Carl Zimmer explains why the brain has two halves. His closing paragraphs are excellent:
No matter how lateralized the brain can get, though, the two sides still work together. The pop psychology notion of a left brain and a right brain doesn’t capture their intimate working relationship. The left hemisphere specializes in picking out the sounds that form words and working out the syntax of the words, for example, but it does not have a monopoly on language processing. The right hemisphere is actually more sensitive to the emotional features of language, tuning in to the slow rhythms of speech that carry intonation and stress.
Neuroscientists know that the hemispheres work together and that they do so by communicating through the corpus callosum. But exactly how the hemispheres cooperate is not so clear. Perhaps paired regions take turns being dominant. That is known to happen in some animals. For instance, dolphins use this strategy to sleep and swim at the same time: One hemisphere remains active for hours, then fades while the other takes over. Bird brains switch as well. In order to sing, a songbird makes the two sides of its lungs open and close. The two hemispheres of the bird’s brain take turns controlling the song, each dominating for a hundredth of a second.
The intimate cooperation between the two hemispheres makes it all the more remarkable that a person can survive with just onea sign that the brain is far more malleable than we once thought. After a hemisphere is forced to manage on its own, it can rewire itself to handle all the tasks of a full brain. In fact, two hemispheres can cause more trouble than one if they cannot talk clearly to each other. Neuro scientists have linked some mental disorders, including dyslexia and Alzheimer’s, with a breakdown in left-right communication.
The two sides of the brain may be a legacy that we inherited from our wormlike ancestors. But their delicate balance of symmetry and specialization is now woven into the very essence of human nature.