Reptile Scales Gave Rise to the Hair on Your Head

June 28, 2016 | Erica Tennenhouse

Placodes are visible before the development of hair, scales, and feathers
Photo credit: UNIGE 2016 (Tzika, Di-Poï, Milinkkovitch). Placodes (spots stained in dark blue) are visible before the development of scales and feathers in chickens (left) and crocodiles (right).

And bird feathers too!

Bird feathers, reptile scales, and mammal hairs might look and feel vastly different from each other. But new research finds that they all share a common ancestor — the skin appendages of a reptile that lived roughly 300 million years ago.

The evolutionary link between feathers, scales, and hairs has been debated for decades. Key to the controversy has been the observation that mammal hair and bird feathers develop from similar primordial structures called placodes — tiny bumps of thick tissue in embryos — that appeared to be lacking in reptile embryos. This implied that placodes arose independently in birds and mammals, or alternatively, that reptiles lost them over time.

SEE ALSO: Scientists Have Grown “Dinosaur Legs” in Chicken Embryos

In their recent paper, published in the journal Science Advances, researchers report the first findings of placodes in three reptiles: Nile crocodiles, bearded dragon lizards, and corn snakes.

These structures are particularly hard to spot in developing reptiles because they exist for a much shorter time — only around 12 hours in reptile embryos — than they do in bird or mammal embryos

“If you don’t look at the right place at the right time you don’t see any,” study co-author Michel C. Milinkovitch told the New York Times. “If you look too early you see nothing, if you look too late it’s already a scale.”

During their study, the researchers also investigated a rare scaleless bearded dragon that has been bred for the pet trade. DNA analysis revealed that a mutation on a gene known as EDA was responsible for the lack of scales in these reptiles. Mutations on this gene are also associated with baldness in mice and humans, which again suggests a link between scale and hair development.

When the EDA gene is malfunctioning in lizards, the researchers found that they fail to develop a proper scale placode; this parallels the inability of mammals or birds affected with similar mutations in the same gene to develop proper hair or feather placodes.

As Milinkovitch explains in a press release, these findings lend strong support to the idea that “the reptilian scales, the avian feathers and the mammalian hairs, despite their very different final shapes, evolved from the scales of their reptilian common ancestor.'”

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