We didn't evolve brains to think. At least not originally. Instead, our central nervous system is something like a complex thermometer that gradually grew into self-awareness.
OK, so that's a bit of an oversimplification, but many of the brain's oldest functions are rooted in directly maintaining the body's chemical and temperature balances. In our remote ancestors the central nervous system and its evolutionary precursors appeared in multicellular organisms that had grown too large for those internal equilibriums to be kept up by purely local cell processes. Not surprisingly the hormone and temperature managing brainstem is evolutionarily the oldest part of our central nervous system and forms the structural core of the brain.
But there is another almost equally primal reason for having a brain, a function whose mediating cortices sit in the basement of the brain, nestled up against the brainstem: Movement. Many of the brain's other functions such as the generation of emotions, the accumulation of memories, perception of the outside world, and associative reasoning all exist to prompt us to move, whether that motion is speech, tool use, or locomotion such as walking and running. The requirement to generate movement shapes both the hardware and processing software of our senses as well as influencing how we employ our cognitive abilities while going through life.
The Ted talk linked to in the BBC article below looks at some of the cognitive processes behind the ability to move as well as some of the evolutionary history of movement.
BBC - Future - Health - The real reason for brains:
'via Blog this'
Of course we don't necessarily perceive our existence as being about motion. While motion is the ultimate outcome of many of our thoughts, the levels of analysis, perception, and emotion that lead us to move have largely taken on a life of their own. All the processes that make up thought and sensation create a rich subjective inner world, and much of our time within it is devoted to analyzing the meaning of emotions and perceptions as well as the probabilities of outcomes from actions yet to be undertaken. Then there is the fact that motion is a medium. One with a vast variety of expressions and divergent sensory impressions, separate from disparate processes of sensation and reasoning that initiate them.
All this will make it all the more ironic if we ever chose to discard our bodies or spawn software entities motivated by drives bequeathed to us by evolution.
P.S. That article on the Indo-European language and the social sciences is still forthcoming. It's just taking a little more thought and research than anticipated.