Humans have three distinct brain systems that evolved sequentially: the reptilian brain for physical survival, the mammalian brain for emotions and bonding, and the human cortex for abstract thinking

Based on Dr. Paul McClain's Triune Brain Theory research. The reptilian brain developed first at the base of the skull for fight/flight/reproduction, the mammalian brain evolved for live birth species to bond and nurture young, and the cortex developed dense neurons at the front for processing abstract ideas and symbols.