The fur on your canine pals, the scales on scintillating snakes, the colorful feathers on a male peacock. All of these are the integuments or outer layers of animals, and they all have fundamental differences in texture, size and color.
But where did they all come from? Is it possible that these characteristics originate from one common ancestor?
This topic has long been a subject of debate among scientists, who have been looking for evolutionary links between hair in mammals, scales in reptiles and feathers in birds.
Now, a new study in Switzerland has discovered a clue that may finally solve this mystery: all these skin appendages actually come from the same ancestor.
Mutated Gene
Led by Michel Milinkovitch from the University of Geneva, researchers studied the scale-less naked bearded dragon, which lacked the spiked neck frill that its species is known for.
Milinkovitch wondered what might have caused the fluke in the naked bearded dragon, and so he and his graduate students analyzed the animal's DNA.
In the end, Milinkovitch and his team found that the mutated gene that robbed the naked bearded dragon of its scales is the same gene that influences the development of feathers and furs in animals.
Indeed, although these three skin appendages look very different, researchers say they start in the same place.
"They're all inherited from a common structure that then diverged hugely," says Milinkovitch.
What Scientists Know So Far
During early embryonic development, mammalian furs and avian feathers look very similar, and researchers say they form from the same primordial structure known as "placode," which is the thick accumulation of cells on the skin.
This common morphology suggests that furs and feathers have a common evolutionary root. Scientists say it makes sense, given the fact that mammals and birds evolved from a shared ancestor about 320 million years ago.
But scientists would never have thought that this shared ancestor is also the predecessor of modern reptiles. In fact, researchers say birds and reptiles have much more in common than mammals and birds.
Still, the scales of reptiles appear to develop very differently from mammalian hair and avian feathers. What's more, biologists who investigate reptile embryos could not find any general evidence of anatomical placodes.
Milinkovitch says there are two possible claims that could explain why there are no anatomical placodes in reptiles.
First, the placode is actually ancestral for everyone but was lost multiple times in the lineages of reptiles.
Second, it was possible that mammals and birds developed the placodes independently. However, scientists say this theory was least likely, especially because the exact gene known as EDA controlled the development of placode in both mammals and birds.
Conclusion
Nicolas Di-Poï, Milinkovitch's grad student, parsed the genome of the naked bearded dragon and pinpointed the mutation that caused the lack of scales. He discovered that the gene responsible was also EDA, which was found in birds and mammals.
Upon closer look, Milinkovitch and Di-Poï found that the naked bearded dragon indeed possessed anatomical placodes, but that theirs just appeared differently than the version that scientists are accustomed to seeing.
"They are extremely different morphologically," says Milinkovitch. "But if you look past that you can see the homology - that's the beauty of it."
Details of the study are published in the journal Science Advances.
