the science of kissing

"When it comes to humanity's first kiss, or its predecessor in another species, we have no way of knowing exactly how and why, once upon a time, it happened. After all, there are kisses of joy, of passion and lust, of love and endearment, of commitment and comfort, of social grace and necessity, of sorrow and supplication. It would be silly to assume all these different types of kisses developed from a single behavior or cause; in all likelihood, we kiss as we do today for multiple reasons, not just one."