Splitting Hairs: Evaluating the Origin of Human Nakedness

Abstract

Humans are the only extant predominantly hairless species in the primate order and are one of a mere handful of mammals with the same trait. Because hairlessness is not the natural condition of mammals and primates, then something in the evolution of Homo sapiens would have reduced or eliminated the selection for hair and instead selected for nakedness. This area of research has inspired various competing models that attempt to explain the emergence of hominin hairlessness. The key to evaluating such models, and deducing which evolutionary pressures were at play, lies in determining the timing of hair loss. For instance, if the development of clothing, as some have predicted, was the catalyst for humanity’s naked condition, then the chronological data should reflect a later evolution of hairlessness. Here, I examine published molecular and physiological data to evaluate several of these proposed hypotheses. Published genetic, fossil, and experimental data were used to evaluate the fit of each model to determine when hairlessness most likely emerged and which model most likely accurately reflects the evolutionary path of our ancestors. Evidence points to the thermoregulatory hypothesis as the most likely driving force behind the evolution of hairlessness in humans.