Ever wondered why human babies take so long to become functional, while many animals are able to walk and take care of themselves shortly after birth? It comes down to brain and pelvis size. If the baby’s brain was large enough for it to be self-sufficient, both the mother and the baby would die during birth.
When our ancestors started walking on two legs, it reduced the size of the human pelvis, thus making the birth canal smaller. In addition, the human brain got bigger, requiring a larger birth canal. Obviously something had to give. What happens is that the baby is born with an immature brain that’s small enough to allow passage through the birth canal. Then the brain continues to develop afterward.