The Evolution of Rationality

I am a rational being. I am, in other words, capable of reasoning my way to conclusions. I am not perfectly rational, though, and can sometimes be quite unreasonable—or so I am told. This is evidence that I am only human. Chances are that you are, too.

Scientists have come up with explanations for human irrationality. In this post, I will describe the explanation offered by evolutionary biologists, and in subsequent posts, I will describe those offered by neuroscientists and cognitive psychologists.

Ask an evolutionary biologist how we gained the ability to reason, and they will point to evolutionary processes. Before going any further, though, let me say a few words about these processes. They are not guided by an agent. Furthermore, evolution has no goals for us. It is simply an ongoing process of trial and error. Organisms that survive and reproduce pass along their genes; those that don’t, exit the gene pool. This is how the living things around us acquired their anatomy, physiology, and (to a considerable extent) behavioral traits.

Realize that evolution is “backward looking.” Why do we humans have the traits we do? Because having them benefitted our distant ancestors in the environment in which they found themselves. That environment, of course, was radically different than ours. Half a million years ago, for example, our ancestors would have lived on the savannas of Africa. They wouldn’t have had cell phones, social media, and search engines. In fact, they would have counted themselves lucky to have a hand axe.

Life first arose on earth about 3.5 billion years ago. It took the form of microscopic, single-celled organisms. Create your hyper-extended family tree, and you will ultimately arrive at such organisms. Which is pretty amazing, don’t you think?

Subsequently, some of these microbes acquired the ability to sense and respond to their environment. A billion or so years later, multicellular organisms appeared, and by 600 million years ago, some of them were able to reflexively move in response to stimuli, but they lacked reasoning ability. Two-hundred million years ago, mammals appeared. They had brains with primitive reasoning ability. Their brains also had a limbic system that allowed them to experience emotions. The emotions in question would have motivated them to care for their offspring, which in turn would have increased their chances of passing on their genes.

It was only in the last few million years that some of these mammals gained the ability to engage in complex reasoning. This required the development of a prefrontal cortex. The resulting increase in brain size came at a price, though. The brain is a power-hungry organ. Although it represents only 2 percent of your body’s mass, it consumes 20–25 percent of your body’s energy, which is part of the reason that thinking is difficult.

This investment in reasoning ability was apparently worth it, though. By allowing our ancestors to better understand the world around them, the ability to reason allowed them to accurately predict and take steps to benefit from future events. Having this ability therefore increased their chances of surviving and reproducing.

The key thing to remember about this change in brain structure is that the prefrontal cortex didn’t replace the limbic system; it simply took up residence nearby. Your brain is therefore home to both your limbic system and your prefrontal cortex, and both are fully functional. As a result, you are a hybrid creature, with both a rational component and an emotional component. Yes, you have reasoning ability, but you also experience emotions. Not only that, but your rational and emotional components can be at cross purposes.

You are doubtless aware of what I am talking about. You might find yourself wanting a second piece of pie, despite your resolution to forgo dessert until you have lost some weight. Alternatively, you might discover that you have developed a crush on someone. When the object of your crush is nearby, they will captivate your attention, and when they are not present, you might find that you can’t stop thinking about them. Your mind, for a time, will have lost its mind. Crushes are one thing, but when you fall in love, the trajectory of your life will be radically altered. And yet, you did not choose to fall in love; it just happened.

As a result of our hybrid nature, we humans are in a predicament: We must spend our lives dealing with conflicting desires, with our head being the source of some of them and our heart being the source of others. Don’t get me wrong, though. As predicaments go, it is a wonderful predicament to be in. I would not want to live a life that was devoid of emotions. I would not, in other words, want to spend my life in a depressed state. Nor would I want to live a life devoid of reason. Such a life would likely be—to echo philosopher Thomas Hobbes—nasty, brutish, and short.

This is a Substack about thinking more and better. By doing this, you can gain an understanding of the conflicts that arise between your intellect and your emotions, which in turn can help you deal with those conflicts. Find the right balance, and you can flourish in this, the one life you have to live.