The “lizard brain” is not a primitive organ buried beneath emotion and reason. It is the most memorable part of a discredited model—one that persists because it makes inner conflict simple and human superiority feel anatomical.
Hannah Critchlow traces the idea to neuroscientist Paul D. MacLean’s 1959 “triune brain.” His model divided the mind into three evolutionary layers: a reptilian core for survival and sexual impulses, a mammalian limbic system for emotion, and a rational neomammalian cortex that keeps the older systems under control. Anger, impulsivity and short-term thinking could then be explained as moments when the lizard broke through.
The account traveled easily from neuroscience into textbooks, management training and self-help. It offers both a vivid picture and a moral hierarchy: reason is modern and human; emotion and appetite are ancient, animal obstacles. But evolution does not preserve complete ancestral brains and bolt new ones on top.
Reptiles possess structures and capacities once assigned to the “mammalian” limbic layer. Mammals broadly show features MacLean treated as neomammalian. Animals placed lower in the hierarchy solve problems, learn through emotion, communicate in complex ways and defer gratification. The anatomy and the behavior refuse the model’s borders.
That does not mean every part of the brain does the same thing. Neurologist Guy Leschziner stresses that functional specialization is well established, and some functions do have deeper evolutionary histories. What fails is the leap from those facts to three separate systems governing instinct, emotion and logic.
A better picture is integrated rather than layered. Psychologist Lisa Feldman Barrett describes a dynamic brain that predicts bodily needs and budgets energy to keep the organism alive. On that account, emotion and reason are not rival occupants of different evolutionary floors. They emerge from systems working together.
The lizard-brain story endures because its error is useful. It excuses unwanted behavior as primitive circuitry, flatters humans as evolution’s rational endpoint and turns a network into a three-character drama. Its survival in introductory psychology texts matters: it teaches the wrong mechanism of evolution and exaggerates the distance between human and animal minds.
Critchlow’s correction is less tidy but more revealing. The brain is not an onion, the self is not a cortex supervising a reptile, and rationality is not humanity’s private possession. The metaphor tells us more about the story humans prefer than about the organ inside the skull.