Three main stresses that human body learn to deal with during the course of evolution trigger development of brain (and nervous system in general).

These stresses are endurance activity (walk long hours), strength activity (fight or flight response) and famine.

The agent responsible for development of nervous system in these cases is brain-derived neurotrophic factor. It is responsible for creation of new neurons and branching of old ones and directly related to intellectual performance.

So your link to high level machine intelligence is wrong, I believe. I also think I explained above why do you feel smarter after a day of physical activity.