Hair, scales, and feathers seem to have very little in common. But these structures appear to have evolved from a single ancestor—a reptile that lived 300 million years ago—according to new research.
The study could end a long and contentious debate in evolutionary biology, says Leopold Eckhart, a dermatology researcher at the Medical University of Vienna, who was not involved in the work. "This really closes some important questions."
Hair in mammals and feathers in birds have long been known to develop from placodes—patches of thickened skin in embryos that are created by special cells known as columnar cells. These patches had not been seen in reptile embryos, leading scientists to believe that scales were unrelated to hair and feathers. Because birds and mammals evolved from separate lineages, scientists had two hypotheses: Placodes evolved two separate yet identical times in birds and mammals, respectively, or reptiles lost them over time, whereas birds and mammals didn't.
"People were imagining very complex hypotheses to explain the absence of placodes in reptiles," says University of Geneva geneticist Michel Milinkovitch in Switzerland.
Milinkovitch unwittingly waded in to this decades-long debate after seeing a rare, scaleless reptile—an Australian bearded dragon—at a pet market. After purchasing the animal, he investigated its DNA and found that a mutation of the gene ectodysplasin-A(EDA) led to the scalelessness. Mutations on this gene are also known to cause baldness, along with deformed teeth and nails, in mice and humans. This discovery led Milinkovitch and his colleague, University of Helsinki biologist Nicolas Di-Poï, to wonder whether there might be a relation between hair and scale development.
To find out, the team looked at the embryos of bearded dragons, Nile crocodiles, and corn snakes. In contrast with previous research, the team found that the reptiles in fact formed placodes, as it reports today in Science Advances.
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