Your Brain Did Not Evolve to Think

Your brain was not built for thinking. It was built for keeping your body alive in a world where lunch could eat you back, says neuroscientist Lisa Feldman Barrett, author of Seven and a Half Lessons About the Brain.

The familiar textbook story says evolution added layers like sedimentary rock, meaning rock built up in stacked layers. At the bottom sits a so-called lizard brain for instinct, then a limbic system for emotion, named for a word meaning border because it borders the inner part, and then the neocortex, or new cortex, for rationality on top. In this telling, the cortex is what Barrett calls "icing on an already baked cake," holding down a boiling inner beast.

Barrett calls that story a myth. She traces it to ancient Greek stories about morality, not to how brains actually grew. The larger idea behind it, that brains climb a steady ladder from simple animals to complex humans, has been seriously challenged by modern evolutionary biology.

A better clue comes from animals without brains. Consider the lancelet, a small sea animal also called amphioxus, that Barrett describes as basically "a little stomach on a stick." It has no eyes, no ears, and no nose, only a few cells that sense light and dark for daily rhythms, plus a simple cell for balance and a rudimentary sense of touch. It has no real head and no real gut, just a tube. It wriggles to a spot, settles into the sand, and filters passing food. When food runs low, it pops out and wriggles somewhere random where food will probably be thicker.

Everything changed, Barrett says, in the Cambrian Period, the ancient era when animals began deliberately hunting and eating one another. Suddenly it mattered whether the thing ahead would eat the animal or would be eaten by it.

Distance senses became precious. In water, that meant vision and a little hearing, plus smell, which grows stronger up close, and touch, which works at a distance because vibrations travel. At the same time animals developed a head along with a brain and new senses for seeing, hearing, and smelling at a distance, and then a jaw for biting and attacking. The brain did not arrive alone as a thinking machine. It arrived with the senses, the head, and the mouth, as bodies grew bigger and their worlds grew larger and more complicated.

Barrett stresses that scientists do not really know why brains evolved, but her reading of the evidence is that thinking was not the original job. She argues the original job was coordinating a body that had to sense far away, move, eat, and avoid being eaten. Rationality, so prized in the West, rides on top of a system built to run the body in a predatory world, not a lizard brain fighting a human brain.