The term "reptilian brain" refers to the most primitive part of the human brain, known as the reptilian complex. This area is responsible for basic survival functions such as aggression, dominance, territoriality, and basic instincts. It includes structures like the brainstem and basal ganglia, which control automatic behaviors necessary for survival. The concept originates from the triune brain theory, proposed by neuroscientist Paul MacLean in the 1960s. According to this theory, the reptilian brain is one of three layers, with the others being the limbic system and the neocortex. Each layer corresponds to different aspects of behavior and emotional processing in humans.