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The hardest material in the human body isn't our bones or our teeth but is actually the enamel that covers our teeth. Nearly all four-limbed, land dwelling animals have this enamel, which helped scientists to discover that evolved from ancient fish scales. But how did it get from there to here?
Tooth enamel is comprised mostly of a transparent crystalline calcium phosphate called hydroxyapatite. It protects us from cavities and a number of other dental problems. Even though it's the top layer of the tooth. Evolutionary scientists have found evolutionary scientists recently a tiny predator fish called Psarolepis romeri which lived on Earth 410 to 415 million years ago. When scientists dissected a fossilized skull of the fish, they found that parts of its scales and skull were comprised of an enamel-like substance called ganoine. But their teeth, didn't have any enamel on them--they were made out of just dentine. Some fish living today don't have enamel on their teeth either.
Scientists have surmised that the armored scales on ancient fish evolved to become teeth. They believe that if those scales had enamel on them, it would explain how it got on our teeth. Further analysis of the fossilized P. romeri's fskull and scales showed that the ganoine distribution of was not in a uniform layer, but rather scattered. It's possible that the scales didn't re-situate themselves to become enamel-covered teeth, and maybe the enamel production shifted towards teeth as the fish evolved. The modern-day spotted gar fish has ganoine-covered scales. Looking at their genome, they found that gars produce two of the three proteins needed to make enamel, which further supports this theory.
How the enamel that coats your teeth evolved (Science Mag)
"The hardest bit of your body is the enamel coating your teeth. But new analyses of fish fossils, as well as genetic analyses of a living fish species, suggest that this specialized material once served a very different function: to toughen some bones and scales of ancient fish."
Shark Teeth Have Built-In Toothpaste (Discovery News)
"The outside of shark teeth is made up of fluoride, the active component of most toothpaste."