Reptilian brain: do we really have this ancestral structure?

The idea that humans have reptilian brains is very common. It would be a vestige that our ancestors left us in cold blood, scales, and crawling on the ground, in front of few friends.

Lizards, snakes, and turtles were considered very primitive animals, which is why the oldest, most instinctive, and simplest part of our brain has been called the reptilian brain.

Is it true that we have a reptilian brain? Is it true that reptiles are as basic as they have been painted? We will see below.

    Do we have a reptilian brain?

    Outside of the realm of academic psychology and neuroscience, the idea that humans, as well as other mammals, have something like a reptilian brain is very popular and widespread. No, it’s not that this conspiracy theory that there are lizard-shaped aliens among us is true, but that the idea of ​​the reptilian brain says that in our species we still have structures that our sauropsid ancestors had, areas of the brain responsible for very basic, primitive and instinctive behavior.

    We owe this idea to the doctor and neuroscientist Paul MacLean (1913-2007) who proposed his theory of the trio or trio brain, a model to explain the organization of the human brain. His theory attempted to make sense of the existence of contradictory or at least alternative systems in our behavior which, through evolution, had been organized and coordinated in order to harmonize within the same organism.

    This neuroscientist introduced his model in the 1960s, although he developed it to the maximum in his book “The Triune Brain in Evolution” (1990). MacLean believed that in our skull there was not one brain, but three, three brain structures that each acted as an independent but interconnected biological computer that would have arisen throughout evolution by forming brain layers. Each of these brains would have its own intelligence, subjectivity, sense of time and also memory.

    The three components of the Trinitarian brain are said to be the reptilian complex, the limbic system, and the neocortex, which have appeared in the same order throughout evolutionary history. The most basic animal species of reptiles would only have the reptilian complex, while as they moved up the mammalian class the limbic system would be added and on reaching the primates the neocortex would be added, making the brains of humans and primates much more complex than that of reptiles.

    1. The reptilian brain

    The reptilian brain or R complex is essentially made up of the central gray nuclei, the brainstem and the cerebellum., structures considered to be the most elementary of the model. The reason this component is known as the “reptilian brain” stems from the fact that the brain of reptiles is dominated by the brainstem and cerebellum.

    According to proponents of the reptilian brain idea, this structure would be responsible for controlling instinctive behaviors and would focus on behaviors fundamental to survival., including aggression, sense of territoriality, domination and rituals.

    The reptilian brain is overflowing with ancestral memories and is responsible for autonomic or visceral functions such as breathing, heartbeat or vasomotor activity. It would also be involved in balance and muscle movement, taking care of direct and thoughtful responses.

    2. The limbic system

    MacLean first introduced the term “limbic system” in a 1952 study, a term that will become more common over time. The key elements of this second layer in the Trinitarian model would be the septum, amygdala, hypothalamus, cingulate cortex, and hippocampus..

    It is also known as the paleomammal brain to be considered the oldest part of the brain of mammals. This structure is said to be related to anything that involves a more complex emotional response but also related to survival, also involved in memory and social relationships, although it needs to interact with the neocortex to process emotions in their entirety. .

    The limbic system detects and avoids unpleasant sensations (for example, pain) and searches for pleasurable sensations (for example, pleasure), acquiring great importance in the motivation and emotions that we feel when we satisfy physiological functions such as feeding or reproduction. It would also be at the origin of parental behavior.

    3. The neocortex

    The neocortex or cerebral cortex is a structure only found in mammals, including humans, and it is considered to be the most modern mammalian brain proper. According to the model, the neocortex is the most characteristic part of the brain of the most evolved mammals, especially primates.

    This structure is responsible for the performance of higher brain functions, encompassing cognition, sophisticated and precise motor control in addition to sensory perception. The evolution of this structure in mammals is considered a key breakthrough that has enabled humans to have cognitive functions such as language, reasoning, advanced thinking, and intelligence.

