The reptilian brain is a myth that should not be taken seriously and yet is referred to by many writers and is even seen in educational sites for children. It is the idea that we have three brains: a reptilian one, the paleomammalian one and the mammalian one. The story goes that these were acquired one after another during evolution. The details differ with the writer. But it is all a myth based on an idea from the ’70s of Paul MacLean which he republished in 1990. Over the years in has been popularized by Sagan and Koestler among others.
So we get self-help like this: “Because until recently in our history, we had been conditioned to operate and function mainly out of the reptilian brain. We had been operating/ manifesting out of the ‘survival’ mode section of the brain. Once you can understand this concerted mental oppression, you can begin to re-train your mind (free yourself from constant reptilian brain generated reaction) and re-set your innate human gift of creative power.” And information for children like this: “Lower animals, such as fish, amphibians, reptiles and birds, don’t do much “thinking,” but instead concern themselves with the everyday business of gathering food, eating, drinking, sleeping, reproducing and defending themselves. These are instinctual processes. Therefore, their brains are organized along the major centers that control these functions. We humans perform these functions as well, and so have a “reptilian” brain built into us. That means we have the same parts of the brain found in reptiles, namely the brain stem and the cerebellum.”
Before our present knowledge of the brain and of evolution, the triune brain did not seem a bad idea and it was a simple model to understand. It no longer makes sense but it is still out there being passed on like right-brained vs left-brained and other myths.
One problem with the reptilian brain is that we are not evolved from reptiles. The last common link between mammals and reptiles is called amniotes. They were like amphibians but did not need to lay their eggs in water. In other words, they were the first truly land-dwelling vertebrates and all terrestrial vertebrates evolved from them. They did not have a neocortex but they had all the other anatomical parts of the brain. The amniotes evolved into two groups: the diapsids which further evolved into four lines - turtles, lizards/snake, crocodiles, birds; and the synapsids which evolved into mammals. Mammal evolution is separate from reptiles from the earliest terrestrial vertebrates. What is more, the neocortex makes its appearance very early in the synapsids line. The triune story of what animals had what sort of brain is simply not what evolutionary biology has found.
Another problem is the divisions of function that the triune model makes. The reptile brain is said to be only concerned with survival, to be reflexive, to act without thought. It is said to contain the basal ganglia and the lower parts of the brain. This would include the cerebellum and the cerebellum is an important sophisticated part of the brain – concerned with most things we do, not reflexive, and essential to many types of thought. The paleomammalian brain was also called the limbic system (another MacLean coinage) and was supposed to deal with feelings and emotions. But the limbic system includes an important part of consciousness and of memory. The neocortex can do very little without those parts of the brain that were labeled limbic. Finally the mammalian brain was said to be the neocortex but the neocortex cannot really be thought of as a brain, as if it could function without the paleocortex and the thalamus. It was said to do all the thinking.
The model presumes that birds and reptiles cannot feel or think, which is a preposterous idea. And early mammals could feel, it was supposed, but not think, again not believable. Birds and many reptiles (perhaps all) have a brain area which does not anatomically resemble the neocortex but which develops from the same part of the embryonic brain and has the same functions as the neocortex. All the descendants of amniotes have essentially the same architecture of brain with the same functions. There are differences in proportions, sizes, connections, fine-scale anatomy but not a gross difference of kind in the brains of land vertebrates.
Forget all about the triune model of the brain.