The Hidden Implications Of Brain Evolution

Our brains control almost everything we do but have you ever wondered how the brain is constructed, what evolutionary forces shaped it?

People sometimes ask whether a decision is governed by the head (brain) or the heart (emotion) but as we will see this is probably a false distinction.

It might be better to think of our decisions as emerging from a soup of neural connections blending basic instinctive fight or flight mechanisms with our higher cognitive activity.

Can we be certain that our decisions are governed by a rational evaluation of all of the evidence? Or is it more likely that we are frequently flying along, swerving this way and that based on what our gut instinct advises?



The Hungry Brain

The brain of the average human being weighs between 1.2kg and 1.4kg about 2% of total weight. This complex organ controls most of our bodily functions, interprets the world around us, stores memories, creates interpretation and meaning, dreams, thinks, feels and establish a personality.

It is a hungry organ and feeds greedily on blood glucose, but will if necessary get by for a time on ketones or lactates.

It consumes:

  • 15% of the heart’s output
  • 20% of total body oxygen consumption
  • 25% of total body glucose utilisation.

The brain of a Homo Sapiens today is 40% larger than the first humans, Homo Habilis but has been shrinking over the last 28,000 years. The brain of Homo sapiens neanderthalensis, the branch of humanity we call neanderthal, had a brain 11% larger than our own.

This phenomenon of reducing brain size, the result of opposing evolutionary forces, led scientists to develop the notion:

As large as you need and as small as you can. 

In other words, our brain is small but perfectly formed.

Our Matryoshka Doll 3 Brain System

The brain we have today is actually a composite made from three separate systems.

Reptile Brain

The oldest brain lies deepest inside the structure and is sometimes referred to as the housekeeping brain. It is similar in structure to the brains of reptiles and birds and so is also known as the “R-Complex” where “R” stands for reptilian.

The reptilian brain is responsible for controlling our basic functions: hunger, temperature control, fight-or-flight fear responses, defending territory, keeping safe. The reptilian brain is our most primitive brain.

Limbic Brain

As animals became more complex additional structures began to surround the old reptilian formation. Eventually, the reptilian brain was wholly surrounded, encased in a shell or “girdle” of additional brain structures.

The Latin for arc or girdle is “limbus” and so this structure is known as the “limbic system”. We share this brain type with other older mammals like dogs, cats, cows and horses. The limbic system is responsible for emotions like rage, fear, disgust and surprise.

Neocortex Brain

Finally, as mammals evolved into even more sophisticated animals like chimpanzees, dolphins and humans, the brain developed a top layer called the cortex or neocortex. This system is responsible for all of the brain’s higher functions like planning, and complex social interactions including language. Humans possess a composite brain therefore which is made up of three individual systems which have evolved from primitive to complex.

Interestingly, while there is assumed to be an association between the relative size of the neocortex and intelligence, scientists discovered in 2015 that a species of whale, the long-finned pilot whale has the most neocortical neurons of any mammal so far studied.

Implications For You And Me

Our brains evolved sequentially, a bit like a coral reef. One layer was deposited on another and because the direction of evolution was “upward”, it means that the connections made between each brain system also work most effectively in an upward direction. Our neocortex is able to receive a signal from both of the older systems extremely rapidly. Unfortunately, the connections are relatively weak heading downward from neocortex through the lambic brain and onto the reptile brain.

Why does this matter?

It matters because our conscious, rational-self receives signals from our more primitive brains very fast. These arrive perhaps as a strong “gut reaction” or emotional response. Our neocortex then attempts to “explain” these signals to use. Very often this is after we have already reacted instinctively.

Sadly, our neocortex is also very adept at post hoc rationalisation: finding explanations and justifications for our reactions. We might believe we are responding with cool, rational precision when in fact we are merely providing cover for our instinctive response.

Disgust and Fear

Ever wonder why you don’t like spiders, or snakes or the taste of food that has gone off? Fear (snakes, spiders) and disgust (tainted food) are all primitive reactions that have a sound evolutionary foundation. However, disgust can spill over into areas where it might not belong. Demonising sections of the population and “dehumanizing” them so it is permissible to feel disgusted at the “contamination” caused by these groups is a long-standing historical phenomenon.

The question is, how can we prevent these impulses from driving us in directions we don’t really want to go?

Here are two strategies for dealing with the implications of this brain structure.

  1. Develop a mindfulness practice: One of the key benefits of a mindfulness practice is that it eventually becomes second nature to observe the train of thought as it passes through your mind. By developing this impartial and compassionate observation of the mind’s activities, we can create a moment’s pause between the thought and the response. Useful if the “thought” is actually a rationalisation for an emotional reaction to something.
  2. Keep a journal: Journaling regularly has many benefits. Recording what happens during the day and in particular what went well and what you can improve upon allows us to develop a more reflective attitude. Over time we can recognise patterns of behaviour we are prone to and this can help us to avoid making responses we would rather not.


How do you manage your instinctive reaction to events?