Humans are bipeds. The pelvis in ancient human ancestors underwent radical changes in orientation and shape compared to other kinds of primates. Many of these changes serve to adapt our muscle orientations to the requirements of upright stance and bipedal locomotion.
The most significant changes to the pelvis in humans compared to other apes are:
Ilium: The ilium (top portion of the innominate bone) in humans is shorter and broader. It curves around the trunk, whereas in apes it is flat against the back of the trunk.
Greater sciatic notch: This is very wide in apes, a function of their long, tall ilium. In humans, the notch is actually a notch.
Anterior inferior iliac spine: This feature is prominent in the hominin pelvis, absent or small in apes.
Sacrum: In humans, the sacrum is broad and short, in apes it is narrow and long, usually incorporating 6 or more sacral vertebral bodies.