Hairs, feathers and scales have a lot in common

Hairs in mammals, feathers in birds and scales in reptiles have evolved from their common reptilian ancestor.

Mammalian hairs and avian feathers develop from a similar primordial structure called a 'placode': a local thickening of the epidermis with columnar cells that reduce their rate of proliferation and express very specific genes.

This observation has puzzled evolutionary and developmental biologists for many years because birds and mammals are not sister groups: they evolved from different reptilian lineages. According to previous studies, reptiles' scales, however, do not develop from an anatomical placode. This would imply that birds and mammals have independently 'invented' placodes during their evolution.

Professor Michel C. Milinkovitch in the University of Geneva and group leader Nicolas Di-Poï  at the Institute of Biotechnology put this long controversy to rest by demonstrating that scales in reptiles develop from a placode with all the anatomical and molecular signatures of avian and mammalian placodes.

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.

gene mutation affects both teeth, glands, nails, hairs, and lizard scales

During their new study published in the Science Advances, the researchers also investigated a mutant form of the bearded dragon lizard that lacks all scales. By analyzing the genome of this mutant, Di-Poï and Milinkovitch discovered 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.

When EDA is malfunctioning in lizards, they fail to develop a proper scale placode, exactly as mammals or birds affected with similar mutations in that same gene cannot develop proper hair or feather placodes. These data all coherently indicate the common ancestry between scales, feathers and hairs.