Many experts debate the development and evolution of hair, feathers, and scales. To contrast against the idea, a paper written by Nicolas Di Poï and Michel C. Milinkovitch proposed the linkage of all three skin appendages, explaining the common ancestry of amniotes. 

In a 2016 study, researchers have suggested that mammals, birds, and reptiles share a common ancestry to a single reptilian ancestor as they’ve found the existence of a physical placode in reptiles. 

The Significance of Ectodysplasin-A

A journal published in Science Advances revealed the link between birds’ feathers, mammal hair, and reptile scales. Written by Nicolas Di Poï and Michel C. Milinkovitch in 2016, the paper proposes that mammals, birds, and reptiles all come from a shared reptilian ancestor that lived more than 300 million years ago. (Source: Science Advances) 

Prior to the release of this study, many scientists remained divided on the evolution of feathers, hair, and scales. These skin appendages start in a nearly identical manner. In addition to that, only mammals and birds develop placodes, a thickened embryonic structure that hair and feather spur from, while reptiles do not. 

Within this study, researchers thoroughly studied the bearded dragon, a reptile genus that contained lizard species. The experts investigated three lizard species from the bearded dragons. The first bearded dragon is a regular, scale-covered type. The second bearded dragon is the middle ground as it has scales but is of a reduced size as it only contains one copy of a specific natural genetic mutation. The last bearded dragon bears two copies of the genetic mutation and no scales at all, the mutant & scaleless Australian bearded dragon or the Pogona vitticeps.

As Di Poï and Milinkovitch compared the genomes of these three species, they discovered that the size of the lizards’ scales correlated with the amount of ectodysplasin-A or EDA existing in their cells. An increased amount of EDA meant longer scales, while its lack of existence meant the absence of scales. (Source: PBS)

We identified that the peculiar look of these naked lizards is due to the disruption of the ectodysplasin-A (EDA), a gene whose mutations in humans and mice are known to generate substantial abnormalities in the development of teeth, glands, nails, and hairs.

Michel Milinkovitch

(Source: Science Daily) 

The Discovery From the Scaleless Lizard

Their discoveries indicated the existence of physical placodes in reptiles, reasoning that most mammals, birds, and reptiles are descendants of a shared reptilian ancestor.

Indeed, we have identified in reptiles new molecular signatures that are identical to those observed during the development of hairs and feathers, as well as the presence of the same anatomical placode as in mammals and birds. This indicates that the three types of skin appendages are homologous: 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.

Michel Milinkovitch

Their findings continue to link mammals, birds, and reptiles together since the malfunctioning of the EDA hinders a regular scale placode, similarly to how birds and mammals cannot develop proper feathers or hair placodes when they’ve experienced a similar mutation in a gene. (Source: Science Daily)