Enamel originated on scales of prehistoric fish and colonised the teeth much later, an interesting new study has found.
In the study, researchers from Uppsala University in Sweden and the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) in Beijing, China, combined data from two very different research fields - palaeontology and genomics.
Enamel is the hardest substance produced by the body, composed almost entirely of the mineral apatite (calcium phosphate) deposited on a substrate of three unique enamel matrix proteins.
Like other land vertebrates we only have teeth in the mouth, but certain fishes such as sharks also have "dermal denticles" - little tooth-like scales - on the outer surface of the body.
In many fossil bony fishes, and a few archaic living ones such as the gar (Lepisosteus) from North America, the scales are covered with an enamel-like tissue called "ganoine."
Tatjana Haitina, a researcher at the Department of Organismal Biology, Uppsala University, investigated the genome of Lepisosteus, which was sequenced by the Broad Institute, and found that it contains genes for two of our three enamel matrix proteins: the first to be identified from a ray-finned bony fish.
Furthermore, these genes are expressed in the skin, strongly suggesting that ganoine is a form of enamel.
To answer where enamel originated - in the mouth, in the skin, or both at once researchers turned to two fossil fishes.
Psarolepis from China and Andreolepis from Sweden are both more than 400 million years old and have been studied by Qingming Qu and Per Ahlberg of Uppsala University in collaboration with Min Zhu from IVPP in Beijing.
In Psarolepis the scales and the denticles of the face are covered with enamel, but there is no enamel on the teeth; in Andreolepis only the scales carry enamel.
"Psarolepis and Andreolepis are among the earliest bony fishes, so we believe that their lack of tooth enamel is primitive and not a specialisation," said Per Ahlberg, Professor of Evolutionary Organismal Biology at Uppsala University.
"It seems that enamel originated in the skin, where we call it ganoine, and only colonised the teeth at a later point," Ahlberg said.
The study is published in the journal Nature.
