Hair fiber function
Traditionally it was believed that the key role of hair fiber in mammals is to provide protection against heat loss. This notion was based on the fact that hair traps air adjacent to the skin to provide an invisible, insulating layer and in many species hair grows in seasonal molting cycles. Hair also provides indications of sexual development through onset of secondary sexual characteristics, such as beard development in humans. In many cases hair plays the key decision-making factor in mate selection, Because the hair color, distribution or quality etc are generally perceived as indicators of the general health and vitality of an individual. Hair fiber also protects the epidermis from minor abrasions and/or from ultra violet light by forming a barrier. Then there are those specialized hair such as eyebrows and eyelashes that protect the eyes by channeling or sweeping away fluids, dust and debris. Then there are the highly developed nerve networks around hair follicles. These provide sensory, tactile information about the environment via hair fiber manipulation. Thus hair fiber significantly contributes to the survival process of many mammalian species.
As compared to other animal species, humans appear to be relatively hairless. But in no way human beings are completely “hairless”. The hairs that are predominantly produced in humans are in the nature of small vellus hair. In case of humans terminal hair growth is restricted to specific body regions. Much has been made of such a visibly overt change in hair phenotype, and what impact it may have had on the human species. Ideally speaking, the loss of terminal hair results into the reduced heat insulation. The loss also reduces protection from exposure to adverse environmental factors, and reduces our ability to sense our surrounding environment. There are contrasting arguments too; some argue a lack of hair is advantageous as this condition helps streamlining the body and preventing parasites to take shelter in hair bearing skin. There are contradictory theories on the reasons of humans’ loss of their terminal hair producing capacity and resulting advantages or disadvantages thereof. But it is clear that this loss of terminal hair growth did not confer a significant life-threatening impact on our ancestors. The intelligence of primitive men taught them how to adapt to, and take advantage of, their relative lack of terminal hair growth.
The importance of hair fiber for human survival has been diminished. But so far as the human skin health is concerned, the importance of pilosebaceous unit, the hair fiber producing follicle structure plus the associated sebaceous gland, is still acknowledged. Humans retain over 2 million hair follicle structures in their skin which collectively may have significant biological influence. Recent findings have established that the impact of hair follicles goes far beyond their most visible role of forming hair fiber. In addition to that, it has been suggested that hair follicles can be detrimental to health under certain circumstances. See this web stie's section on hair follicles for more details.