What do you and ancient fish have in common? Well, there's one thing that might surprise you; the enamel on your teeth, the hardest tissue in the human body, started out as tough fish scales.

Made mostly of calcium, phosphorous and some other minerals, enamel — which protects our teeth as we chew and insulates them against heat and cold — is found in some form in most land vertebrates.

Turns out some fish, such as sharks, have what scientists call "dermal denticles," small tooth-like scales on their bodies, but they're the exception; most living fish don't produce any enamel.

However, scientists have long suspected some ancient fish sported scales armored with a form of enamel, and a new study shines light on the question of how the substance ended up coating teeth.

Those ancient bony fish of the Silurain Period, a time when marine life was making evolutionary advances, sported enamel coatings on their scales but none in their teeth.

It took a million further years of evolution for the fish to take advantage of the enamel to make their teeth stronger and harder, researchers say.

"This is important because it is unexpected," says paleontologist Per Erik Ahlberg of Sweden's University of Uppsala, author of a study appearing in the journal Nature. "In us, enamel is only found on teeth, and it is very important for their function, so it is natural to assume that it evolved there."

Some scientists had suggested the hardened scales gradually migrated into the mouth over eons of evolution, but the new study suggests it may have been the pattern of enamel production that moved, rather than a movement of already enameled structures.

Rather than moving scales, evolution may have shifted the location of enamel-making proteins to new body parts, from skin to mouth.

The researchers studies fossils of two ancient fish species, Psarolepis and Andreolepis; both have a form of enamel known as ganoine in their scales, and Psarolepis had some denticles on its face, but neither had enamel in the teeth.

"Psarolepis and Andreolepis are among the earliest bony fishes, so we believe that their lack of tooth enamel is primitive and not a specialization," says Ahlberg, noting the Psarolepsis lived almost 450 million years ago. "It seems that enamel originated in the skin, where we call it ganoine, and only colonized the teeth at a later point."

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