Evolution of Long Scalp Hair in Humans

The evolution of long scalp hair in humans is attributed to its initial role in thermoregulation and later as a social signal. Long hair likely emerged around 300,000 years ago, coinciding with anatomically modern humans. The duration of the anagen phase, the active growth phase of hair follicles, primarily determines hair length, with human scalp hair capable of sustaining anagen for 5-7 years. This trait may have developed through the attenuation of a checkpoint mechanism regulating the anagen phase. Genetic factors, such as regionally restricted enhancers, play a significant role in hair growth, with genes like FGF5 and WNT10A being crucial. The study suggests that understanding conditions with altered hair length could reveal the molecular basis of this trait, with potential implications for treating alopecia.