Kissing Could Date Back 21 Million Years, According to Evolutionary Research
Long before modern humans showed up, and long before anyone worried about romance, pathogens, or bad breath, our primate ancestors may have been pressing their lips together. A new study proposes that lip-to-lip contact dates far back as 21 million years ago, offering a surprising glimpse into the origins of one of the most iconic human behaviors.
Before researchers at the University of Oxford could journey into the deep past, they had to wrestle with a deceptively simple question. What actually counts as a kiss? Primate species perform several mouth-related gestures, some affectionate, some not, and many can easily be confused with kissing. So, the team settled on a specific definition of “non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer.”
To identify historical patterns, evolutionary biologist Matilda Brindle and her colleagues drew on scientific papers, field notes, and YouTube clips to gather data on modern primate kissing behavior. Using that material, they built computer models that could simulate mapped primate family trees to estimate how far back this behavior might stretch.
The simulations pointed to a common ancestor of apes and monkeys that lived roughly between 21.5 and 16.9 million years ago. If that timeline is correct, kissing is not a recent cultural spark but an ancient primate relic, Brindle told the Washington Post.
“It would be very unlikely that kissing independently evolved in all of these species of ape that we’re very closely related to. It makes much more sense that this is kind of an ancient trait within our primate family tree.”
The findings, titled “A comparative approach to the evolution of kissing” and published in Evolution and Human Behavior, deepen an already lively debate. Scientists have long disagreed about whether smooching is a learned cultural phenomenon or something we were wired to do long before Homo sapiens emerged around 200,000 to 300,000 years ago.
“Kissing seems a bit of an evolutionary paradox,” Brindle told New Scientist. “It probably doesn’t aid survival and could even be risky in terms of helping pathogen transmission.”
Despite that risk, several species, including chimpanzees, gorillas, and rhesus macaques, routinely smooch. That interspecies overlap is part of what led researchers to suspect that kissing didn’t originate with humans at all. Instead, it may have trickled down from early primates who used mouth-to-mouth contact in ways that are still not fully understood.
The study also suggests that Neanderthals likely participated in this behavior. The models indicate an 84 percent chance that our extinct relatives kissed.
“Obviously, that’s just Neanderthals kissing; we don’t know who they’re kissing. But together with the evidence that humans and Neanderthals had a similar oral microbiome and that most humans of non-African descent have some Neanderthal DNA, we would argue they were probably kissing each other, which definitely puts a much more romantic spin on human-Neanderthal relations.”
Though the act may be ancient, the earliest known human depiction of a romantic kiss is much younger: a clay tablet from Mesopotamia, dating to roughly 4,000 years ago. The translated inscription reads, “My upper lip becomes moist, while my lower lip trembles! I shall embrace him, I shall kiss him.”
Even with newly extended evolutionary timelines, the purpose behind kissing remains up for debate. Brindle described two leading hypotheses. One suggests the behavior may boost reproductive success by triggering arousal or allowing potential mates to evaluate biological cues through smell or taste. The other theory points to grooming, proposing that kissing may have emerged from caretaking behavior and eventually became a tool for building or repairing social bonds. “Chimpanzees will literally kiss and make up after a fight,” Brindle said.
“It’s important for us to understand that this is something we share with our non-human relatives. We should be studying this behavior, not just dismissing it as silly because it has romantic connotations in humans.”
Yet not all researchers find an evolutionary explanation convincing. William Jankowiak, an anthropologist at the University of Nevada, Las Vegas, pointed out that romantic kissing is far from universal. His 2015 study found that romantic kissing appears in only 46 percent of 168 documented cultures.
“If kissing were essential to mate evaluation or to reproductive success, why have so many [human] cultures ceased to practice it?”
With so many variations in cultural norms, researchers are still wrestling with the central question of whether or not kissing is an inherited behavior or a human invention. Catherine Talbot, co-author and animal cognition researcher at the Florida Institute of Technology, sees the new findings as a starting point rather than a conclusion.
“The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behavior or cultural invention. This [new study] is the first step in addressing that question.”
If these early results hold up, the simple act of a kiss might represent one of the oldest surviving threads of primate affection—one that began long before humans even walked the earth.
Moumita Basuroychowdhury is a Contributing Reporter at The National Digest. After earning an economics degree at Cornell University, she moved to NYC to pursue her MFA in creative writing. She enjoys reporting on science, business and culture news. You can reach her at moumita.b@thenationaldigest.com.
