Taken from SUSANNAH CAHALAN's review of "The Science of Kissing: What Our Lips are Telling Us" by Sheril Kirshenbaum (Grand Central Publishing). Read the whole thing here.

The history of the kiss

The first written kiss was documented in 1500 BC — when there wasn’t even a word for a mouth-to-mouth exchange. The earliest mention comes from India’s Vedic Sanskrit texts, which talked about “smelling with the mouth” and reads “young lord of the house repeatedly licks the young woman.”

By the 3rd century AD, the most important sex-text to hit the world, the Kama Sutra, laid out the guidelines for a good kiss, devoting an entire chapter to the act. It told readers where to kiss the body: “The forehead, the eyes, the cheeks, the throat, the bosom, the breasts, the lips and the interior of the mouth.” And it laid out three types of kisses: “the nominal kiss, throbbing kiss, touching kiss.”

In the Middle Ages, kissing again underwent a makeover, where it was employed as a legal way to seal a document. Men were often illiterate, so instead of writing their names, they would draw an X and kiss it to make it legal (and that’s where we get x as the symbol for kiss).

Kissing lost its allure during the 17th century, when the Great Plague was at its height in London, as people opted to tip their hat, wave or bow to avoid making physical contact. It recouped its vogue in Hollywood, around the time that “French Kiss” entered English vocabulary; now heroes and heroines were often waiting for a kiss to complete the narrative.

It may feel like the most natural thing in the world, but kissing is far from easy. To undertake the task, six major muscles around the mouth help to pucker up the upper lips, pull up the corners of the mouth, and pull down the lower lips. And don’t forget the effort it takes to cross your fingers that it’s worth it.

It’s not just the mouth; the whole body is involved in the act of kissing. When lips hit lips, five of the 12 pairs of cranial nerves, responsible for smell, taste, vision and facial expressions, are excited. Blood vessels expand, cheeks flush, pupils dilate, and the heart pumps a higher level of oxygen to the brain.

Meanwhile, the tongue, with its nearly 10,000 taste buds, is taking in the other person, while the lips, jam-packed with nerve endings, send messages to the limbic system of the brain, responsible for love, passion and lust.

All the while, neurotransmitters and hormones — like dopamine (brings us feelings of pleasure), oxytocin (fosters feelings of attachment), serotonin (also regulates emotion like dopamine) and adrenaline (which boosts heart rate and prepares body for sex) are coursing through our systems. These things are responsible for the butterfly, roller-coaster-ride effect that some kisses have.

About two-thirds of people tilt their heads to the right. Although scientists don’t know for sure why, some speculate that it is something we adopt in utero and others believe that the behavior stems from which side our mothers nursed.

It’s not easy, causes stress on the body, and there’s always the problem of spreading germs, so why do we do it?

One theory is that the act of pursing lips reminds us of the calmness, comfort and attachment of nursing, a Freudian thought that might actually have some validity, Kirshenbaum says. Another theory along the same lines says that kissing reminds us of an old habit of “premastication” wherein a mother chews the food and then feeds her baby via the mouth.

But, really, the most important and obvious reason why we kiss is that it facilitates reproduction. Women, who according to studies place more emphasis and importance on a kiss, use the mouth-to-mouth moment as a way to judge the taste of the tongue, lips and saliva to see if she is with an adequate mate.

Sense of smell doesn’t just provide a window into hygiene habits, it also gives women access to the unseen DNA of their chosen mate. According to recent studies, women can smell when a man’s group of genes that manage the immune system, called MHC, are matched well to her own. Scientists theorize that kissing may be so ubiquitous because it gives women an instant check on if there is chemistry, literally (or less poetic terms, if they would make good children together).