How Did Humans End Up Smooching on the Lips? It May Have Started Out With a 21-Million-Year-Old Kiss

Our ancient primate relatives—including Neanderthals—may have enjoyed a nice peck on the lips. But researchers still don’t know why we do it

Sara Hashemi

– Daily Correspondent

November 21, 2025

Kissing, for all popularity, is a bit of a mystery. Scientists have long debated when humans’ ancestors first put their lips together, and whether the act is simply a cultural trait.

A new study suggests giving someone a peck has a long history, dating up to around 21 million years ago, long before modern humans existed. The work was published in the journal Evolution and Human Behavior on November 19.

“Kissing seems a bit of an evolutionary paradox,” Matilda Brindle, an evolutionary biologist at the University of Oxford and co-author of the study, says to Chris Simms at New Scientist. “It probably doesn’t aid survival and could even be risky in terms of helping pathogen transmission.”

But humans aren’t the only primates that like to smooch. Some monkeys and apes, including gorillas, chimpanzees and rhesus macaques, also kiss. So, Brindle and her team speculated that kissing may have been a behavior passed down from ancestors that lived long before modern humans (Homo sapiens), which evolved around 200,000 to 300,000 years ago.

First, the researchers had to figure out what constitutes a kiss. That task proved challenging, because several mouth-to-mouth behaviors resemble kissing, according to an Oxford statement. They settled on defining of kissing as “non-aggressive, mouth-to-mouth contact that did not involve food transfer.”

Brindle and her colleagues then scoured the scientific literature—and even YouTube videos—to collect data on kissing behavior in modern primates. With that information, the team mapped out a family tree of primates and ran computer simulations of various evolution scenarios to estimate the probability of different ancestors kissing.

The results suggested that kissing evolved in a common ancestor that lived around 21.5 to 16.9 million years ago.

“It would be very unlikely that kissing independently evolved in all of these species of ape that we’re very closely related to,” Brindle tells the Washington Post’s Leo Sands  “It makes much more sense that this is kind of an ancient trait within our primate family tree.”

Neanderthals may have also smooched one another, as the researchers found an 84 percent chance that humans’ extinct relative engaged in the behavior.

“Obviously, that’s just Neanderthals kissing; we don’t know who they’re kissing,” Brindle explains to New Scientist. “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.”

Researchers still don’t know why we kiss. Brindle offers New Scientist two hypotheses. One is that it enhances reproductive success by sexually arousing animals or allowing them to assess the quality of potential mates. The second idea is that kissing evolved from grooming, and that the behavior helps strengthen social bonds.

“Chimpanzees will literally kiss and make up after a fight,” Brindle says to the outlet.

William Jankowiak, an anthropologist at the University of Nevada, Las Vegas who was not involved in the study, tells the Washington Post that while it’s useful for scientists to look at kissing in other primates, the study hasn’t answered an important question about the behavior.

“If kissing were essential to mate evaluation or to reproductive success, why have so many [human] cultures ceased to practice it?” he asks. Jankowiak co-authored a 2015 study that found romantic kissing is present in only 46 percent of 168 sampled human cultures.

Still, the new research may open a path toward finding the answer. “The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behavior or cultural invention,” says Catherine Talbot, a study co-author and animal cognition researcher at the Florida Institute of Technology, in the statement.

“This [new study] is the first step in addressing that question.”