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Triune brain theory (Paul D MacLean) proposes that the brain is made of up three structures that have progressively evolved as animals evolved on this planet. I reference the Triune brain as it is a convenient model for physical-emotional-mental bodies. The triune theory is now considered to be incorrect and an over-simplification. I still find it a useful tool for viewing the brain structures so I’m going to continue using it but as my diagrams show, the brain is not made up of 3 distinct areas; it is all one interconnected entity and when we do subdivide it there are at least 6 or 7 main lobe areas (PFC, Parietal, Temporal, Occipetal, Insular, Limbic, Cerebellum, brainstem, etc.).
Going back to the Triune model, the reptilian brain is at the core carrying out the basic survival instincts and physical body functions for movement, respiration, etc. It was thought that this brain structure was the first to develop, and for the early reptiles on the planet (like the dinosaurs) it was all they had. All sensory inputs and motor outputs go through the reptilian brain.
Evolution gradually resulted in the Limbic brain developing around the Reptilian brain. The Limbic brain supported more complex physical bodies, and also a degree of interaction between individuals of the species. Humans share this brain structure with all other animals and mammals.
However, it is the neocortex that really sets humans apart from other mammals – this is the “thinking” brain. Sensory input passes through the reptilian brain to the structures in the limbic brain and then on up into the neocortex.