The roots of mammalian hair go far back into evolutionary history, according to a new study.
Hair, which provides insulation and protection, is seen as one of the main evolutionary innovations that led to the rise of mammals. But the origins of hair date back to an unknown reptile ancestor that lived more than 300 million years ago, in the Paleozoic era, the new study says [National Geographic News].
Previously, biologists had considered the possibility that hair evolved from scales or feathers, but the paucity of fossils showing the evolution from reptiles to mammals has made the question a hard one to examine. So in this new study, published in the Proceedings of the National Academy of Sciences [subscription required], researchers ignored the fossil record and looked instead to the genetic record of living animals: namely, a chicken and an anole lizard. Researchers identified the gene that codes for a protein called hair keratin in mammals, and then looked for those same genes in the feathered and scaly animals. They found several previously unidentified hair keratin genes in the animals, and realized that the genes coded for proteins used in claw and skin formation. This
suggest that keratin genes "are not restricted to mammals and suggest that the evolution of mammalian hair involved the co-option of pre-existing structural proteins." Keratin genes, they conclude, likely emerged in the last common ancestor of all amniotes — the group of four-legged vertebrates spanning mammals, reptiles and birds [Wired News],
and mammals later adapted to use the genes to grow hair. Developmental biologist Denis Headon
says that the study shows "that the components required to make hair fibers were already encoded in the premammalian genome." The remaining question, he adds, is the origin of the follicle, the assembly unit of mammalian hair, which is absent in birds and reptiles [ScienceNow Daily News].
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