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Inside you are three animals: a primitive crocodile, a panicky mouse, and a pragmatic human being. No, this is not related to the popular story of the two wolves fighting inside you—one good, one evil, the winner being the one you feed; rather, these three metaphorical animals are thought to occupy the inside of your head, fighting for control over your body.
We have all heard that we have a lizard brain. In fact, popular wandering hero Jack Reacher is said to have a highly reliable lizard brain, a sort of primitive instinct that alerts him to what is really going on. On top of this ancestral brain, we are said to have evolved a second brain, full of emotions, shared with mammals like mice and horses.
Lizards are detached, but mammals? We experience fear, anger, joy, defiance, and longing. And crowning this limbic system is what makes us uniquely human: a third brain capable of speech, of contemplation, and of complex thoughts.
Even I was still carrying around this notion until last week, and it’s no wonder: it’s being taught in universities. A recent study took a look at 20 introductory textbooks on psychology. The majority mentioned brain evolution—which is a good thing—but almost all of those made the mistake of teaching students about the lizard brain.
It will come as a shock, then, that neurobiologists have known since the 1990s that this simple and enticing idea is just not true. Its origin goes back to 1959—although it draws from Western cultures’ obsession with trinities—and it got a boost from a well-respected science communicator: none other than Carl Sagan himself.