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
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
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.