Modified: 2020-06-10
Some definitions are in order at this point. The central nervous system (CNS) includes the brain and the spinal cord.
The peripheral nervous system is everything else. The autonomic nervous system is a part of the peripheral nervous system, and it is the part that regulates functions that run without our conscious control, such as breathing, heart rate, and digestion. The endocrine system is outside of the nervous system. It regulates and integrates functions like physical growth and menstruation in human females. Its functions are indirectly controlled by the nervous system.
Now let us look at the brain more closely. If you look at the brains of a fish, a frog, a cat, and a human you will note the following. All but the human brain are laid out horizontally. The human brain is laid out at nearly a right angle. Why? The answer is that when humans evolved into creatures that walked upright, their heads rotated forward and the brain had to follow. Also, as we go from fish to frog to cat to human, we see more bumps or convolutions on the brain. Why? The answer in this case is that as the brain got bigger and as the head stayed about the same size, the brain had to fold up to fit inside the constant size of the head. Imagine a two-dimensional analog, a coastline like Louisiana's. If you measure the straight line distance from Mississippi to Texas and then compare it to the distance you get by actually get by sailing the entire coastline, you find that the latter is much longer. The same is true in three dimensions in the brain. You may ask why did we not just evolve a bigger head? The answer to that question tells us something very fundamental about evolution.
Evolutionary changes are constrained by physical and temporal factors. In this example, the constraint is the size of the human female's birth canal. As any woman who has had a baby can probably testify, the baby's head is big enough as is. Further, the bones of the baby's head deform during delivery to make it smaller, in order to aid its passage. So, while making the head bigger might work in principle, that solution does not work in this case. So, the evolutionary solution was the convoluted brain.
If you were to take the neocortex, or outer layer of the brain, and spread it out on a wall, it would cover about a 2 X 3 foot area, like a typical poster. So, the evolutionary solution does cram quite a bit of brain tissue into our head.
Finally, what about the old saw that we only use part of our brain? It is simply not true. Evolution does not work that way. Structures and behaviors evolve because they must in order to ensure reproductive success. So, the brain evolved its large size because of some evolutionary pressure or pressures. There is some debate as to whether or not language or vision was more important in this regard. Today, many people may not have to use all of the power of their brains because they live and operate in environments made simpler by civilization. Remember that humans have only lived in civilized environments for a very small portion of our species' existence. When we get hungry we go to the supermarket; we do not have to snare a rabbit, kill a mastodon, or catch a fish. When someone threatens to kill us we call the police; we do not have to kill the person first. So, life may be simpler now than it was in prehistoric times, and some of us may not use all of the brain power we have available to us. But, that brain power evolved because we needed it, and some environments require all of it for us to survive and reproduce.