“Within the relatively short period during which our ancestors and the ancestors of our great ape relatives have been evolving separately,” Diamond explains, “along with posture and brain size, sexuality completes the trinity of the decisive respects in which the ancestors of humans and great apes diverged. . . . Recreational sex . . . was as important for our development of fire, language, art, and writing as were our upright posture and large brains” [emphasis added].
