I was intrigued by Arthur Robson's explanation for why "intelligence and longevity were simultaneously exaggerated in humans." ("The Biological Basis of Economic Behavior" 2001. Journal of Economic Literature 39: 11-33):
Not only has human brain size increased dramatically over the last two million years, but human mortality also seems to have fallen substantially. Indeed, if chimpanzee mortality rates approximate those for human evolutionary ancestors, the hunter-gatherer investment in children could not possibly be recouped without such a fall in mortality.
Economists often point out that firms only give out free samples if they expect to stay in business enough to earn back their upfront losses. Robson essentially applies the same logic to human evolution. Parents only raise slow-maturing, big-brained offspring to adulthood if they expect their kids to enjoy the survival advantages of a bigger brain for enough years to make it worth the wait.