A kiss is the climax of the movies, the moment that crowns a victory, the gesture of passion and also of betrayal, the one that seals a wedding and the one that says goodbye at the boarding gate. Kissing can be an everyday act, yet we rarely stop to think about how strange it is to press one mouth against another. A study by scientists at the University of Oxford, published in the journal Evolution and Human Behavior, asked where it came from, and the answer took the researchers 21 million years back.

In search of the oldest kiss

For a long time, asking about the age of the kiss meant asking about archives. And the result of that search leads us to a written reference found on cuneiform tablets from ancient Mesopotamia. The oldest recorded kiss in human history happened there 4,500 years ago.

But the oldest record is not the origin. A kiss leaves no fossil and gets trapped in no permafrost, and the fact that it was written down thousands of years ago only proves that kissing was already a well-established practice. The origin, the moment the act first appeared, has to lie much further back.

The first thing the team led by Matilda Brindle, an evolutionary biologist at the University of Oxford, had to do was decide what counts as a kiss and what does not. Since they were going to compare very different species, they needed a definition that worked for all of them, so they settled on this one, non-aggressive mouth-to-mouth contact with no food transfer. With that definition, the team found kisses everywhere. It documented mouth-to-mouth contact in chimpanzees, bonobos and orangutans, but also far from the primates, in wolves, prairie dogs, polar bears and Galápagos albatrosses.

Bonobos and chimpanzees are the most like us in this, because they kiss in sexual and platonic contexts and even as a sign of reconciliation, some with tongue. Brindle was surprised by how far the gesture reached. “It’s important for us to understand that this is something we share with our non-human relatives,” she told the BBC, adding that it shouldn’t be dismissed as silly just because in humans it sounds like romance.

The method

Here is the interesting part. The researchers used a method called Bayesian modeling, simulated different evolutionary scenarios along the primate family tree and ran the model ten million times so the estimates would be robust. In other words, they built a simulator of the past and let it run until they could see at what point in the family tree the gesture might have appeared. That said, it’s worth keeping in mind that this is a well-founded reconstruction, but still an estimate of probabilities.

Did we kiss Neanderthals?

Every human of non-African descent carries between 1 and 2 percent Neanderthal DNA, proof that interbreeding happened. But the most telling clue is in the mouth. Modern humans and Neanderthals share an oral microbe, Methanobrevibacter oralis, whose strains diverged between 112,000 and 143,000 years ago, long after the two species split. That this microbe kept passing from one species to the other so long afterward implies close and repeated contact. That exchange could have happened in three ways, by sharing food, by premastication, or by kissing. According to the Oxford model, with an 84.3 percent probability, we most likely kissed too.

What is a kiss for?

From a biological standpoint, the kiss is almost a contradiction. It transmits diseases, exposes us to microbes, and offers no obvious reproductive advantage. And yet here it still is, after millions of years.

One of the hypotheses the study revisits is that kissing lets us read, whether we notice it or not, the genetic fitness, the immune compatibility, the health and even the cycle phase of the other person. Another possible explanation sees it as an act of trust, a way of weaving a social bond.

But the question remains open. Catherine Talbot, co-author of the study, puts it this way. “Although kissing may seem like an ordinary or universal behavior, it is only documented in 46 percent of human cultures. Social norms and contexts vary so much from one society to another that it raises the question of whether kissing is an evolved behavior or a cultural invention.” And that, she says, is only beginning to be investigated.

That is the mystery of a gesture we feel as so much our own and yet share with bonobos, with polar bears, and with distant ancestors who kissed in Miocene forests that no longer exist.