What Is the Purpose of the Brain?

Why are our brains located in our heads? Wouldn’t they be safer if they were deep in our chest, similar to the location of our hearts? Brains, regardless of how small or simple, have evolved at the best possible location to perform two basic functions: survival and the procreation of the species. The back half of the brain processes the information gathered by your sensory systems, and then sends this information to the front half of the brain to consider its options and instruct the muscles to contract in order to obtain food, find receptive mates for reproduction or avoid predators.

Brains want us to eat

For the past 600 million years, since they first appeared in a single-celled common ancestor of the human, brains have always been located at the front end of the feeding “tube”, which in humans and many other organisms is the tubular system (the alimentary canal) that extends from the mouth to the anus. Worms, fish, birds, reptiles, dogs, and you: all simple feeding tubes. Your brain makes it possible for you to find food by sight, sound, and smell and then to organize your behavior so that the front end of your feeding tube can get close enough to taste the food and check it for beneficial or potentially harmful contents before you ingest it. Once the food is in your feeding tube, it is absorbed and becomes available to the cells of your body.

Your entire feeding tube and associated organs, also known as the gastrointestinal system, use nearly 70% of the energy you consume just to make the remaining 30% available to the rest of your body. Your brain uses about 25% of the available consumed energy, and your other organs that allow you to reproduce and move around your environment (including your muscles and bones) utilize about 15%. As you can see, very little energy is left over for other tasks in the body. These percentages give you some idea of the priorities—thinking, sex, and mobility—that billions of years of evolution have set for your body to achieve.

Our brain uses most of its energy to organize our behavior to socialize with others in our species in order to find a mate with whom to reproduce. That’s our inherent biological imperative, regardless of whether or not everyone responds to it. You know one manifestation of this imperative as dating, and it requires a very, very large and complex brain to pull this off successfully. Meanwhile, your brain has evolved some interesting neurotransmitter chemicals that allow you to enjoy dating—dopamine and an opium-like chemical. Both play a critical role in rewarding your brain—and, therefore, you—for consuming high-calorie food, such as the quintessential dating meal of cheeseburgers and fries at the local diner, and for having sex, often the quintessential dating result. Eating and having sex are obviously excellent ideas if your purpose is to maintain and propagate your species.

Brains want us to be fearful

Feelings of fear are probably the most important survival feature our brain has ever evolved. How does your brain decide to induce fear? This critical task is processed by a small almond-shaped structure, the amygdala, which lies deep within the bottom of the brain, not far from your ears. The amygdala receives information from many brain regions, your internal organs, and external sensory systems, such as your eyes and ears. The amygdala integrates this information with various internal drives, such as whether you are hungry or thirsty, or in pain; it then assigns a level of emotional significance to whatever is going on.

For example, when the amygdala becomes aware that you are alone and hearing unfamiliar sounds in the dark, it initiates a fear response, such as panic or anxiety. It then activates the appropriate body systems, the release of hormones, and specific behaviors to respond to the (real or imagined) threat. The amygdala also is activated by sensory stimuli that seem ambiguous or unfamiliar to us, such as unfamiliar sounds or people. In response to ambiguous or unfamiliar stimuli, we become vigilant and pay closer attention to what is happening in our immediate environment.

Almost without fail, and regardless of the nature of the information gathered by your vigilant brain, the amygdala usually comes to the same conclusion, be afraid and prepare to fight. If everything is assumed to be dangerous until proven otherwise, you are much more likely to survive the novel experience and pass on your be-fearful-first genes. Thus, humans often fear everything that is unfamiliar or not-like-me: we may fear people who look or dress differently, unfamiliar places, unfamiliar odors, things that go bump in the night, people who stare at us for too long, heights, enclosed small spaces, dark alleys, etc. You get the idea.

Brains evolved to find mates, find food and avoid becoming food; the emotion of fear helps to make that possible.