I've read some books that theorized about the evolution of the mind. What was selected for was pattern finding. Our ancestors (as well as us) needed this ability to adapt to a sparse savannah lifestyle. Constantly roaming, looking for food. Pattern recognition helps us to determine what is safe/dangerous, good to eat/not good to eat, where water may be found, where food my be found, etc...
Also, a lot of our minds (the conscious mind) is devoted to social skills. As our social groups became more and more complex, we needed larger brains to determine social status and other things. What I read spoke of compartmentalization of the brain. Our early ancestors shaped tools without understanding what they were doing. They didn't think about it, they did it. Much like a bower bird. It's like when you try to explain to someone how to perform a complex task. You struggle to find the words to explain it and most of the time eventually give up and have to show them. It's not your conscious mind that knows how to do these things, it's the darker portions.
There is a lot of debate going on about how the mind works (left-brain/right-brain, id/superego/ego, etc...). The truth is we don't know. But our rational mind tries to explain it, even if it's unexplainable. Free will falls in this category as well. Do you actually mean to do the things you do? Or does your rational mind merely explain to yourself why you did what you did?
It's interesting to note a difference in selection criteria. Women largely select their mates not on their intelligence (solving quadratic equations and the like) but rather on social skills. Which is more taxing to the mind? Does selecting for social intelligence result in increased scientific type intelligence? If not, then we're in trouble.
It's possible that future evolution might reduce the size of the brain. As resources grow more scarce, the brain might be too expensive to maintain at it's present size. Our descendants might be as pea-brained as our ancient ancestors.
Patterns, my friend, it's all about patterns.
