A major transformation in vertebrate evolution took place when breathing shifted from being driven by head and throat muscles — like in fish and frogs — to the torso — like in reptiles and mammals. But what caused the shift? A new study posits that the intermediate step was locomotion. When lizards walk, they bend side-to-side. The ribs and vertebrae are crucial to this movement, and the mechanics follow the same pattern as when they inhale and exhale.