Fact: Cytomegalovirus, found in saliva, is generally innocuous unless you happen to be a child in utero, whom it can afflict with a range of maladies from a birth defects to death.
Fact: Kissing helps build up a woman's immunity to Cytomegalovirus, thus protecting the child she carries.
Conclusion: Kissing evolved to spread the "love bug," not human affections.
I guess we should all be thankful to "Lucy" and her mate for the foresight to endure a little "saliva-swapping" for the sake of the species. But come to think of it -- how did all this teleology get smuggled into a process that is blind and undirected?