Enamel is the hardest tissue in our bodies, made almost entirely of calcium, phosphorus and other minerals.

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The enamel that covers your teeth originated in an unlikely place: the scales of ancient fish.

Scientists say they figured this out by examining the fossils of long-dead fish and the DNA of a range of creatures alive today. The researchers make their case in a report published online Wednesday by the journal Nature.

Enamel is the hardest tissue in our bodies, made almost entirely of calcium, phosphorus and other minerals. It protects our teeth when we chew and shields them from pain when encountering things that are very cold or very hot.

Nearly all four-limbed creatures have enamel, including mammals, birds, amphibians and reptiles. So do so-called lobe-finned fish, some of which evolved to walk and live on land.

Some types of primitive fish had similar kinds of tissue covering the exterior of their bodies. And some had enamel-like substances on their exterior and on parts of their teeth. The genes needed to make all of these hard tissues are largely the same, and most are clustered together in the genome.

By comparing the teeth and outer skeletons of various groups of fish, researchers determined that enamel first arose in fish that had skeletons made of bone. (Other fish, including sharks and rays, have skeletons made of cartilage.)

Scientists then turned their attention to a fish species called Psarolepis romeri, which lived in present-day China roughly 445 million to 420 million years ago. This fish intrigued them because it was one of the oldest species known to sport enamel.

However, the researchers wrote, the one part of the fish where enamel was conspicuously absent was the creatures’ teeth.

Psarolepis makes clear that a single species can have enamel on some parts of its body but not others. With that in mind, scientists turned their attention to another prehistoric fish: Andreolepis hedei.

Andreolepis, which lived about the same time as Psarolepis, has been a puzzle to scientists. Specimens have been found with enamel on their scales but not on bones in the skull. Some researchers have argued that these specimens must represent two separate kinds of fish. But the example of Psarolepis means that Andreolepis could be a single species with enamel on its scales, but not elsewhere.

Pulling it all together, the study authors concluded that “Enamel originated on the scales, before colonizing the dermal bones and finally the teeth.”