According to a new study, published this week in the journal Nature, enamel evolved in the skin and colonized the teeth much later.
Early evolution of enamel. Colored stars indicate first appearance of a tissue. Image credit: Qingming Qu et al.
Enamel is the hardest substance produced by the human body, composed almost entirely of the mineral apatite (calcium phosphate) deposited on a substrate of three unique enamel matrix proteins.
Like other land vertebrates humans only have teeth in the mouth, but certain fishes such as sharks also have tiny tooth-like scales (dermal denticles) on the outer surface of the body.
In many fossil bony fishes, and a few archaic living ones such as the spotted gar (Lepisosteus oculatus), the scales are covered with an enamel-like tissue called ganoine.
A group of scientists from Uppsala University in Sweden and the Institute of Vertebrate Paleontology and Paleoanthropology in China investigated the genome of the spotted gar 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.
But where did enamel originate – in the mouth, in the skin, or both at once?
The answer to that question is provided by two prehistoric fishes – Psarolepis romeri from the Devonian of China and Andreolepis from the Silurian of Sweden.
In Psarolepis romeri 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.
“We show that Psarolepis romeri, a bony fish from the Early Devonian period, combines enamel-covered dermal odontodes on scales and skull bones with teeth of naked dentine, and that Lepisosteus oculatus has enam and ambn genes that are expressed in the skin, probably associated with ganoine formation,” the scientists wrote in the Nature paper.
“The fossil evidence, further supported by the Silurian bony fish Andreolepis, which has enamel-covered scales but teeth and odontodes on its dermal bones made of naked dentine, indicates that this tissue originated on the dermal skeleton, probably on the scales.”
Prof. Per Ahlberg of Uppsala University, senior author on the study, explained: “Psarolepis romeri and Andreolepis are among the earliest bony fishes, so we believe that their lack of tooth enamel is primitive and not a specialization.”
“It seems that enamel originated in the skin, where we call it ganoine, and only colonized the teeth at a later point.”
The study is the first to combine novel paleontological and genomic data in a single analysis to explore tissue evolution.
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Qingming Qu et al. New genomic and fossil data illuminate the origin of enamel. Nature, published online September 23, 2015; doi: 10.1038/nature15259
