That coat of enamel protecting our pearly whites originated from an unexpected source: primitive fish scales, according to new work published in Nature this week. Researchers studying both ancient and living fish found that the mineral originated in the skin and then colonized teeth much later on.
The hardest tissue produced in our bodies, enamel is unique to vertebrates, ranging from coelacanths to people. It’s composed of calcium phosphate deposited on three matrix proteins. While we only have enamel on our teeth, a tissue that resembles enamel called ganoine can be found on the tooth-like scales of fish both living and extinct. But researchers still aren’t sure whether enamel originated in the teeth and then spread to the scales, or if it was the other way around.
To investigate the origins of enamel, Uppsala University’s Per Erik Ahlberg and colleagues combined genomics with fossil data. Based on their genetic analyses, the ganoine present on living species of armor-plated fish, such as the spotted gar (Lepisosteus oculatus), is equivalent to enamel. Its genome contained genes for two of our three enamel matrix proteins, and they’re expressed in the skin.
Furthermore, the extinct 400-million-year-old fish Psarolepis romeri from China and also Andreolepis hedei from Sweden have enamel-coated external plates (scales as well as dermal bones on the face). But Psarolepis also sported enamel-free teeth. That means enamel was originally present on the body surface but not 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," Ahlberg said in a statement. “It seems that enamel originated in the skin, where we call it ganoine, and only colonized the teeth at a later point.” Further analyses can help pinpoint the exact timing and mechanism that made it possible for enamel to colonize teeth.
Psarolepis romeri by Nobu Tamura CC BY-SA 3.0 via Commons