http://liberalarts.tamu.edu/html/news--first-kiss-how-we-learned-to-lock-lips.html?utm_source=tamutimes&utm_medium=email&utm_campaign=2012-02-14
My gut tells me that since there are many mammals that lick their young, I would suspect that to be more the root of kissing. It is often a tender "look" on the mother's face as she grooms her young. People probably got the idea that using your tongue and lips to convey similar feelings was worth a try.