If you studied hard in school, you might recall a biology lesson about what makes mammals special: They nurse their young with milk, harbor unique middle ear bones, and sport hair on at least some parts of their body. But a new report published today in Current Biology reveals that genes that encode key hair-building proteins are also present in reptiles and birds. These findings, the study's authors say, suggest a new scenario for the evolution of hair.
The study focuses on the origins of keratin proteins, the building blocks of mammalian fingernails, claws, and some types of horns. Reptiles and birds have keratins, too, in beaks, scales, and feathers. But the so-called hair keratins, hardened with the amino acid cysteine and used to build hair shafts, are exclusive to mammals. As such, scientists have speculated that hair-keratin genes emerged after mammals split off from other animals.
That history may need some revising, says molecular biologist Leopold Eckhart of the Medical University of Vienna. When he and colleagues searched the genomes of a lizard and a chicken, they found several previously unidentified cysteine-rich hair-keratin genes, some of them exclusive to the reptile-bird group. Instead of making hair, however, two of these genes encode proteins used mostly in the skin and claws. "[It's probably] more appropriate to call these proteins claw keratins instead of hair keratins," says Eckhart.
Based on the results, the authors say, hair-keratin genes likely evolved before the split between mammals and reptiles. The common ancestor of mammals, birds, and reptiles would have had hair-keratin genes, probably expressed in skin and claws. After the separation, reptiles developed their own variants of hair keratins, while mammals adapted the genes to create hair.
Denis Headon, who studies skin development at the University of Manchester in the U.K., 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.