The enamel that gives our teeth their bite evolved from the scales of ancient fish that lived more than 400 million years ago, fossil and genetic evidence suggests.

Enamel is the hardest tissue produced in the bodies of people and other vertebrates, including fish.

But until now, scientists were unclear about whether enamel originated in teeth or evolved from a tissue resembling enamel called ganoine, present on the scales of many fossil fish and a few primitive fish alive today.

To help resolve the question, researchers from Sweden and China examined fossils of two primitive bony fish from the Silurian Period, a time of evolutionary advances in marine life.

They found an enamel coating on their scales, but no enamel on their teeth. Only millions of years later through evolutionary processes did fish exploit the enamel to make teeth harder and stronger, they report in the journal Nature.

"This is important because it is unexpected. 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," said palaeontologist Professor Per Erik Ahlberg of Sweden's University of Uppsala.

"As the wolf in 'Little Red Riding Hood' said: 'All the better to eat you up with!' [Teeth] are hugely important as food-processing structures."

Enamel, shiny and white, is one of the main tissues in teeth in most vertebrates, and is composed almost entirely of calcium phosphate.

Evidence from fossil and primitive fish

The spotted gar fish is a primitive fish that has enamel in its scales ( Brian Gratwicke/Wikimedia Commons )

Fish are the ancestors of terrestrial vertebrates including amphibians, reptiles, birds and mammals, including humans.

"Although this tissue on our teeth is used for biting or shearing, it was originally used as a protective tissue such as in living primitive fishes including gars and bichirs," added palaeontologist Dr Qingming Qu of Uppsala University and the University of Ottawa.

Fossils showed a fish called Andreolepis that lived 425 million years ago in Sweden had a thin enamel layer on its scales.

Another called Psarolepis romeri, from 418 million years ago in China, had enamel on its scales and also the bones of the face. Neither had tooth enamel.

The genome of the spotted gar, Lepisosteus oculatus, a freshwater fish from central and southern United States largely unchanged since the age of dinosaurs, provided more clues.

Gar scales, like those of Andreolepis and Psarolepis, are covered by a shiny enamel-like substance.

The researchers pinpointed genes relating to enamel formation that were active in the gar's skin, indicating that this substance really is a kind of enamel.

"The genetic evidence strengthens the hypothesis that ganoine is homologous with enamel," they write.

But, the researchers said, further phylogenetic analyses of primitive fish are needed to confirm the exact timing and mechanism through which enamel colonised the teeth.

ABC/Reuters