Descent of Man

Charles Darwin first illustrated an evolutionary explanation of beards in ‘The Descent of Man’. 

In which he hypothesized that the process of sexual selection may have led to beards. Females find hirsute suitors sexy. Explanations for the existence of beards include signalling sexual maturity and signalling dominance by increasing perceived size of jaws, and clean-shaved faces are rated less dominant than bearded. Detractors may deny it, but modern biologists have reaffirmed Darwin’s theory with evidence that the ladies find mates with beards more attractive than their clean-shaven brethren.