The reptilian brain, according to a classic theory of brain science, has corresponding structures in the brains of mammals, including humans. According to the “triune brain” theory, the reptilian brain, concerned with instinct and survival, developed first in evolutionary history. Creatures such as mammals developed more complicated brain structures on the foundation of the reptilian brain, allowing for thought, emotion and self-awareness. Brain studies have since shown that the triune brain theory is oversimplified at best; however, it remains popular with the media and the general public.
During the 1960s, neuroscientist and physician Paul D. MacLean’s research into brain structures revealed that the basal ganglia, a group of structures in the base of the human brain, resembled the brain of lizards and other reptiles. This, coupled with the knowledge that the basal ganglia are strongly involved in motor functions, led MacLean to believe brain development corresponded to evolutionary development. Reptiles developed first in evolutionary history, followed by mammals and then humans, so he reasoned that the brain could likewise be divided into sections based on developmental complexity.
In MacLean’s theory, the basal ganglia, which he called the reptilian brain, controlled baser instincts such as aggression and territoriality, behavior that can be observed in reptiles as well as mammals, including humans. The intermediate brain structures, which he called the “limbic system,” controlled higher functions necessary to rearing the young but were not necessary in reptiles, which generally lay eggs rather than give birth to and raise live young. The neocortex, found only in higher mammals, allowed the development of language, reasoning, and conscious thought in humans.
Subsequent discoveries in brain and animal science have shown the triune brain theory is not a precise model. Creatures such as birds, for example, are capable of using rudimentary tools and language, despite their lack of a neocortex. Some brain functions once believed to be controlled by the reptilian brain have since been found to involve various areas of the brain. Evolutionary development is also not as simple as once thought, further disputing MacLean’s developmental model.
The triune brain and the reptilian brain remain fixtures of popular culture and belief about brain functions. Astrophysicist Carl Sagan’s popular science bestseller “The Dragons of Eden” gave the triune brain theory wide exposure during the 1970s. In his groundbreaking graphic novel “Elektra Assassin,” comics artist Frank Miller gave his character Elektra the ability to function only with her “reptilian brain,” allowing her to act instinctively and ruthlessly in the presence of danger.