the trunk upon the femur and of the femur upon the tibia (through the iliotibial tract) in holding the erect posture.
The gluteus maximus is, in fact, the heaviest and
Montagu A. The Buttocks and Natural Selection. JAMA. 1966;198(1):51. doi:10.1001/jama.1966.03110140101027