Anatomically, modern humans evolved from our chimplike ancestors around 100,000 years ago, although it took another 50,000 years for our brains and culture to evolve sufficiently to make us capable of language, planning, and creativity. But this extended, complex history has a downside: the more recently emergent aspects of our brains—which give us astonishing powers of thought, logic, imagination, empathy, and morality—must share skull space with the ancient brain equipment that we've inherited from our mammalian and reptilian forebears over the past several million years. So even today, one of the most basic human challenges is integrating and coordinating the complex and highly specialized systems that comprise our brains.
The neocortex, for example—the part of the brain that organizes our powers of conscious thought, imagination, and empathy—must coexist and cooperate with primitive survival networks conserved by natural selection through hundreds of thousands of generations. This means that beneath our newer equipment, capable of composing sonnets and developing computers, are structures driven by primitive instincts, unconscious impulses, and primordial fears. Within our skulls, the reptilian, ancient mammalian, and modern human brains attempt to coexist and cooperate, at least enough to get us through the day.
The American physician and neuroscientist Paul MacLean first discussed this structure, which he called the "triune brain." The basis of his theory is that the contemporary human brain resembles a site inhabited by successive civilizations, and embodies a living record of our deep evolutionary history. At its core is the reptilian brain, responsible for arousal, homeostasis, and reproduction. The paleomammalian ("old-mammal") brain, involved with learning, memory, and emotion, surrounds it. The neomammalian ("new-mammal") brain, required for conscious thought and self-awareness, sits atop the other two. These levels roughly conform to the common distinction of brainstem, limbic system, and cortex. Though MacLean's theory has many significant limitations, it can both assist us to better understand some of the most distinctive features of human experience and give us fresh insight into how psychotherapy influences brain function.
MacLean suggested that our three brains don't necessarily work well together because each of them processes information in a distinctive manner and has a unique agenda. The functions of the reptilian brain, which drive our instincts and behaviors—fear, rage, eating, mating—retain a good deal of executive control over our actions, while only a small region of the cortex is capable of conscious awareness and articulating its strategies. This means that multiple levels of the brain often vie for dominance simultaneously and in conflict with each other, without our conscious awareness—an idea that parallels Sigmund Freud's conception of the relationship between the conscious and unconscious minds.