Scientists have long debated the origins of teeth. Did they evolve from body scales that migrated into the mouths of ancient vertebrates and became adapted for eating—an idea known as the "outside-in" hypothesis? Or did they evolve independent of scales, originating deep within the oral cavity and ultimately mounting onto the jaws—known as the "inside-out" hypothesis? A new study by scientists at Penn State provides evidence for the "outside-in" hypothesis.