Researchers studying the embryos of lizards, snakes, and crocodiles reveal that reptile scales, bird feathers, and mammal hair all evolved from a common reptilian ancestor. The findings are published in Science Advances this week.
For decades, scientists have debated whether or not scales, feathers, and hair share a common evolutionary origin – despite vast differences in how they develop and the proteins they contain. However, the fossil data for intermediate forms of these three so-called skin appendages have been lacking. And then there’s also the issue with “anatomical placodes”. These are tiny patches of thickened epidermis that’s usually present early on during embryonic development. While these microstructures are found during feather and hair growth, they haven't been observed in the development of scales. If reptile scales didn’t develop from placodes, some researchers argued, then scales didn’t give rise to bird feathers or mammal hair. That would imply that our hair evolved from a different structure in early land vertebrates altogether.
But perhaps placodes have simply been overlooked in reptiles since they’re hard to identify in embryos. Using skin tissue analyses and molecular techniques, University of Geneva’s Nicolas Di-Poï and Michel Milinkovitch examined the embryos of Nile crocodiles, corn snakes, and bearded dragons.
Placodes, spots stained in dark blue, can be seen before hair, scales, and feathers develop in (left to right) a mouse, snake, chicken, and crocodile. Tzika, Di-Poï, Milinkovitch UNIGE 2016
Reptile embryos, they discovered, develop tiny structures that possess all the morphological and molecular characteristics of anatomical placodes – just like with birds and mammals. “The three types of skin appendages are homologous,” Milinkovitch said in a statement. “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.”
Additionally, scales, hairs, and feathers also share the same signaling pathways. The team found that a key gene in mammal hair development – called ectodysplasin A, or EDA – is also an important driver of reptile scale development. If EDA malfunctions in mammals and birds, the animals can’t develop proper hair or feather placodes, and bearded dragons with the same mutation won’t develop the proper scale placodes. A bearded dragon with one mutated EDA gene has scales that are reduced in size. Meanwhile, a dragon carrying two copies of a natural EDA mutation (one from mom, one from dad) becomes scaleless. You can see these mutants and a “normal” bearded dragon in the top photo.
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