Philematologists, the scientists who study kissing, aren’t exactly sure why humans started locking lips in the first place. The most likely theory is that it stems from primate mothers passing along chewed food to their toothless babies. The lip-to-lip contact may have been passed on through evolution, not only as a necessary means of survival, but also as a general way to promote social bonding and as an expression of love.

But something’s obviously happened to kissing since the time of the chewed-food pass. Now, it’s believed that kissing helps transfer critical information, rather than just meat bits. The kissing we associate with romantic courtship may help us to choose a good mate, send chemical signals, and foster long-term relationships. All of this is important in evolution’s ultimate goal—successful procreation...