Hairlessness

Modern humans are described as functionally hairless because they lack the coats of easily visible, protective, and insulating body hair or fur possessed by most other mammals. They nevertheless have very fine body hairs over the surface of the body that grow from typically mammalian hair follicles. Modern humans have retained visible patches of hair on tops their heads, and in their axillae and pubic regions. The epidermis of the hairless surfaces of the human body exhibits barrier functions that prevent dehydration and protect against varied environmental insults. Hairlessness is correlated with enhanced abilities to dissipate heat from the body’s surface by radiation and evaporation. The loss of functional body hair in the human lineage was accompanied by the evolution of permanent dark (eumelanin) pigmentation on exposed skin surfaces. This process was probably completed by about 1.2 million years ago, when all variation in the amino acid sequence of the melanocortin 1 receptor (MC1R) locus was lost as the result of a selective sweep. The evolution of hairlessness was correlated with an increase in the density of eccrine sweat glands on the surface of the human body. The details of the genetic mechanisms whereby body hairs were made thinner and eccrine sweat glands more numerous are not yet known. The evolution of hairlessness in humans greatly reduced the potential for communication of emotional states via piloerection and may have been associated with further differentiation of the muscles of facial expression and elaboration facial expressivity.