Judas did it to Jesus, Britney did it to Madonna, Prince Charming did it to Sleeping Beauty. Birds do something like it, bees don't, and Bonobo apes have been observed doing it for 12 minutes straight (they prefer the tongue-sucking method). This Valentine's Day, you might be doing it, too.
Kissing is the universal language. It is mirrored in the animal kingdom and in human terms it is arguably the most evocative behaviour we exhibit towards each other. Studies show that people remember the details of their first kiss more clearly than they do of any other of life's firsts; including first sexual experience.
Despite its deep roots in human culture, until recently kissing was a mystery. Scientists have only started to study it in the last few decades and there is still no definitive answer as to why we kiss; except that perhaps we do it because it feels good.
In fact, though, the mouth-to-mouth kissing we are familiar with is a relatively recent global phenomenon. Just over 100 years ago there were still many cultures in the world where smooching was alien. Nineteenth-century explorers discovered several civilisations which had never encountered kissing and instead had their own idiosyncratic romantic practices.
In his 1864 book, Savage Africa, British explorer William Winwood Reade described how an African princess he fell in love with thought he was trying to eat her when he approached her for a kiss. Reade also described how one tribe he encountered greeted each other by talking in baby language and patting each other's chests; "the kiss is unknown among the Africans", he surmised. In 1929, anthropologist Bronislaw Malinowski visited the Trobriand Islands and discovered that lovers there would go through several phases of sucking and nibbling during intercourse before biting off each other's eyelashes at the point of orgasm.
Many cultures newly introduced to the Western kiss found it abhorrent. Around the same time that Reade was bothering African royalty, another traveller, Bayard Taylor, studied Finnish tribes and observed that while naked inter-sex bathing was commonplace, kissing was considered indecent.
By the 1970s, however, these remote no-kissing zones had all but been wiped out. A study found that 90 per cent of the world's cultures kissed mouth-to-mouth; that figure is now assumed to be closer to 100 per cent. At some point in the past 40 years, mass media homogenised kissing.
Philematologists – people who study kissing – record that the earliest reference to kissing-like behaviour is found in the 1500BC Vedic Sanskrit texts from India. Although there was no word for 'kiss' back then, the Hindu documents refer to the act of 'smelling with the mouth'. Kissing was big in India. Another Hindu text describes lovers 'setting mouth to mouth' and in the fourth century BC, the epic Indian poem Mahabharata describes affectionate mouth kissing. The act is also referenced in seventh century BC Babylonian stone tablets and in ancient Greek in the works of Homer. Several centuries later the lip kiss was well on its way to global dominance thanks to invading Roman soldiers who introduced the practice to the nations they conquered.
Over the years kissing has challenged great thinkers from Jonathan Swift to Charles Darwin, who believed that puckering up was an innate human act encoded in our genes.
Modern thinking is that kissing is both nature and nurture, and has evolved over human history. One theory suggests that cavemen licked each other's cheeks as a way of obtaining salt and another theorises that, in the days before lightbulbs, people had to get close and sniff each other to recognise family members. This brush of the face with the nose is thought by some to have evolved into the European-style social kiss.
According to Rutgers University Anthropologist Helen Fisher, kissing evolved to facilitate three essential needs: sex drive, romantic need and attachment. Each is a component of human reproduction and kissing bolsters all three. In this theory, kissing helps people find a partner, commit to them and stay with them long enough to have a child.
Sheril Kirshenbaum is the scientific queen of kissing. She is a biologist from the University of Texas and author of The Science of Kissing. She says: "The truth is we don't know exactly when and where kissing began but it is probably something that arose and disappeared throughout human history for a variety of reasons. Humans seem to have an instinctive drive to connect with each other in this way but the style and shape of it is informed by culture and experience.
"When an infant is born, his or her first experiences of love and comfort and security usually involve some kind of kissing, so from a neuroscience perspective we are hard-wired at an early age to associate these positive emotions with lip contact."
Before formula milk, the most sustained contact a baby had with its mother was usually through nursing and in many cultures, before baby food was widely available, parents pre-chewed food and passed it into their infant's mouth by tongue.
As an individual grows, kissing develops to fulfil a range of functions. In children it is used playfully as a way to bond with peers. Research by Goldsmiths College found that five-year-old boys used playful hugging and kissing in the same way that adolescent boys play-fight to reinforce masculinity. The study, carried out on reception classes in two schools, found that boys were far more likely than girls to make physical contact.
