[Stevie Bain (OP)]

Jerry asks:

It is true that we only use a small part of our brain, and if so, how can this be explained from an evolutionary perspective?

Can you help?

[chiralSPO]

No, we use essentially all of our brains. Only a small portion is dedicated to conscious thought, but the rest is busy doing things like keeping you alive (regulating heartbeat, breathing, sleep cycles etc.), processing sensory stimuli (audio and visual processing is very energy-intensive) and subconscious processes like reading body language, knowing where you are in your environment, and knowing what shape your body is in.

[Janus]

The whole "we only use a portion of our brains" idea comes from the misreporting of a study done long ago.  The study was an attempt to map the different functions of the brain.  At the time they were able to assign definite functions to about 20% of the brain.  This was reported as "Scientists say we only use 20% of our brains".   Of course, the conclusion of this study was nothing of the sort.  Just because they were limited by the methods of the time to mapping only a portion of the brain, that did not mean that the rest of the brain did nothing.
Then, as the years went on, through continual repetition, the idea that we used just a small percentage of the brain became embedded in the public mind, In addition, the percentage decreased.

Today we have much more sophisticated means of tracking brain activity and have no doubt that we make use of all of it. 

[mrsmith2211]

Why is it when you are cruising down the road in your car you only us a small part of your engines potential? Same concept I think, the capacity is there when you need it but driving down the road at 30mph you have no need for total motor power.