Kissing, Cliterally

Tracy Clark-Flory interviews Sheril Kirshenbaum, author of The Science of Kissing:

Certain parts of the female anatomy, especially with our primate ancestors, were enhanced with red, and it especially had to do with the female being ready to reproduce. As our ancestors began to walk upright, rather than males being attracted to the female's posterior, they began to focus on the breasts and the lips -- they call this "genital echoes." In research on lip color, men consistently choose the women wearing the bright red lipstick as the most attractive -- there's this power to making the lips slightly redder. There's a lot of evidence to back up the existence of the makeup industry.

Kirshenbaum also connects kissing to breastfeeding:

Nursing is a very pleasurable activity. The lips are so sensitive to stimulus, and the hormone oxytocin, which is involved in social bonding and attachment, is stimulated in the infant and the mother during nursing. We start to associate this bonding with lip pressure.