Scientists disprove 70-year-old lizard brain myth

Scientists say the idea that our brains evolved like a layered cake—with a primitive “lizard brain” buried under newer, smarter parts—is wrong. In a study published Thursday in *Trends in Cognitive Sciences*, researchers argue that evolution doesn’t stack brain areas like floors in a building. Instead, it juggles two rival wiring systems, enlarging one and shrinking the other depending on an animal’s needs.

The 70-year-old “triune brain” model, popularized by neuroscientist Paul MacLean in the 1960s, painted a picture of three layers: the reptilian brainstem for survival, the mammalian limbic system for emotion, and the neocortex for reason. It became a staple of pop psychology and even school curricula. But advances in brain imaging, genetics, and evolutionary biology over the past decade have made that picture look too neat. “The triune model is a cartoon,” says Dr. Lori Marino, a neuroscientist at Emory University and lead author of the new paper. “It oversimplifies how brains actually change across species and over time.”

The team reviewed hundreds of studies on brain structure and behavior in mammals, birds, reptiles, and fish. They found that evolution doesn’t add new layers. It re-wires existing circuits in two broad styles: one that emphasizes quick, reflexive decisions (like fleeing from danger), and another that supports slower, more flexible planning (like solving a puzzle). Animals that need speed—like a gazelle outrunning a lion—develop more of the fast system. Animals that rely on memory and strategy—like a crow using tools—expand the slow system. The result isn’t a stack, but a dynamic tug-of-war, where each species tunes its brain according to its lifestyle.

Next, the researchers plan to test their idea with experiments on living animals and computer models of brain networks. They hope to map how these two wiring styles interact in real time. If they’re right, it could reshape how we understand brain injuries, mental health, and even artificial intelligence. A brain that isn’t just a stack of layers could inspire new ways to treat disorders like ADHD or PTSD, where fast and slow systems seem out of balance. The myth of the “lizard brain” may finally be laid to rest.