This is taken from page 154 of Steven Pinker's book How the mind works as supporting argument against the assertion that We only use 10% of our brain.

the same disadvantages would face any creature pondering whether to evolve a humanlike brain.

First, the brain is bulky. That design compromise kills many women during childbirth and requires a pivoting gait that makes women biomechanically less efficient walkers than men. Also, a heavy head bobbing around on a neck makes us vulnerable to fatal injuries in accidents such as falls.

Secondly, the brain needs energy. Neural tissue is metabolically greedy; our brains take up only 2% of our body weight but consume 20% of our energy requirements and nutrients.

Third, brains take time to learn to use. We spend much of our time either being children or caring for children.

Fourth, simple tasks can be slow. (he goes on to note how insect reflexes are much faster than human)

I created this node/quotation in order to argue against We only use 10% of our brain. It works well as such. I agree with both mcc and Mr.Option, though I get the impression that they think that they are disagreeing with Mr Pinker. Far from it. I used a brief quote from a large work which does discuss these issues. It's a great book, go read it. I highly recommend it if you are into this kind of thing.

I quoted thus to help point out that it makes no sense at all to have a brain that gets only 10% use in spite of these costs. So we would not have evolved this brain unless it was fully utilised to great benefit.

This data also leads to a different conclusion: the particular level of average human intelligence is a compromise, an evolutionary stable point not just between "too smart" and "not smart enough"; but a balance point between intelligence and its biological costs.

The costs and benefits are however on a balance sheet drawn up by the hundreds of thousands of years of survival of the fittest in the paleolithic. This has all changed for modern first world people - our access to food is not limited (indeed the overabundance of food is a problem for some) and the benefits of intellignce have been increased.