The triune brain theory was developed by Paul MacLean in the 1960s. It says that the human brain is divided into 3 parts that correspond to evolutionary stages of development. - The earliest stage is the reptilian brain, which controls autonomic functions such as breathing and heart rate. (This evolved around 300 million years ago). - The next stage is the limbic brain, which evolved with mammals, and is the seat of emotions, and our fight or flight responses. (This evolved around 100 million years ago). - The most recent stage is the neocortex, which allows us to reason and plan. These divisions were possibly inspired by Plato's separation of the soul into three parts: base instincts, emotions and reason. However, the triune brain theory was rejected by the 1990s. Using more sophisticated technology (MacLean only used microscopes), scientists could compare the genes in human neurons with those of other animal species. What they found is that they are quite similar. The current understanding rejects the notion that the brain has three layers. Instead, what it says is that as brains get bigger, they reorganize. So while a rat might have just one somatosensory region (giving a sense of touch), humans have four. A visual inspection might suggest that our brains have extra layers, but molecular genetics shows that reptiles and nonhuman mammals have the same kinds of neurons as we do. --- Related: [[Jungian archetypes arose in the reptilian brain]] --- Sources: [Pearce. 2008. *Paul MacLean, 94, Neuroscientist Who Devised ‘Triune Brain’ Theory, Dies*](zotero://select/items/1_JJ7MEMV6) Barrett, Lisa Feldman. _Seven and a Half Lessons about the Brain_. Boston: Houghton Mifflin Harcourt, 2020.