It's a Jungle in There


We're Not as Evolved as We Think.

September/October 2008


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…

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