Eli5 why is there a very small amount of biped species?

Question

Eli5 why is there a very small amount of biped species?

In: Biology

Answer 1

Birth! I won’t go much into it because I don’t know but the bipedal position requires a smaller pelvis whereas a quadruped can have a wider pelvis. This causes human babies to be born “earlier/ more underdeveloped” than quadruped babies – because smaller head circumference, easier to pass through the pelvic diameters. So the theory is that the bipedal position requires babies to be born earlier and therefore need more time to develop post natal.