Danish husband and wife researchers Troels Pank Arboll and Sophie Lund Rasmussen stress that “the behavior did not emerge abruptly or in a specific society, but appears to have been practiced in multiple ancient cultures,” including Egypt.

Arboll and Rasmussen note that “the act of kissing may have played a secondary and unintentional role throughout history” by enabling disease-causing microorganisms to spread from one mouth to another. But the kiss, they write, “cannot be regarded as a sudden biological trigger” that led to societies being deluged by pathogens.

The two researchers launched on their search for the earliest kiss last summer while discussing a paper on the ancient DNA of the herpes simplex virus 1 at the dinner table. The herpes paper had noted a shift in the transmission of the virus during the Bronze Age (2,000 to 700 B.C.), “potentially linked” to new cultural practices “such as the advent of sexual-romantic kissing.”

New? Bronze Age? Really?

“I said to Sophie that I knew we had something older. And then I started digging a bit into that,” said Arboll, an assistant professor of Assyriology at the University of Copenhagen who studies ancient accounts of medical diagnoses, prescriptions and healing rituals.

“We’re a very nerdy couple,” explained Rasmussen, an ecologist at the University of Oxford’s Wildlife Conservation Research Unit (WildCRU) and Aalborg University in Denmark.

Arboll had little trouble finding accounts of kissing from Mesopotamia written in both the Sumerian and Akkadian languages. He and Rasmussen noted too a calcite sculpture in the British Museum called the “Ain Sakhri Lovers,” which was found in caves near Bethlehem and is estimated to be about 11,000 years old.

“It had been known for decades in my field,” Arboll said of the written accounts of kissing from Mesopotamia.

“The thing about Assyriologists,” Rasmussen said, “is they like to argue with each other, but they don’t really talk to other people.”

That could explain why some experts adopted India as the place of origin for kissing, though Arboll has a different theory: “I think one of the sources they cite are manuals like the Kama Sutra,” he said, referring to texts estimated to have been published between 2,400 and 1,700 years ago. “That’s obviously very appealing for discussing sexual behavior, I imagine.”

In her 2011 book “The Science of Kissing: What Our Lips Are Telling Us,” Sheril Kirshenbaum reported the earliest literary evidence for human kissing “dated back to India’s Vedic Sanskrit texts around 1500 B.C.” But she’s convinced the behavior goes back much further in history.

“We see so many similar behaviors across the animal kingdom — including in our closest relatives, like bonobos,” she said. “I suspect our species has been kissing for as long as we’ve been on Earth.”

There may have been practical reasons for humans to kiss, Rasmussen said. “I came across research suggesting that the purpose of kissing, why it could have evolved, is that it serves as an opportunity to evaluate your partner,” she said. “If you kiss somebody with poor teeth, they tend to have bad breath.”

Kissing may have allowed couples to bond and strengthen their relationship. “And of course also for sexual arousal,” said Rasmussen. “So when you want to mate and pass on your genes, it’s very convenient.”

Enjoying a romantic kiss outside the confines of marriage appears to have been frowned upon by the Mesopotamians. Arboll and Rasmussen came across the story of a married woman “almost led astray by a kiss from another man.” Worse, kissing someone who was not supposed to be sexually active, such as a priestess, they write, “was said to deprive the kisser of the ability to speak.”

Such prohibitions may have had the unintended benefit of protecting the good health of the kiss-deprived. The Danish researchers point out kissing’s likely role in spreading herpes simplex virus 1, Epstein-Barr virus and human parvovirus B19, which causes a rash. Such pathogens, the authors wrote, “can infect humans through a range of different transmission routes, including saliva, making any act of kissing a potential means of spreading infection.”

To date, though, science has had relatively little to say about kissing, according to Kirshenbaum, an academic specialist at Michigan State University.

A scientific literature search for “kiss” or “kissing” sends you through funky territory before you ever encounter two sets of lips. PubMed’s search engine begins with the KISS1 gene and its product kisspeptin, and proceeds to a cellular process called kiss-and-run. Google assumes that in seeking the “earliest known kiss,” you hope to find the earliest publicity photo of the rock band Kiss, or the earliest known Kiss tribute band.

“Here’s something that touches all of us,” Kirshenbaum said, “yet science has barely scratched the surface of it.”

The human fascination with kissing history comes as no surprise to Laura Weyrich, an associate professor of anthropology at Pennsylvania State University who co-authored one of the studies cited by the Danish researchers. Her paper, published in Nature in 2017, examined DNA from the dental plaque of Neanderthals and used it to infer information on their behavior, diet and disease.

Weyrich’s study noted that humans and Neanderthals may have traded microbes, which could have happened through the sharing of food or water sources — or through kissing.

In some interviews about the Neanderthal paper, Weyrich dispensed with the term “kissing” in favor of the more earthy alternative, swapping spit, and “biologically, it really is swapping spit,” she said.

Never let it be said that scientists lack romance.

There is some debate among researchers about whether kissing began in one place and spread or had “numerous independent origins,” as Arboll and Rasmussen write.

“My hunch is that kissing arose or was discovered amongst elite in complex societies (hierarchal, market systems with writing) and diffused outward,” William Jankowiak, a professor of anthropology at the University of Nevada at Las Vegas, wrote in an email.

The practice of kissing, he said, was in keeping with “the elite pursuit of pleasure.”