Combining fossil and genomic evidence, researchers show that modern day tooth enamel likely evolved from a protein covering the scales and dermal bones of extinct fishes.
Enamel is the hardest tissue produced by the human body. This complex of proteins and minerals covers the teeth of almost all living tetrapods. Enamel is found only on teeth, but surprisingly, recent research has shown that it probably did not evolve there.
To understand the evolutionary origin of enamel, collaborating researchers from Sweden and China examined a seemingly unlikely dataset: the fossilized scales of 400 million year old Silurian Period fish and the genomes of their living relatives.
Previous paleontological research has shown the extinct fish, which are the ancestors of all amphibians, reptiles, birds and mammals, had a layer very similar to enamel covering their scales and dermal bones, but not their teeth.
The primitive living relatives of these fish, such as gar, also have this covering on their scales and lack enamel on their teeth. Genetic evidence from these extant organisms has shown the protein making up this layer, ganoine, is a homolog (a related protein) of enamel.
The researchers, who published their findings in Nature, compared the chromosomal arrangement of gar ganoine genes with human enamel genes. They found that ganoine genes followed a similar pattern to enamel genes, though there was evidence of evolutionary mechanisms such as deletions and rearrangements on the chromosome.
They also analyzed fossils spanning millions of years to conclude that, over time, the enamel-like layer found on scales began to show up in other tissues such as skin and teeth and likely eventually evolved into modern day enamel.
Their analysis provided strong evidence for the fishy origin of enamel, but the steps it took to make the transition are far from understood.
The authors note that more detail phylogenetic analyses, such as more accurate species trees and an evaluation of other genes such as regulatory regions is required to make definitive conclusions about enamel evolution.