Did Enamel Evolve in the Skin?
We are all familiar with tooth enamel. It is the hardest substance produced by the human body and is an essential part of our oral health. Damage to the enamel is one of the major cause of visits to the dentist but most people are less aware of the substance’s evolutionary importance. Today almost all land animals have enamel on their teeth and it is the resilience of this heavily mineralised tissue which has allowed animals to adapt to so many different habitats and diets. Now new research carried out by Uppsala University, Sweden and the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, has combined fossils and genomics to examine the origin of biological enamel and have come to a surprising conclusion.
Enamel is a surprisingly complex tissue, made up from a combination of the mineral apatite (calcium phosphate) and three ‘matrix proteins’. Today we, and most land animals, have enamel only in our mouths but some fish also have tiny scales, called dermal denticles, which cover their skin and which have a structure surprisingly similar to our teeth. One particularly primitive modern fish is the gar of North America and like many fish the gar is heavily covered in scales but these are somewhat unusual in being covered in a substance called ‘ganoine’, which acts almost like a natural veneer. By examining the genes which controlled the formation of the ganoine the researchers discovered that the gar possesses genes for two of the three matrix proteins found in our teeth. These proteins are expressed in the fish’s skin, strongly suggesting a connection between the ganoine of the denticles and the enamel of our teeth. This is also the first time these genes have been identified in a bony fish.
The second strand of evidence for enamel’s dermal origins comes from the fossil record. In particular two species; Psarolepis from southern China and Andreolepis from Sweden. Both are primitive bony fish from around 400 million years ago and both had scales covered in enamel. In the case of Psarolepis these denticles even covered the face, but in neither case did the teeth have similar enamel. This means that the lack of tooth enamel is almost certainly the original condition and that enamel first evolved as protection for the scales and only later migrated to the mouth.
Reference: Qu, Q., et al. 2015. New genomic and fossil data illuminate the origin of enamel. Nature Letter. doi:10.1038/nature15259
Featured Image: Lepisosteus oculatus. Wikimedia Commons