Evolution of the pelvis in primates is the widening and the shortening of the blade called the ilium.

http://en.wikipedia.org/wiki/Pelvis

 Evolution
The most striking feature of evolution of the pelvis in primates is the widening and the shortening of the blade called the ilium. Because of the stresses involved in bipedal locomotion, the muscles of the thigh move the thigh forward and backward, providing the power for bi-pedal and quadrupedal locomotion.