Don't forget the evolution of opposable lips.
Not only were lips necessary for early humans to articulate sound and speech, but some of the more inventive of our ancestors realized that human lips were a tactile sensory organ, and could be erogenous when used in kissing and other acts of intimacy.
"Through making a narrow funnel with the lips, the suction of the mouth is increased. This suction is essential for sucking on a straw to drink liquids. Lips can also be used to suck in other contexts."
As the opposable thumb developed the hand into a fist for the more aggressive of our Urfathers, others (you know the type) were experimenting with their opposable lips, "finding that their lips had many nerve endings and reacted as part of the tactile senses. Lips, they soon discovered, were very sensitive to touch, warmth, and cold, and therefore an important aid for exploring unknown objects."
If early man enjoyed clenching his fist, yelling "Drop that Babe," and then pounding the shit out of some other guy to win her, imagine how much more pleasure he got when his opposable lips delved into hers.