The potential evolutionary¹ link between hairs in mammals, feathers in birds and scales in reptiles² has been debated for decades. Today, researchers of the University of Geneva (UNIGE) and the SIB Swiss Institute of Bioinformatics, Switzerland, demonstrate that all these skin appendages³ are homologous: they share a common ancestry⁴. On the basis of new analyses of embryonic⁵ development, the Swiss biologists evidenced molecular⁶ and micro-anatomical signatures that are identical between hairs, feathers and scales at their early developmental stages. These new observations, published today in Science Advances, indicate that the three structures evolved from their common reptilian⁷ ancestor. Mammalian hairs and avian feathers develop from a similar primordial⁸ structure called a 'placode': a local thickening of the epidermis⁹ with columnar cells that reduce their rate of proliferation and express very specific genes¹⁰. 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. 

 

The single evolutionary origin of placodes revealed 

 

In 2015, a team from Yale University (USA) published an article showing that scales, hairs and feathers share molecular signatures during their development. These results fueled an old debate between two schools. One defends that these molecular signatures suggest a common evolutionary origin of skin appendages, whereas the other proposes that the same genes are re-used for developing different skin appendages.

 

Today, Nicolas Di-Poï and Michel C. Milinkovitch at the Department of Genetics and Evolution of the UNIGE Faculty¹¹ of Science and at the SIB put this long controversy¹² to rest by demonstrating that scales in reptiles develop from a placode with all the anatomical and molecular signatures of avian and mammalian placodes. The two scientists finely observed and analysed the skin morphological and molecular characteristics during embryonic development in crocodiles, snakes and lizards¹³. 'Our study not only provides new molecular data that complement¹⁴ the work of the American team but also reveals key microanatomical facts, explains Michel Milinkovitch. 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.'

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