Mammalian hair has a surprising evolutionary origin, according to a study published in PNAS this week: the reptilian claw.

Evolutionary biologists have long conjectured about the origin of hair -- did this defining mammalian characteristic evolve from features such as scales and feathers, or did it occur much later in mammalian evolution. "Because it's very difficult to find fossils" showing an intermediate stage between scales and hair, said first author Leopold Eckhart from the Medical University of Vienna, "we followed a completely different strategy." Eckhart and colleagues looked at homologous genes in the molecular components of hair and then searched for the homologous genes in the chicken and reptile genomes by mining published data. (The green anole lizard is the first full genome of a reptile published.)



A hair keratin-like protein (labeled red) detected in the growth zone of the claw of the green anole lizard.
Many biologists have thought that "hair is an evolutionary innovation that occurred only in mammals," said Eckhart. Previous studies failed to show genes coding for hair keratin in chickens and other non-mammals. However, Eckhart's team showed that the one major component of hair -- cystein-rich alpha keratin -- is also expressed in the claw of the green anole lizard, suggesting these structures appear much earlier. The findings imply that there were several "steps in the evolution of hair," said Eckhart. "The first step, is the evolution of these keratins."

Some researchers, however, are unconvinced by the novelty of this paper's findings. While the study adds to existing evidence that keratins are an important evolutionary stepping stone in the development of hair, that proposal is not new, Dominique Homberger from Louisiana State University wrote in an Email to The Scientist. Also, she added, the evolutionary link between hair and claws isn't clear from the work. "It may be that similar keratin proteins are found in hard-cornified portions of hair and claws," she wrote, "but this does not mean that hair evolved from claws."

Image courtesy of Karin Jaeger and Leopold Eckhart, Medical University of Vienna