I don't think there's any more reason to think that we've been kissing forever than to think we've been nodding up and down for "yes" and side to side for "no"

I don't think there's any more reason to think that we've been kissing forever than to think we've been nodding up and down for "yes" and side to side for "no" forever. There are many people in the world who do not kiss, or who nod up and down for "no" and side to side for yes. Just because a behavior is shared by 99% of people doesn't make it necessarily intrinsic, or even historically likely or useful. Also, as far as I can tell from anthropological films of rural tribes/bands with less contact, none of them ever kiss. Mouths that aren't constantly scrubbed out are gross.