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Did Hairlessness Stimulate an Increase in Hominin Brain Size? Insights from the Cutaneous Neurosensory Interface and Comparative Vertebrate Morphology

Mitsuhiro Denda, Gopinathan K Menon and Peter M Elias*

The basis for the late stages of brain growth in our immediate human ancestors is unknown, but generally attributed to the development of language associated with increasingly complex social settings. We present arguments for an additional or alternative potential contributor to endocranial expansion. As ancestral humans emerged from tropical forests into open savannahs, loss of their body hair facilitated the dissipation of body heat. But their newly-exposed cutaneous surfaces would have been bombarded by exogenous stimuli that could have stimulated an increase in brain size required to centrally process a plethora of new sensory information arriving from the periphery. Two lines of evidence support this hypothesis. First, the common embryologic origin of epidermis and central nervous system (CNS) from the primitive neuroectoderm, endowed the skin with a broad suite of neuro mechanisms that mirrors the CNS, including outward-facing ‘physical’ receptors that monitor changes in visual, acoustic and olfactory stimuli, as well as many neuromediators that impact memory, mood and wakefulness in the CNS. Second, two groups of marine organisms, cephalopods and electric fish, exhibit large brain: body mass ratios associated with heightened cutaneous sensory capabilities. Human nakedness also stimulated new psychosocial and stress responses, mediated by cutaneous oxytocin and glucocorticoids, respectively. Together, these insights suggest that the onset of hairlessness could have stimulated an increment of endocranial expansion, shortly before the emergence of modern humans.