Brain evolution is not well understood. What we do know, however, is that the brain is more complex than any other known structure in the universe. Weighing in around 2.9 pounds, on average, this spongy mass contains over 300 distinct brain regions. Each region has a specific set of functions, but works together with other brain areas as a network to enable more advanced functions.
On a much smaller scale, your brain is made from approximately 160 billion cells, about half of which are nerve cells that transmit messages. Between 86 billion to 17.2 trillion of these messages are sent every second, communicating information relating to memory, emotions, language, sensory experience, thinking, control, and movement.
But how did the brain become to be this way? And what changes happened to our brain over the course of human evolution?
Key Takeaways
- Brain evolution is not well understood. What we do know is that it is the most complex organ in the human body.
- One simplified model for understanding how the human brain evolved is the Triune Brain Model. It states that the reptilian brain, limbic system, and neocortex were added over the course of evolution.
The Triune Brain Model and Brain Evolution
One basic model for understanding how the brain evolved is the Triune Brain Model. Formulated by American neuroscientists Paul MacLean, the Triune Brain Model divides the brain into three distinct areas: the reptilian brain, limbic system, and neocortex.1 The Triune Brain Model states that each of these three structures were sequentially added to the forebrain over the course of evolution—that is, the reptilian brain came first, then the limbic brain, and lastly, the neocortex.2
The innermost structures form what is called the reptilian brain, which first appeared in reptiles like snakes, lizards, crocodiles, turtles, and tortoises. This region is the most primitive area and consists of the brainstem, the posterior stalk-like part of the brain that connects to the spinal cord, and the cerebellum, which coordinates muscle movement, maintains posture, and balance. The overall function of these two areas is to dictate basic impulses.
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The limbic system bridges these basic survival impulses and processes them to promote emotions or behaviors. This area of the brain has been linked to feelings of motivation and reward, learning, memory, hunger, and thirst.
It is also known to be responsible for the fight or flight response, an automatic reaction to a potentially life-threatening situation. If you come across a potential threat, such as a predator, and you fear that a fight lies ahead, the limbic brain will light up, and nerve cells will send messages to speed up heart rate and breathing. Should running be the better option, you also have plenty of oxygen and increased blood flow. And in case the predator did not see you, your brain will assist you to keep tense, rigid muscles.
The neocortex is the newest addition to the brain and is involved in higher functions, such as sensory perception, spatial reasoning, conscious thought, and human language. It is a structure with great processing power and is responsible for acquiring new skills, abilities, personalities, and memories.
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The Human Brain Evolved to Survive
The functions of the reptilian brain, limbic system, and neocortex serve to keep you alive. As you take in information from your internal and external environment during a dangerous situation, the modern brain can make split-second decisions and signal behaviors to help you survive.
The brain evolved to be this way because for the majority of human existence, staying alive was difficult. There were no cures for diseases and illnesses, you had to hunt and gather your own food, and enemy tribes and predators could attack at any time. To increase the likelihood of producing offspring, the brain learned survival functions.
Today, many of us don’t get into situations where these types of responses are necessary—with the exception of functions related to physiological needs like water, food, shelter, and sleep, of course. The fight or flight response is useful when we come across a predator in the wilderness, but it not needed most parts of the Western world.
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Life is safer than ever before, but what is interesting is that many people still find them in a survival state throughout the day. When we watch TV and come across a commercial that invokes fear, the brain thinks we’re in danger, the fear centers of the brain light up, and our body prepares for a situation ahead (Marketers are also skilled at making you feel nostalgic. You can find out more about that here).
When we find ourselves in survival mode, is difficult to gain back control. That is, when the reptilian and limbic parts of the brain are fully engaged, the rationality provided by the neocortex will not have a significant impact on our behavior. This is because the more primitive parts of the brain have had more time to evolve, and are, therefore, stronger and more advanced than the newest additions of the brain.
References
- MacLean, Paul D. The Triune Brain in Evolution: Role in Paleocerebral Functions. Kluwer Boston, Inc., 1990.
- This article aims to provide you with a general overview of brain evolution, but the Triune Brain Model has been subject to criticism over the years. If you want to learn more about that, I suggest you read this paper.