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.