We have reason to believe that man first walked upright to free his hands for masturbation.

The line is a deliberately outrageous comic inversion of a familiar evolutionary narrative: humans are often said to have evolved bipedalism to free the hands for tool use, carrying, or other adaptive advantages. By substituting masturbation as the “reason,” the joke punctures solemn scientific storytelling and highlights how easily people retrofit grand explanations to bodily behavior. It also plays on taboo and surprise—using a private, stigmatized act to undercut respectable discourse—while implying a satirical view of human motivation: that pleasure and self-interest can be as powerful as lofty ideals. The humor depends on the clash between academic phrasing (“We have reason to believe…”) and crude subject matter.