Smooth as a baby's bottom

There is a saying "smooth as a baby's bottom". As I was changing G.'s nappy today, it occurred to me that there is a very sound evolutionary basis for this feature. A baby with a hairy bottom would not work. Back at the dawn of time, the hairy bottom strain must have been eliminated early. Poor old hairy bottom cavedad was on his 53rd wipe as the sabre tooth tiger sidled up.