    The Trinitarian model considers the human brain to be the end result of a process of stratification of these three structures., that is, the reptilian brain, limbic and neocortex have been added throughout the history of evolution to form an increasingly sophisticated brain and result in that of our species. The reptilian would be the deepest, the most primitive and the most instinctive; the limbic would be the emotional component, involved in the motivation and care of the offspring; and the neocortex would be the rational and modern brain, allowing us to use higher intellectual capacities.

      Is this model true?

      As we mentioned, the idea of ​​the reptilian brain is prevalent in popular culture, being taken up by many people. However, the truth is that the Trinitarian model is considered a myth and while some aspects of the components mentioned in it make sense, the truth is that no study has succeeded in clearly separating emotions and rationality, other than the line separating our survival instincts, emotions and the cognitive aspects are not very well defined.

      The MacLean model has been completely discarded since the 2000s. The idea that evolution has added new and more complex structures because it is contrary to what evolutionary scientists know about the evolutionary process. In fact, what has happened throughout evolution is that the brain has reorganized the existing circuits, giving them more complexity and new functions. The appearance of new brain structures does not suddenly happen as if a new lobe of nothingness is growing inside us, what has happened is that the old structures have changed.

      The idea of ​​the Trinitarian brain, in particular the reptilian brain, is rejected by most current neuroscientists, especially among those with notions of comparative neuroanatomy. As proof, we have the case of the central gray nuclei, structures of the reptilian brain that can be found in mammals such as the lion. In this feline, the lymph nodes are proportionately larger than those found in the telencephalon of most reptiles..

      Another aspect that dismantles the theory of the reptilian brain is that it is found not only in reptiles, but also in groups of vertebrates considered before the appearance of reptiles. Let us quote for example the fishes and the amphibians, animals in which one also finds the telencephalon, made up here of very developed olfactory bulbs and of a hindbrain.

      Then we have the case of the structures of the limbic system. According to the MacLean model, this structure is properly mammalian but ancient, being in all mammals but not in reptiles, explaining why mammals are warmer and more sociable while reptiles are cooler or emotionless. This idea is wrong, as other vertebrates have shown affection and interest in their offspring, as is the case with certain species of fish, birds, and reptiles, such as crocodiles.

      And finally we talk about the case of the neocortex. While it is true that other vertebrates do not have this six-layered stratification-like structure, that does not mean that they do not have homologous regions, something like their own functional crusts. For example, the areas that make up the telencephalon of birds are connected to each other and perform similar functions to those reserved for the neorch in the Trinitarian model.

      Proof of this is that even in popular culture it is known that there are very intelligent birds like crows, able to use sticks to remove insects from holes, to put stones in a test tube with water because raising the level and being able to drink it or even discriminate between different colors, associating some with food and others with no. That is, they exhibit memory, perception, learning ability and discrimination of stimuli.

      In short, the idea of ​​the reptilian brain is quite outdated. Neither are reptiles as basic and simple as they are painted, nor do we have a purely reptilian brain that functions primitively.. Certainly, we have motivations associated with instinct, but more than being due to the existence of three semi-independent brains, it is because we have a whole evolutionary history that has retained functions considered appropriate for the survival.

      In addition, the notoriety that has been given to reptiles, considering them cold, unintelligent and devoid of emotions completely dismantles the model. There are very intelligent and fast reptiles that exhibit reproductive and affectionate behaviors. These animals, which are the only ones we can say with absolute certainty that have reptilian brains, aren’t as basic as MacLean originally called them. It is clear that the Trinitarian brain model, while popular, is still a myth and that we humans have human brains.

      Bibliographical references

      • Smith, CU (2010) The trigonal brain in antiquity: Plato, Aristotle, Erasistratus. J Hist Neurosci 19 (1): pp. 1 – 14.
      • Alonso, JR (2017). The myth of the reptilian brain. Neurosciences: José Ramón Alonso’s blog.
      • MacLean, PD, 1952. Some psychiatric implications of physiological studies on the frontotemporal part of the limbic system (visceral brain). Electroencephalon. Clin. Neurophysiol, 4: p. 407 – 418.

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