"We hypothesize that enamel originated on the scales, before colonizing the dermal bones and finally the teeth," the authors write. They think that the enamel-like ganoine started just protecting scales of ancient bony fish, creating armored plates like the spotted gar still has today. The teeth of these fish were simply unprotected, "naked dentine". Dentine is the hard, white part of the tooth just under our enamel--still sturdy, but not as tough as enamel. Somewhere along the evolutionary line, some of these ancient fish began to incorporate ganoine/enamel into other hard surfaces of their bodies, including the tooth-like structures called odontodes which they had on the outside of their mouths. Eventually, this theory says that fish began evolving with ganoine/enamel on their actual teeth, a trait that has been passed down the evolutionary line to almost all toothed creatures today, from humans to crocodiles.