In non-human hairy primates, hairs serve to retain heat in cold climate and keep out the heat in a hot environment, thus helping in thermoregulation which has survival value in maintaining the species under extreme climatic variations occurring in the African Savanna mosaic habitat. It protects the body against trauma and also against ultraviolet damage. Hair coloration helps to camouflage against predators and in some instances serves as a sexual attractant like the mane of the male lion. Endowed richly with nerve fibers, it has tactile and communicative functions. Some animals additionally have specialized sinus hairs in the skin, the lips, the snout and above the eyebrows. These tactile hairs called vibrissae have more than 2000 sensory nerve endings and in nocturnal animals may almost function as eyes. Pilo erection may signal either a state of light or attack. In non-human primates, hairs also provide the means for the newborn to cling to the mother. So, in animals, hairs not only have survival value, but also to a certain extent help in the propagation of the species.
When it is so important in primates, why was there a reduction of hair cover in early hominids? Actually, the number of hairs in human beings is not that much lesser than in apes, there being about 60 hairs per cm2 on our naked skin. But these human hairs are small, miniaturized and have less pigments than in apes. The reduction in density and size of hairs in humans helps in thermoregulation by evaporative loss of heat through sweating. Non-hairy skin will not pose a barrier to evaporation while plenty of hairs would reduce evaporative heat loss. The ability to sweat profusely in primitive man enabled him to forage for food in the hot summers of the African Savannas. So, this function of sweating to get rid of the heat of the day must have proved much more important than simply to retain heat with thick hairs in the cool nights of the dense forests of the African continent where primitive man is presumed to have his origin.
In human beings, specialized hairs such as eye lashes and hairs inside the nostrils and external ears afford some protection from the environment. Eyebrows prevent sweat from getting into the eyes. Scalp hairs may assist in stabilizing the temperature of the brain. Hairs can also excrete toxic substances like arsenic, and are thus of use in forensic medicines. As every dermatologist knows, the psychological functions of hairs are immeasurable.
The hair follicle is an important model system for studying basic biological problems. Humans have a much longer anagen and shorter telogen than mammals. The hair follicle stem cells are likely to replace embryonic stem cells as the cells of the future due to easier availability, abundance and lack of ethical issues. The hair follicle plays a role in epidermal homeostasis, wound healing and skin tumorigenesis. Genetic studies have revealed numerous genes involved in follicle formation, growth and cycling. Repigmentation in vitiligo patches often starts around hair follicles.
So, even though hairs have no longer survival value in Homo sapiens, when seen through Darwinian eyes, yet they do serve useful functions and are not completely vestigial!