It is often said that we only use 10% of our brain. This is typically a preamble to a motivational speech, encouraging us somehow to use the other 90%. Well, you'll have to find some other way to get motivated, because, Flowers for Algernon and Limitless notwithstanding, it isn't true.
There are anecdotes of individuals with brain damage through disease or trauma who recover most of their functionality. Recovery may take years, and may involve intense ongoing physical therapy, but there he stands when some thought he would never walk, and speak he does when some thought he would never speak again. This demonstrates the plasticity of the human brain, but it doesn't indicate percentage of use. In fact, we know the damaged portion was being used, else there would be no need for years of recovery. The other sector of the brain, call it region Y, that takes over the function of region X, may have sacrificed some of its own functionality to make room for the functions of X. These could be memories jettisoned that aren't critical to life and aren't missed, or high school algebra, which hasn't been used in years. In other words, these anecdotes shed no light on the question of overall brain utilization. To make matters worse, they are the exception, not the rule. Just asked Gabby Giffords, who has made a miraculous recovery, but is certainly not the woman she was before. closer to home, my grandfather was never able to speak clearly or move his right arm after his stroke.
In fact we know, without experiments or tests or PET scans, that we use all of our brain, because evolution would not tolerate anything less. Your brain is a hungry organ, consuming 25% of your oxygen, and ipso facto, 25% of the caloric content of your food. If 90% of our brain was unused, one of our ancestors, through chance mutation, would have shut down some of those unused cells long ago. That particular hominid, eking out a living on the plains of Africa, would have had an advantage over the others, requiring less food and less oxygen. He could run faster and longer (sending more oxygen to his muscles), and survive a famine where some of his relatives might not. His children inherit the beneficial mutation, and so it goes. A hundred thousand years later, this mutation has become the norm, and at that point another mutation shuts down even more of the unused brain. After a million years, we would retain only that portion of the brain that was necessary or advantageous for survival. Our demand for oxygen and food would be almost a quarter less, with no negative side effects. This did not happen, and so we really do use, and need, our entire brain.
The advantages of a smaller brain extend well beyond food and oxygen. A smaller brain allows for a smaller head, which simplifies the process of child birth. The female pelvis has been reshaped to support the baby's enormous 10cm head, but even with these anatomical changes, childbirth is still a horrific ordeal. This is further complicated by the metabolic demands of feeding the fetus during the third trimester, which would again be less if the fetus had less brain to support. (Remember that the brain is a much larger percentage of a baby than an adult.) It is hard to know for sure, but throughout prehistory, 4% of women may have died in childbirth, and each one of these deaths often represented the end of her genetic line, since the children were not cared for, or became an additional burden on kin. A smaller brain has so many adaptive advantages, that the presence of our very large brain after millions of years assures us that it is an invaluable organ in its entirety.
Since functionality and size cannot be compromised, might the brain operate with less power? As you might imagine, evolution has ratcheted energy efficiency up to an unprecedented level. This is of course difficult to quantify, but by some accounts, the brain is 30,000 times as energy efficient as our best integrated circuits. As a humorous thought experiment, imagine your brain, in the same space, with the same functionality, based on silicon. You would have to burn 10,000 calories per minute, instead of 2,000 calories per day. Of course nobody can find that much food, nor take in that much oxygen. To top it off, your head would reach the boiling point in half a second. Fortunately for us, the brain is built upon ultra-efficient wetware.
Another curious feature of the brain is that it feels no pain. Once again this is clear when viewed through the lens of evolution. Pain causes us to favor injured limbs until they have a chance to heal, and to avoid situations that would repeat those injuries in the future. However, if your brain is injured, because an animal has bitten through your skull, death is immediate. You can't favor the right side of your brain until it heals, nor can you avoid that situation again, because you have shuffled off your mortal coil. Surgeons sometimes take advantage of this oddity by operating on a patient's brain while he is awake. Local anesthesia is required to open the skull of course, but once inside, the surgeon can poke and prod, and note the effects of his actions on speech, vision, and cognitive skills, while the patient sits in relative comfort. A brain tumor can be removed while minimizing collateral damage.
At the other end of the scale, what is the most sensitive part of the body? If you're a man, you can answer this one right away - those delicate testicles. The slightest tap can cause you to recoil in pain. Why should this be? If your hand is injured, you can survive, and have children, and raise them to adulthood, but if your testicles are damaged, that is the end of your genetic line. Evolution is going to place a premium on these organs. If they are profoundly sensitive, a boy will learn to protect them by the age of 4. This young man is more likely to have and raise children, and so it goes, not just in humans, but for all mammals.
Well somehow I managed to segue from brain to balls, so let me close with a quote from Robin Williams.
"The problem is, God gave man a brain and a penis, and only enough blood to run one at a time."