The American physician and neuroscientist Paul MacLean, who made outstanding contributions in both the fields of psychology and psychiatry, proposed an evolutionary theory that fit with the idea of a brain in continuous evolution: the theory of the three brains, with a layer, the more primitive, destined to survive; another to feel, and a last, the most evolved and modern, oriented to think. It is what is known as the triune brain or triune brain. It is likely that at some point you have read or heard about this theory. We break it down first.
MacLean’s Three Brains
Reptilian brain: The deepest layer, the lizard brain, would be the one that, supposedly, we humans inherit from the ancient reptiles, and it houses our most basic instincts, those of survival.
Limbic brain: In a stage or intermediate layer we would find the limbic system that would contain ancient regions related to emotions, inherited from prehistoric mammals.
Neocortex: The most superficial layer would be that of the neocortex (or new cortex), which would represent the human rational brain, that which gives us the source of rational thought as an exclusively human quality.
Is this triune brain theory true?
Is it true that the human being has three different brains according to the degree of evolution? The answer is easy and forceful: absolutely no. This is a completely wrong idea and, unfortunately, it is widespread in many fields of science. It has many inconsistencies in its approach, so, no, we cannot consider it as valid.
Although MacLean determined this creative solution to our intelligent brain, it has no scientific backing . However, his evolutionary story about the brain did catch on in certain sectors of the scientific community because they were plausible with Charles Darwin’s ideas on the evolution of human cognition. Even the science popularizer Carl Sagan himself talks about these three brains in his popular 1977 book “The Dragons of Eden” which, remember, won a Pulitzer Prize. But as early as the 1990s, scientists had completely rejected this three-layer brain theory.
But our brain is not a layered piece of land that can be analyzed geologically, so to speak. As Lisa Feldman Barrett explains in “Seven and a half lessons about the brain” (Ed. Paidós, 2021), our brain got bigger over evolutionary time and, therefore, its internal structure was also reorganized. So we neither have a primitive inner lizard, nor the emotional brain of a beast, nor a new cortex that made us intelligent.
And why is this theory still popular?
While it is true that these MacLean applications have been discarded decades ago, their popularity, especially in the networks, is due to the fact that this evolutionary story is congruent, even comforting for us. It is a way of making us understand that our reasoning has overcome our most primitive and animal part and, therefore, we have conquered the planet and become the most intelligent and developed species that exists. So the next time you are told about your reptilian brain, remember that it is just a romantic story from the past.
