MacLean’s triune brain theory: what is it and what does it propose?

Is it true that the human being has three brains differentiated by their degree of evolution?

The MacLean's triune brain theory proposes that the human species, throughout evolution, has been changing in terms of its brain morphology, but instead of seeing it as a global and unified process of change, it describes it as a process in which new and independent brain structures emerged, each operating with their own individual characteristics.

In this article we will review what exactly Paul MacLean's triune brain theory is, and see why it is not valid for the scientific community.

What is MacLean's triune brain theory?

According to MacLean, in his book The Triune Brain in Evolution, our current brain is the sum of a process of superimposition of layers, which appeared consecutively over the millennia and were placed one on top of the other, without the previous ones ceasing to exist.These layers appeared consecutively with the passing of the millennia and were placed one on top of the other, but without the previous ones ceasing to exist.

Thus, MacLean's theory of the triune brain affirms that each of these structures has its own individual functioning logic, very different from that of the other layers, taking into account that the upper layers are the most evolved.

Let us see below what these layers are according to the triune brain theory.

1. The reptilian brain

This would be the first of the three layers that exist in our brain, and represents the most basic instincts. It represents the most basic instincts of the human species; hunger, sexual reproduction, sleep, sleep, sleep, sleep, sleep, sleep, sleep, sleep, sleep and sleep.hunger, sexual reproduction, sleep, survival instinct and fight.

This brain would be composed of the first structures to appear, which are the basal ganglia, the brainstem and the cerebellum. All these structures are located in the lower part of our encephalon..

The followers of this theory affirm that the reptilian brain contains a great amount of information in its individual memory; this information would be conformed by ancestral rites and esoteric beliefs without any type of scientific foundation.

2. The limbic system

This second structure, or second brain, according to the theory, would be in charge of all the sensations that we experience when we carry out any activity.. For example, when we eat something we like very much, when we have sex with a person we like, or when we delight in a beautiful landscape.

These emotions are in charge of the limbic system, which supposedly according to the triune brain theory is the next structure of the nervous system to evolve after the appearance of the reptilian brain.

This layer is composed of the cerebral amygdala, the septum, the hypothalamus, the cingulate cortex, and the hippocampus.

3. The neocortex

This layer is the most recent in human evolution and is also known as the modern brain. It is unique to mammals.

The neocortex is in charge of everything related to abstract thinking, logical and rational thinking, apart from communicative processes.Apart from the complex communicative processes that we use to communicate in modern society.

It is composed of the cerebral cortex, which is made up of gray matter, where there are a large number of neurons that are in a constant process of connection with each other.

Considerations about the theory

Taking into account what we have seen so far, it can be said that MacLean's triune brain theory assumes that our present brain is nothing more than a summation process between individual layers that appeared in our species making us more and more rational..

Each of these layers, as we have already seen, has its own characteristics; even if the functions of one have to do with the functions of another structure, they operate autonomously. That is to say that no layer has control over the other, but they could, for example, make the subject aware of his desires and thus make him channel them appropriately.

This theory has not been well received in the scientific community, and most specialists in the field of neuroscience have expressed their disapproval of this postulate. Let's see why.

What does the science say?

For neuroscience experts, the theory of the triune brain theory has too many inconsistencies in its approach to be considered valid..

It is now known that the specific functions that this theory grants to the aforementioned structures do not belong to the most evolved vertebrates, that is to say, that similar behaviors also occur in other species.

For example, birds, without needing to possess the limbic system, have a great instinct of protection towards their young, which is a characteristic attributed to the second brain (limbic system) according to MacLean's theory.

In addition, there are intelligent mollusks such as octopuses, whose brains have nothing to do with those of vertebrates, capable of social and rational behavior.

More recent discoveries also refute the theory that the neocortex emerged as the last stage in the evolution of the modern mammal. It is now known that the earliest evidence of gray matter in the higher brain areas were found in primitive mammals..

In other words, these structures did not appear in an ascending order as the author of the triune brain theory suggests, but rather they all existed in the same brain, which evolved in a general way, and not part by part.

Moreover, the way in which evolution works does not consist of an accumulation of traits that do not influence each other. In other words, the changes that occur due to mutations that are reflected in new features in the structure of the brain do not occur individually, but in interaction with the rest, so that a part that previously specialized in certain mental processes may become responsible for others if another new set of nerve cells appears.

Bibliographical references:

• Gardner, R.; Cory, G. A. (2002). The evolutionary neuroethology of Paul MacLean: convergences and frontiers. New York: Praeger.
• Lambert, K. G. (2003). The life and career of Paul MacLean: A journey toward neurobiological and social harmony. Physiology and Behavior. 79 (3). Elsevier: pp. 343 - 349.
• Velásquez Burgos, B. M., Calle, M. G., & Remolina De Cleves, N. (2006). Neuroscientific theories of learning and their implication in the knowledge contruction of university students. Tabula Rasa, (5): pp. 229 - 245.