Ingrid Bergman is said to have uttered one of the most notable definitions of the act of touching lips: “A kiss is a lovely trick designed by nature to stop speech when words become superfluous.”

Some scholars, though, have tried to remove any ambiguities from words related to the kiss by consulting cuneiform text on clay tablets dating back more than 4,500 years.

A commentary article published on May 18 in Science recounts how textual references to kissing are appearing further back in time, drawing nearer and nearer to the invention of writing itself. The first documented evidence of kissing is often cited as coming from a text from India in 1500 B.C.E. But the article points to a “substantial corpus of overlooked evidence” about the kiss in Mesopotamia and Egypt from at least 2500 B.C.E., discovered in tales of smooching depicting both gods and commoners.

A kiss can leave behind more than just the magic of the moment. Memories may be supplemented by oral herpes or the Epstein-Barr virus. It is surmised that Neanderthals and modern humans could have touched lips more than 100,000 years ago because of the presence of the microbe Methanobrevibacter oralis in both species, says Troels Pank Arbøll, an Assyriologist at the University of Copenhagen who studies the history of medicine in ancient Mesopotamia and co-wrote the article in Science. Arbøll spoke with Scientific American about the ancient origins of the romantic kiss.

[An edited transcript of the interview follows.]

What is the first documentation of the romantic kiss?

The earliest evidence is in a language called Sumerian, which doesn’t have any modern language relatives. And then shortly after that, we also found some passages written in an ancient Semitic language called Akkadian that is related to Hebrew and Arabic today. Somewhere before 2500 B.C.E. we start finding these mythological texts written down— narratives about gods as well as incantations. It’s in these narratives that we find copulation and kissing, so that shows us it’s clearly a sexual kiss.

A few centuries after these early references, we start getting more private documentation outside of states and temples where we see references to the sexual kiss practiced by people in society.

There are two types of kissing. What’s the difference between the two?

We differentiate between what is called the “parental kiss” or the “friendly kiss”—so that would be parents kissing their children, as well as friends kissing each other, and acts of submission, which we have a lot of in ancient Mesopotamia, where you kiss the feet of the ruler, for example.

The sexual-romantic kiss: that’s kissing in relation to a sexual act or in relation to love.

Do all societies kiss?

In anthropology, studies have shown that the sexual-romantic kiss is not universal in all human cultures, while the parental kiss seems to be relatively ubiquitous. Researchers have found a tendency in modern societies for the sexual-romantic kiss to be practiced where there is a high degree of social stratigraphy, lots of [social] classes, where people will come into contact with others that they don’t know.

It’s difficult to say if [kissing] was universal. The societies that created these written references are also often societies with a high social stratigraphy. So it could confirm the same tendency we see today, but it might be an accident.

The first documentation of a kiss isn’t the same thing as the first kiss, so what do we know about when kissing really started?

In previous studies it was suggested that the earliest documentation was from India, and the idea was that kissing was brought out of India to other areas in a diffusion process. But now that we have this evidence from Mesopotamia and Egypt, you can’t really talk about a single point of origin in historic times. When we look in prehistory, we can see a few figurines that seem to indicate that kissing was practiced back then. It’s been suggested that Neandertals and humans kissed.

How do we know that?

A study that we cite found a microbe that had been transferred to humans from Neandertals that is transmitted through saliva. It might have been from kissing, although the author also stated quite explicitly that it could have been that they shared food.

How about our closest primate relatives?

Bonobos engage in kissing for sexual arousal, and chimpanzees use it to determine mate suitability, in the sense that there are some chemical [scent and taste] cues in terms of determining compatibility. That seems to indicate there is something very fundamental there or going way back in human history.

What is the biological purpose of a kiss?

It has, at least to some extent, to do with mate suitability—something like your breath might be indicative of having bad teeth, and that might be indicative of bad genes. And then there is sexual arousal, of course, which also has to do with passing your genes forward.

What role does kissing play in disease transmission and an awareness of disease in ancient cultures?

At least from the ancient point of view, kissing didn’t seem to have played a major role. People did have an idea of contamination. For example, there was this letter in ancient Mesopotamia that told people not to drink from the cup of a certain sick woman or lay on her bed. But it’s different from how we would conceptualize it, because it was influenced by religion, and people didn’t have any germ theory of disease. Kissing doesn’t seem to have played a major role in these beliefs, though it is entirely possible that one avoided kissing a sick person.

In terms of which diseases, from a modern medical perspective, would have been spread through kissing, there are some clear examples. One, of course, is herpes simplex virus 1 (HSV-1). But our argument is that kissing is distributed over a wide geographic area and has been practiced for a very long time, and must therefore have represented a constant effect on disease transmission. It wasn’t something that would have suddenly accelerated transmission, because it was used throughout those societies.

Have there always been societal rules around kissing? What would happen if you kissed a priestess in ancient Mesopotamia, for example?

There is one reference from Iraq, in something like 700 B.C.E., that attributes the lack of ability to speak to having kissed a priestess—perhaps someone who is not meant to be sexually active.

There are indications in ancient texts that society would have tried to regulate kissing. For example, it was frowned upon to kiss your partner in the street, and there are indications that it was probably preferred that you keep kissing as something that is done among married couples. But the thing is, when society tries to put these regulations into place, it’s usually indicative of the fact that it was common. So it’s most likely that there was a lot more kissing going on than we necessarily find.

 Why has it been easy to miss some of the history of kissing?

There are a lot of these everyday behaviors that often go unrecorded. If people wanted to look back on our era of history in 1,000 years, I guess they would produce a very different picture than how most people have actually lived. It’s always a matter of the survival of evidence. As we go further back in history, we have fewer and fewer texts.

It’s always interesting to push back history as far as we can, and it has a bearing on how we study culture and wider phenomena. But I think there’s also something generally human that we can recognize across space and time.