Author and editor Judy Wearing has revisited her long-standing fascination with human hair in her new book, Anxious and Hairy: The Evolution and Culture of Human Hair. Wearing's investigation began during her PhD studies at Oxford, where she questioned the purpose of various body hair patches, particularly pubic hair.
Wearing explains that hair, or fur, first evolved around 245 million years ago with cynodonts, the warm-blooded reptilian ancestors of all mammals. This early fur provided insulation, preventing heat loss from their bodies. Fossil evidence also suggests that wolf-like Cynognathus had whiskers, which are sensitive tactile hairs providing sensory input.
Human hair, skin, and nails are all based on the protein keratin, which also forms cat claws, bird feathers, and sheep wool. Wearing notes that hair growth is an evolutionary modification of an ancient skin-development process seen in reptiles, where they grow scales. She suggests that hair can be viewed as a form of long, thin scales, both composed of keratin.
Despite appearing less furry than primate relatives, humans have the same number of follicles as chimpanzees, between two and four million, found everywhere except the palms and soles. The difference lies in the majority of human hairs being very thin and fine. Mammary glands, another defining trait of mammals, evolved as an adaptation of certain hair follicle glands.