When people kiss, they exchange millions of bacteria. Scientists believe that kissing tests the compatibility of partners - that the purpose of kissing is to exchange saliva. But where did we get such behavior from?
In stories, a kiss can turn a frog into a prince or wake a sleeping beauty from a coma. Symbolically, kisses are a key point in personality development - at the moment when they are at the crossroads between child and adult, preparation for romance and reproduction.
In relationships, kissing creates a physical intimacy that may touch us more than words of love. Kisses are also sacred to many. Prostitutes, the movies tell us, don't kiss on the lips – they save that level of intimacy for their real lover.
But kissing can also be unromantic. A kiss on the child's forehead is gentle and caring, while a kiss on the cheek can be a greeting. Kissing someone's feet or the ground in front of them shows humility and submission — a way of showing respect and awe to gods and tyrants.
The problem is that kissing becomes a bit gross when you zoom in on the mouth. A 10-second kiss exchanges 80 million oral bacteria. Phew. In this way, the disease is very often transmitted.
So why do we do it?
The first kiss in the history of mankind
Troel Arbol, an Assyriologist from the University of Copenhagen, Denmark, says: "The first evidence we have of mouth-to-mouth kissing dates back to around 2.500 BC. It appears in a mythological text from Mesopotamia, ancient Iraq. The text describes two gods mating and they kiss. It's definitely a sexual encounter," Arbol told DW.
Arbol and co-author of the text Sophie Rasmussen presented a new view of the ancient history of kissing and published it recently in the journal Science. They have a new theory that romantic kissing developed in multiple ancient cultures over several millennia. In addition to Mesopotamia, texts also depict erotic kissing in India and Egypt from 1.500 BC.
Arbol argues that sex and kissing began to appear more frequently when writing evolved from an administrative tool into a way to tell stories. Kissing, for example, is dramatically depicted in the Epic of Gilgamesh, one of the oldest written stories, written around 2.000 BC. As in contemporary interpretations, intimacy is transformative – the wild man Enkidu meets the holy prostitute Shamhat – the sexual act with her transforms him into a man.
While kissing is found very early in some kind of erotic literature, Arbol also finds it in the daily documentation of life in ancient Mesopotamia.
"For example, an unmarried man or woman should be separated and not kiss. Societies have tried to regulate the romantic kiss. The fact that it is regulated clearly means that it was a common practice in everyday life," said Arbol.
Is kissing innate?
Arbol hypothesizes that romantic kissing may not be an innate human behavior, but rather develops in complex societies as a learned mating behavior.
"It appears that kissing is not universal across cultures. It coincides with the increased complexity of social interactions," he said, citing both historical texts and recent data that 46 percent of human cultures do not kiss in a romantic sense.
But some anthropologists believe that kissing is innate, at least unromantic. Behaviors such as licking and sniffing are common in mammals such as cats, dogs, elephants and monkeys. Dwarf chimpanzees (bonobos) kiss like humans, on the lips, for comfort and companionship, even after a fight as a sign of reconciliation.
Experts believe that romantic kissing may have evolved from this type of kissing in higher mammals. It is the same during our development. We first experience our parents' love through kisses and hugs before we redirect the behavior to our adult lovers.
Sickness and kissing
New evidence suggests that kissing may have played a role in facilitating disease transmission throughout human history. Kissable pathogens such as herpes simplex virus 1 (HSV-1) and Epstein-Barr virus (also known as kissing disease or glandular fever) have been found in human remains.
Arbol points to evidence that HSV-1 changed in the Bronze Age, possibly due to the increasing prevalence of romantic kissing. More recently, kissing has played a role in the transmission of the COVID-19 coronavirus, prompting China to ban kissing and the French to avoid greeting each other.
For Thui Do, who specializes in oral health microbiology and works at the UK's University of Leeds, it's not surprising that kissing is a channel of disease transmission.
"We all have 800-900 different types of microbes living in our mouths," Do told DW. "When we kiss, we exchange a lot of saliva and all kinds of microbes. There is a risk of disease transmission if viruses such as hepatitis and HSV-1 are exchanged."
Pleasure and sex
However, it's really not all that repulsive. Do explained that a healthy mouth requires a balanced microbial environment, and kissing and sharing microbes with our partners can actually be an important way to maintain a healthy diversity of microbes in the mouth.
"Certain types of bacteria such as streptococcus salivarius can help reduce inflammation. People with a high number of health-relevant species tend to have healthier mouths," Do said.
She thinks kissing doesn't just benefit oral health.
"The mouth is the gateway to the whole body — it's connected to the microbiome of our gut and skin. So when you kiss, it can have a positive effect on the microbiome throughout the body, even our brain and mood," Do said.
Scientists believe that kissing is a way of testing potential partners. Kissing allows us to assess a partner's genetic fitness or general health by taking biological cues from saliva.
If the saliva is compatible, sex can follow. Tactile inputs from the tongue and lips trigger an ancient whole-body response. The brain's pleasure and reward centers are engaged, accompanied by the release of hormones such as oxytocin, dopamine and serotonin. Your skin flushes, your heart rate quickens, your pupils dilate, your eyes glaze over. Time to close the bedroom door.
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