ABSTRACT
While evolutionary biologists have not yet been able to identify precisely when hair first appeared, it is clear that it evolved at least 200 million years ago ( 1 ). Because the most obvious benefit of hair in the wilderness was insulation, conventional wisdom has been that hair originated as a part of the development of endothermy or warm-bloodedness ( 2 ). Although not without controversy, accumulating scientific data now suggest that the development of hair was a part of a more complex process of skin evolution that took place during the transition of mammalian ancestors from aqueous to terrestrial environments. During this transition, glands were internalized as the skin became less mucogenic, and the evolution of keratins and lamellar bodies reduced cutaneous water loss, allowing for survival in a dry environment ( 1 ). Simultaneously, genetic changes led to the proliferation of epidermal appendages whose initial function was as mechanosensory organs, and whose most direct descendents are vibrissae ( 1,3 ). These appendages provided an additional benefit of mechanical protection, but as primitive mammals moved out of aqueous environments and became endothermic, the insulating
capacity of the proto-hairs became increasingly vital ( 4 ). Subsequent genetic changes in the keratins and the keratinassociated proteins (KRTAPs) over time led to the current morphology of hair, and provided an adaptive advantage that played a critical role in the ascendance of mammals ( 5 ).
