The "reptilian complex," also known as the reptilian brain, refers to the most primitive part of the human brain, which includes structures like the brainstem and basal ganglia. This area is responsible for basic survival functions such as aggression, dominance, territoriality, and basic emotional responses. It plays a crucial role in instinctual behaviors necessary for survival. This concept is part of the triune brain model proposed by neuroscientist Paul MacLean in the 1960s. According to this model, the reptilian complex is the foundation upon which more advanced brain structures, like the limbic system and neocortex, evolved. These higher brain areas are responsible for more complex emotions and cognitive functions.