It's the sucking reflex combined with another primitive response, the rooting reflex, that allows infants to breastfeed. The rooting reflex works by turning the infant's head to face anything that strokes its mouth or cheek. As soon as something grazes the newborn's lips, the sucking reflex is activated. While the tongue then does a lot of the work, the lips are vital to maintain a tight seal so that the infant can swallow.

That means feeding, whether from breast or from bottle, is not a passive behaviour on the part of a newborn infant. It's more like a conversation, with each side doing his or her part in a dance elaborately choreographed by evolution. At the centre of that dance are the lips.

Read my lips

The lips are of course also important in the act of eating other foods, and in speech. In linguistics, the lips are two of many places of articulation, or spots in the mouth and throat that aid in blockage of air moving out from the lungs. Bring your two lips together and you can form the sounds p, b, and m. To make the sounds f or v, bring your lower lip to your upper teeth. To make a w sound, move the back of your tongue towards the roof of your mouth while also moving your lips closer together.

Speech is arguably a critical part of human life, but it's not quite as much fun as kissing. Kissing isn't universal, though it does pop up in some 90% of cultures. Darwin himself noted that there are cultures in which kissing is conspicuously absent. "We Europeans are so accustomed to kissing as a mark of affection that it might be thought to be innate in mankind," he wrote in The Expression of Emotion in Man and Animals, "but this is not the case…[it is] unknown with the [Maori] New Zealanders, Tahitians, Papuans, Australians, Somals of Africa, and the Esquimaux [Eskimo]."