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Bruce Hood Ph.D.

Why Do We Need a Brain?

It turns out that we evolved brains for acting.

Whenever I speak to the general public about the brain, I usually begin with a simple question, “Why do humans need a brain?” At first, this seems like a silly question with an obvious answer. “You need a brain to stay alive,” is the most common response and indeed this is true. You would be dead without your brain, which is why “brain dead” is usually the legal definition of death. Someone is brain dead when there is a lack of reflexive responses controlled by the brain stem, the most fundamental structures at the core of the brain that control the vital functions.

However, keeping you alive is not the sole responsibility of the brain. The same could also be said of your other major organs including the heart, liver and lungs. While it is true that these can all be successfully transplanted whereas a brain transplant is neither possible nor desirable (the topic of forthcoming blogs), it is not the case that to be alive depends on having a brain. There are many animals that are alive that do not have brains. They may have simple nervous systems but they do not have brains as such. There are even some animals that start off with a brain that they later discard. The classic example is the sea squirt that begins life as a tadpole-like creature, swimming around the ocean in search of a suitable rock upon which to attach. It has a simple nervous system to coordinate movements and even a rudimentary eye spot to “see,” but when it finally attaches to the rock, it no longer needs to move around and so digests its own nervous system.

That’s the answer to why we need a brain. The primary purpose of a brain is to move around our environment in a meaningful way. In fact, one could even argue that most of the brain is dedicated towards actions. If we consider that the basic building block of the brain is the neuron, then it comes as a surprise to most to find out that the majority of neurons are not in the association cortex where “higher” thought processes are generated. Of the estimated 86-100 billion, around 80 percent are to be found in the cerebellum, the bulbous structure at the base of the brain at the back that controls our movements.

People often assume that animals with bigger brains are more intelligent because they have more brain cells. It is true to some extent but it is not the number of neurons that determine intelligence but rather the number of connections between the neurons. So the ‘association cortex’ is just that, the 3mm thick outer layer of the brain that stores information for processing through the vastly interconnected networks of associated activity. And this of course, is the secret to the processing capacity of the brain. Each neuron has up to 10,000 connections, which means that the number of potential different patterns of neural activity is virtually infinite. With just 500 neurons you have the potential for more different patterns than the estimated number of atoms in the observable universe!

Our big brains are mostly made up of connections between neurons, around 160,000 km—enough in an individual human brain to circumnavigate the equator four times. The purpose of all these connections is to store patterns of neural activity that are the basis for sensations, perceptions, cognitions and other important brain functions we possess. However, we must remember that these abilities all serve the primary function of action. Without the ability to act, we would be completely at the mercy of the environment, which is why we evolved a brain to act on the world. After all, Mother Nature doesn’t select for a good idea, it’s action that speaks louder than words.


About the Author

Bruce Hood, Ph.D., is the Chair in Developmental Psychology at the University of Bristol.