> Third, brains take time to learn to use. We spend much of our time either being children or caring for children.
Not quite.
The fact is that the human brain is large enough that, evolutionarily, you simply can't fit a brain that size through a pelvis. Any pelvis capable of doing so would be a huge liability, and would be evolutionarily selected against. It just doesn't work.
The way humans get around this is having the children born with the brains only partly formed. It isn't that it takes a long time to "learn to use" the brain; it's just that the brain isn't finished being created yet. So we wind up with humans leaving the womb as virtual vegetables, while some other less "advanced" species are able to function relatively well immediately after birth, because humans can only form the bare framework of a brain before birth or else the birth just doesn't take place at all.
The funny thing is, while evolution (as a blind process) could not possibly have been "planning" this, this has all turned out to be almost more of an asset than a "cost". Because humans are essentially helpless for the first four years of life, and not even really fully capable for the next ten or so after that, humans are *forced* to stay with their young and keep them safe or the species dies out. This winds up evolutionarily selecting toward things like motherliness, which leads to caring and more complex emotions. Between this, the extreme difficulty of caring for a child in a wild environment, and the necessity of "learned" knowledge rather than instinctive, humans are forced toward intraspecies interaction in ways that eventually become complex family structure, and then complex social structure, and then civilisation.