Hairs, feathers and scales have a lot in common: Study

Earlier, researchers have considered hair in mammals and feathers in birds to be different from scales in reptiles as reptile embryos lack placodes (thickened skin patches are created by special cells termed as columnar cells). A research paper published in the journal Science Advances has unveiled that a bird’s feathers, a reptile’s scales and a mammal’s hairs have originated from a common ancestor. They look different features, but the researchers said that these skin appendages may come from same origins.

Study researchers from the University of Geneva (UNIGE) and the SIB Swiss Institute of Bioinformatics, Switzerland have found that these skin appendages are homologous, which means that they share a common ancestry.

The researchers carried out the analysis of embryonic development and noticed that molecular and micro-anatomical signatures were similar between hairs, feathers and scales in the early developmental stages. The assessment indicates that these structures in different organisms have evolved from their common reptilian ancestor.

Avian feathers and mammalian hairs have been formed of a similar primordial structure known as placode. This aspect has confused the researchers a lot as birds and mammals are not sister groups. Then, a 2015 research paper unveiled that hairs and feathers share molecular signatures. But then also there were many theories regarding the evolution.

In the current research paper, the study researchers have put those thoughts to rest. As per them, scales in reptiles have developed from a placode having all the anatomical and molecular signatures of avian and mammalian placodes.

The researchers have based their findings on the basis of assessment in crocodiles, snakes and lizards. Researcher Michel Milinkovitch the Department of Genetics and Evolution of the UNIGE Faculty of Science said, “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 mentioned that one thing becomes clear that the three skin appendages he reptilian scales, the avian feathers and the mammalian hairs, that have completely different shapes, but have evolved from the scales of common reptilian ancestor.

In the study, the researchers also assessed the bearded dragon, a species of lizard that comes in three variants. After comparing the genome of these three variants, the researchers have come to know about the gene affected by this mutation.

Disruption of the ectodysplasin-A (EDA), a gene whose mutations in humans and mice results into abnormalities in the development of teeth, glands, nails and hairs, was found to be the reason as to why lizards have such look.

The researchers said that when EDA is not in proper form in lizards then it fails to develop a proper scale placode. If it is the case with mammals or birds then they would not be able to develop proper hairs or feather placodes. It ultimately shows the common ancestry between scales, feathers and hairs.

The researchers said that investigating the origins of the skin appendages is a tough task as it is very difficult to fossilize the skin. Owing to which, they have focused on the structure and mechanisms behind feathers, scales and hairs at a molecular level.

The researchers now expect to widen the research area, including as how the specific characteristics of scales, hair, and feathers have evolved to what they are now. But there are many other researchers who think that more work needs to be done in the future.

According to a report in ScienceMag by Ben Panko, Hair, scales, and feathers seem to have very little in common. But these structures appear to have evolved from a single ancestor—a reptile that lived 300 million years ago—according to new research. The study could end a long and contentious debate in evolutionary biology, says Leopold Eckhart, a dermatology researcher at the Medical University of Vienna, who was not involved in the work. “This really closes some important questions.”

Hair in mammals and feathers in birds have long been known to develop from placodes—patches of thickened skin in embryos that are created by special cells known as columnar cells. These patches had not been seen in reptile embryos, leading scientists to believe that scales were unrelated to hair and feathers. Because birds and mammals evolved from separate lineages, scientists had two hypotheses: Placodes evolved two separate yet identical times in birds and mammals, respectively, or reptiles lost them over time, whereas birds and mammals didn’t.

A report published in the CS Monitor said, This doesn’t imply at all that feathers evolved from hair or that scales evolved from hair or that hair evolved from scales, et cetera,” cautions Richard Prum, an ornithologist at Yale University who was not part of this study but who has studied these same developmental structures.”

So Prum and his colleagues proposed that a placode is “not just an anatomical feature, it’s an information center, and that that is the appropriate definition,” Prum explains to the Monitor. That would mean that scaly reptiles, birds, and mammals all have the same mechanism underlying their distinctive skin appendages, even if the physical expression of a placode didn’t exist. But it does, says Milinkovitch. “We found the anatomical placodes” in reptile embryos, from snakes to lizards to crocodiles.

“The coverings all develop from the same primordial structure that might have first emerged in the common reptilian ancestor of birds, mammals and reptiles, the new research — published in the journal Science Advances — strongly suggests. “What we found is that they (hair, feathers, scales) all start from the same micro-anatomical structure, called a placode, made of a local thickening of the epidermis due to epidermal cells taking a columnar shape,” Michel Milinkovitch, who co-authored the paper with Nicolas Di-Poï.” according to a news report published by Seeker.

Reptiles, birds and egg-laying mammals were already connected because all are known as amniotes, referring to their type of egg (amniotic) in which their embryos are embedded. Now it’s known that the three animal groups share basic skin features as well, so it is likely that their common ancestor had placodes, too.

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