NEW YORK — Birds do it. Bees do it. No, not that! We're talking about kissing, the simple gesture with a wallop that spans time and place but remains largely unexplained.
Anthropologists have their theories. So do neurologists, biologists, psychologists and endocrinologists. Einstein was interested. Darwin, too. So why doesn't anybody know how it all began and why we do it in the first place?
Sheril Kirshenbaum, a researcher at the University of Texas at Austin, has compiled a motherlode of fragmented studies and observations from historians and sociologists, brain experts and animal-watchers in a surprisingly slim and definitely curious new book, "The Science of Kissing: What Our Lips Are Telling Us."
Her conclusion? Inconclusive. The act of "osculation" – in technical parlance – is ingrained in more than 90 percent of cultures around the world. If they don't place lips on lips – or lips elsewhere – they lick or nibble with the same goals in mind.
If we could unravel its origins, Kirshenbaum surmises, we could unlock a trove of evolutionary and physiological mysteries that might carry the kiss from merely interesting to incredibly valuable.
Scientists can't decide whether kissing is instinctual (newborns pucker for their first taste of mother's milk), cultural (learned, that is, for joy or survival) or deeper still (ingrained in our very DNA) – or all of the above.
They suspect, Kirshenbaum said, that the practice has come and gone through the ages and might have surfaced as an outgrowth of sniffing as a way to suss out the familiar. The first kiss as greeting, according to some anthropologists, might have been a nose-to-nose exchange to recognize, reconnect or check on a person's health through smells.
The color red may also play a prominent role in the rise of the kiss. The hue takes us back millions of years to "red as reward" for ancestors in search of ripe fruits amid leaves and bush. It's possible that over all those years of man learning to walk upright, he also became hardwired to appreciate the flashy color, primed to seek it out wherever it occurred – including the everted, red lips on a woman's face and other parts of her anatomy.
Gender when it comes to the kiss was something of a surprise to Kirshenbaum.
"The gender differences stood out," she said. "I don't like gender stereotypes at all but I saw so much research along that divide. Men tend to describe kissing as more of a means to an end, hoping it leads to more, whereas women tend to place a lot more emphasis on the act of kissing itself."
Kissing was first documented in human societies around 1500 B.C., in India's Vedic Sanskrit texts that serve as the basis of the Hindu religion. One describes the practice of smelling with the mouth. Another recounts how a "young lord of the house repeatedly licks the young woman," which could symbolize a kiss or related caress, said the kissing detective Kirshenbaum.
By the end of the Vedic period, she said, Satapatha Brahmana talks of lovers "setting mouth to mouth," and early Hindu law reprimands a man for "drinking the moisture of the lips" of a slave woman. From there, kissing – in India, at least – marched on to the Kama Sutra, a sex guide of the third century that covers a whole lot of ground and includes an entire chapter on kissing alone.
India wasn't only the only hotbed of kissing. In the world of Herodotus, according to his fifth-century B.C. text the "Histories," Persian kisses ranged from lip on lip for equals to the ground or feet by an exorbitantly lower status person to a higher one.
A Babylonian creation story recorded on stone tablets in the seventh-century B.C. – based on much older oral legends – includes references to a kiss of greeting and a kiss of the ground or feet in supplication, Kirshenbaum said.
The Roman emperor Caligula had subjects kiss his feet, which was also a custom throughout the Middle Ages. Charles Dickens was no fan in 1861, when he found foot-kissing of popes in the Catholic Church a "slavish self-abasement."
Mythology, literature, the visual arts are all full of kisses, let alone both testaments of the Bible. Derided by some through the ages as dirty – or worse – lips on lips went where Europeans ventured, and Western-style kissing spread to much of the world. Kissing turned into handshaking during the Great Plague years in 1660s London.
Kirshenbaum estimates that today more than 6 billion of us – East and West – lock lips socially or romantically on a regular basis. The German language alone has 30 words for kissing, including one, "nachkussen," which is a kiss to compensate for those that have not occurred, according to Kirshenbaum.
Human lips, she learned, are packed with nerve endings that are extremely sensitive to pressure, temperature and other means of stimulation. They're the perfect little engines – and erogenous zones – since the slightest touch stimulates a very large part of the brain.
More than half of men and women – 59 percent of men and 66 percent of women – have ended a relationship because of a bad kiss, according to one study cited by Kirshenbaum.
"It really serves as a litmus test for our future together," she said. "Very often we feel like we're with the perfect person and our lips meet and often it doesn't feel right. There's something not magical there."