What is the significance in the changes to our ancestors' pelvic structure?

Question

Why are these changes significant and what do they tell us about our evolution?

Answer 1

The homnin pelvic structure began to narrow and lengthen as our main form of locomotion became bipedalism. A narrow pelvis allows this movement to be more energy efficient. Walking upright on two legs also meant that our organs needed to sit in the bowl-shaped structure of our pelvis.

At the same time, our cranial capacity was gradually increasing in size, from 700cc to our modern 1300cc. With bigger brains came bigger heads, which was not advantageous to the female bipedals. Childbirth became more challenging and deadly as our babies' heads grew larger and our pelvic structure (and subsequently the birth canal) narrowed.

In summary, our anatomically modern pelvic structure is the result of the battle between these two evolutionary demands (bipedalism and childbirth).