Thirty-five thousand years ago is about the time that our direct Cro-Magnon ancestors were displacing Neanderthals in Europe. They had something going for them — more agile minds? language? imagination? Maybe the source of their success was not reproductive efficiency, as such, but eroticism. That is to say, maybe the conceptualization of sex was a driving engine of cerebral facility and language. The Playboy bunny. The Harlequin romance. Foreplay. Dirty dancing. Maybe sexual fantasy prepared the way for art and religion and technological innovation. Maybe the brain evolved as a sexual organ, and then found other things to do.