Some time ago Sciencewoman, who has a new baby, posed me a question about the evolutionary origins of affectionate versus erotic kissing. I'm not really sure what she was getting at, but my sister, who also has a new baby, chatted with me recently about how her son is a very affectionate little (open-mouthed) kisser. So, it got me thinking.
Of course, my perspective is heavily influenced by my knowledge of primates and also the relative lack of scientific investigation on the subject of kissing. In other words, what I am about to say about kissing represents my best theory, albeit a speculative one.
What we know about babies, humans or other primates, is that they love to explore the world through their mouths. Lips contain some of the highest concentrations of nerve endings found anywhere in the body, which suggests that we are programmed to derive, if not seek, much sensation from our lips. For infants, this makes nursing rewarding on a sensory level. I don't think it matters terribly, from an infant's perspective, what those lips land on - as long as at some point they land often enough on mom's nipples to receive enough food to stay alive. I'd argue that infants are programmed to explore life open mouthed. Only later do we learn to associate that sensation with other kinds of physical pleasure. We also learn later when it is not okay to associate open-mouthed exploration with pleasure.
As human adults, we've learned that exploring others' bodies with our tongues, lips, and mouths is inappropriate to do with just about everyone, and even for those granted special oral stimulation recipient status, we have learned that there is a time and place for that kind of activity. Not so for another great ape - the bonobo.
Bonobos, also called pygmy chimps, are close genetic relatives of chimpanzees and humans. In fact, bonobos are as closely related to humans in terms of DNA as they are to chimpanzees. Although they look a lot like chimps to the untrained eye, there are some marked differences between the them. They've got more human-like faces and expressions, including bright red lips. Bonobos are also gentler with each other. Unlike chimpanzees, bonobos have never been known to engage in gang warfare or organized hunts. They live relaxed lives by comparison even though they also live in the same large social groups as chimpanzees, along with all of the stressors - food competition, social hierarchy, and the like.
Rather than allowing competition to boil over into aggression, bonobos "make love, not war" as one well-known primatologist (Frans de Waal) has said of them. Basically bonobos are a very physically affectionate bunch. They kiss each other open-mouthed, including with tongue. They have sex, including oral, with each other without regard to age or who is male and who is female. In other words, homosexuality and pedophilia are the norm.
Some scientists have speculated that bonobos are "paedomorphic" which is to say that they retain infantile or youthful traits into adulthood. Paedomorphic traits are those that are characteristic of infants and children. Humans are also believed by some to be paedomorphic. For a comprehensive explanation of paedmorphism, see Stephen Jay Gould's 1977 book Ontogeny and Phylogeny.
I believe there is no huge evolutionary difference between open-mouth exploration of other's bodies, including "French kissing" and erotic kissing, and that of the non-erotic "affectionate" kissing. We begin life biologically prepared to do the first and only learn later that our culture reserves that form of affectionate expression and sensation-seeking for people we are romantically involved with. Biology prepares us to follow the rules that culture enforces.
The rules of display vary immensely from culture to culture and even in sub-cultures. Some allow open-mouthed kissing in public; others expressly forbid even closed-mouth kissing in public. Some allow open-mouthed kissing between people of the same sex; others forbid it and punish it with death. Most forbid open-mouthed kissing between an adult and a child. Yet, it still occurs.
But back to the beginning of the story - kissing of any kind is ultimately tied up with the original function of the lips - to nurse and to eat. In some cultures, mothers (and I imagine fathers too) masticate food before giving it to their babies to eat. When there is no Gerber baby jar to reach for when the baby can finally begin to have solid food and is being weaned from milk, masticating food in a parent's mouth is a great way to solve the problem. Later it may be the only way to get a teething baby the most precious form of nutrition: meat. Rather than passing masticated food to the baby with fingers that may be dirty, food is passed mouth-to-mouth. It's easier and cleaner and very pragmatic, just the solution evolution favors.
So how then do you get from kiss-sharing food to adult romantic kisses? Perhaps kissing can be explained by paedmorpism. It's just a childish ritual adults have stretched into adulthood because it's what our species does, kind of like bonobos. But I think there's a little more to it. I think it's hi-jacked behavior - an exaptation, not an adaptation. Originally, kisses solved an important problem of getting food, but once humans started to form pair bonds and parent with a mate, kissing took on a new ritual, that of courtship.
If you consider that adult courtship rituals all over the world involve the kind of caregiving behaviors we see parents displaying for their kids, you'll recognize kissing is an effective form of courtship and pairbonding because it signals a desire and ability to care for the children that union might produce.