Why has Darwins natural selection endowed the human brain with an abundance of spare capacity?
It did not “overshoot”. Competition drives advancement. In a large society, not everyone needs to be stronger to survive, and intelligence is valued as an attribute. Natural and sexual selection would ensure that those with higher intelligence would have more offspring, perhaps because they could use their intelligence to either become wealthy and powerful, or to help others in their community, but it is a trait that is advantageous. It also is unburdened by the physical limits of size and strength, that is, a very large man could very well have a bone structure that will not support him, or that will make him strong but slow. It is unrestricted in the sense that it does not have a physical capacity. .
