Scientists have long pondered whether or not the hair of mammals, the feathers of birds, and the scales of reptiles had a common evolutionary link, and now, after decades of debate, they finally have an answer, thanks to new research published today in the journal Science Advances.
In that study, experts from the University of Geneva (UNIGE) and the SIB Swiss Institute of Bioinformatics demonstrated that hairs, feathers, and scales have identical molecular and micro-anatomical signatures during the early stages of their developments, suggesting that all three types of skin features likely evolved from a common reptilian ancestor.
Avian feathers and mammalian hairs both develop from a similar structure known as a placode, or a localized thickening of the epidermis with cells that reduce proliferation rate and express a specific set of genes. This has long puzzled experts, however, as the two types of creatures did not evolve from the same reptilian lineage and thus are not technically sister groups.
However, previous research has shown that reptilian scales do not develop from an anatomical placode. This has led to speculation that these structures probably evolved independently in both birds and mammals during the course of their development, but as the study authors explain in a statement, there’s actually something else at work here.
Feathers, scales, and hair are all more closely related than we thought.
Molecular signals, mutations reveal link between skin features
The UNIGE and SIB teams, led by science faculty members Michel C. Milinkovitch and Nicolas Di-Poï, have demonstrated that reptile scales actually develop from a placode after all, and that it possesses anatomical and molecular signatures identical to avian and mammalian placodes.
The researchers observed and analyzed both the morphological and molecular mechanics of skin in the developing embryos of snakes, lizards and crocodiles, and found evidence that supports an earlier Yale University-led study which discovered that scales, hairs and feathers share molecular signatures during the course of their development.
“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,” Milinkovitch explained. “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.”
Furthermore, the researchers also analyzed the bearded dragon, a lizard species which comes in three different variants: one with normal scales, one with reduced-sized scales and one that lacks scales completely due to a genetic mutation. By comparing each of their genomes, the scientists were able to discover the specific gene affected by the aforementioned mutation.
“’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,” Milinkovitch noted. When EDA malfunctions in lizards, the researchers found that it is unable to develop a proper scale placode. A similar mechanism has been observed in humans and birds, furthering the link between the three different types of skin features.
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Image credit: Thinkstock
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