In adulthood, kissing also has multifaceted purposes. When a woman kisses a man, the contact enables her to gauge his suitability as a mate by picking up on his hormonal markers.
Kirshenbaum explains: "Our sense of smell tells us a lot about other people and whether they may be a potential partner and even a genetic match. It happens on a subconscious level and kissing puts us in the closest proximity possible to get a sample. Women have a stronger sense of smell and taste and when we are kissing we use the information we get from our senses. It is nature's ultimate litmus test."
Endocrinologists have found that through kissing, women can sample a section of a potential partner's genome called the major histocompatibility complex (MHC). The MHC are codes for the immune system and women are attracted to the scent of men with a different MHC to themselves.
"The advantage is that if you pair up with someone who has a diverse MHC, your children will be healthier and more likely to survive," adds Kirshenbaum. It is also believed that kissing allows partners to smell each other's pheromones.
Kissing styles vary from culture to culture – Inuits prefer a sucking motion – but the open-mouthed French kiss has distinct advantages for men. Saliva contains small amounts of testosterone and it is thought that if a man kisses a partner repeatedly with an open mouth, over time he will pass on a quantity of the hormone. Women are more sensitive to testosterone than men and over weeks and months this raised level of testosterone will increase her libido and make her more sexually receptive.
Results of a psychological study on student's attitudes towards kissing at Albany University in New York, led by Gordon Gallup, reinforce these biological theories. The men surveyed overwhelmingly described kissing as a means to a sexual end whereas women reported that kissing allowed them to gauge how a prospective partner felt about them and whether the relationship was worth pursuing.
So why does the act of kissing feel so good? Biologically, when we lock lips, our bodies erupt with a cocktail of feel-good neurotransmitters such as endorphins, dopamine, serotonin, adrenaline and oxytocin; also known as the love hormone. It is also an activity which engages all the senses.
Kirshenbaum says: "People remember their first kiss more than their first sexual experience probably because kissing is such an active process. Our brains are totally engaged during kissing. The amount of neuro real estate associated with our lips is disproportionately enormous and when we have those really important kisses we are most likely coding them so we can remember the experience later."
Although there's no formula for the perfect kiss, Kirshenbaum explains that the best kisses rely on the right frame of mind. "Stress and kissing don't mix," she says. "There needs to be anticipation and an understanding of the desires between partners."
Kissing is an evolving act and 22nd-century kisses may look very different than the smackers we give each other today, relying more on technology than on human touch. Scientists in Japan (where else?) have already invented a kissing machine.
Earlier this year inventors at the Kajimoto Laboratory in the University of Electro-communications in Tokyo unveiled a prototype machine designed to mimic the feeling of a French kiss. The 'Kiss Transmission Device' recorded the movement of a kisser's tongue using a straw-like device and mimicked this movement in the mouth of a recipient using another machine. The 'kiss information' recorded by the device for different individuals can be freely replayed.
"For example, if a popular entertainer uses this device and records it, that could be hugely popular if you offer it to fans," says researcher Nobuhiro Takashi. "The elements of a kiss include the sense of taste, the manner of breathing, and the moistness of the tongue. If we can recreate all of those I think it will be a really powerful device."
At present our clunky technological attempts to recreate kissing are about as romantic as a clinch with Wall E, but perhaps in the not-too-distant future we could all own an electronic mouth, download a Britney kiss from iTunes and snog each other via Skype.
Secret world of snogging
* The word kiss comes from the Old English cyssan from the proto-Germanic kussijanan or kuss, which is probably based on the sound kissing can make.
* On Valentine's Day last year, a couple in Thailand (pictured below) locked lips for 46 hours, 24 minutes, and nine seconds, making it the longest kiss ever recorded.
* A woman in China partially lost her hearing after her boyfriend reportedly ruptured her eardrum with a passionate kiss. Apparently, the kiss reduced the pressure in the mouth, pulled the eardrum out, and caused the breakdown of the ear.
* French kissing involves all 34 muscles in the face. A regular kiss involves only two.
* Passionate kissing burns 6.4 calories a minute.
* It is possible for a woman to reach an orgasm through kissing.
* During the Middle Ages, witches' souls were supposed to be initiated into the rites of the Devil by a series of kisses, including kissing the Devil's anus.
* The mouth is full of bacteria. When two people kiss, they exchange between 10 million and 1 billion bacteria.
* The average person spends about 15 days kissing throughout the course of a lifetime.
* Alfred Wolfram holds the record for kissing the most people. In 1990 he smooched 8,001 people in eight hours.