The brain evolved in layers, and each layer still shapes behavior

Kaku describes the brain's structure as an evolutionary accumulation rather than a single unified design: a deep "reptilian" core governing basic survival instincts like fear and aggression, wrapped in a mammalian layer supporting emotion and social bonding, topped by the human prefrontal cortex responsible for planning, reasoning, and impulse control.

This layered model matters practically because it explains recurring tensions in human behavior — a rational plan formed in the prefrontal cortex can be overridden in an instant by a fear response originating in the older, faster-acting reptilian structures, which is why panic or rage can feel like they're operating almost independently of conscious intention. The newer layers don't replace the older ones; they sit alongside them, sometimes cooperating and sometimes competing for control.

Kaku uses this framework to explain why purely rational appeals often fail to change behavior driven by fear or instinct, since the relevant neural machinery for those responses evolved long before, and operates largely independently of, deliberate conscious reasoning. Takeaway: much of what feels like irrational behavior is really older brain structures functioning exactly as evolution designed them to.