Bizarre Origins of Kissing Trace Back 21 Million Years to Apes — And Possibly Neanderthals
What seems like a simple human gesture may actually carry roots far deeper in our evolutionary past. A new study has uncovered evidence that kissing evolved in the common ancestor of humans and other large apes around 21 million years ago — long before modern humans or Neanderthals walked the Earth. Published in Evolution and Human Behavior, the work is the first attempt to reconstruct the evolutionary history of kissing across primates.
By mapping documented kissing behavior onto the primate family tree, the team found that the trait likely originated between 21.5 million and 16.9 million years ago and continued through the lineages that gave rise to today’s great apes.
“While kissing may seem like an ordinary or universal behaviour, it is only documented in 46 percent of human cultures,” said Catherine Talbot, co-author of the study, in a press release. “The social norms and context vary widely across societies, raising the question of whether kissing is an evolved behaviour or cultural invention. This is the first step in addressing that question.”
Read More: Our Prehistoric Ancestors May Have Snacked on Mushrooms Just Like Some Primates Do Today
The Evolutionary Puzzle of Kissing
Biologically, kissing is an odd behavior. It spreads pathogens and doesn’t offer the obvious advantages that grooming or food sharing do. Still, humans and several great apes do it.
What’s more, the behavior hasn’t been studied much from an evolutionary perspective. Anthropologists have documented it in certain human cultures, and primatologists have described versions of it in chimps and bonobos, but few researchers have looked at it across the larger primate family.
Kissing also seems to reach beyond early apes. The study found that Neanderthals were likely kissers, a conclusion supported by earlier evidence that humans and Neanderthals shared oral microbes and interbred.
Tracking Kissing Across Primates
To understand where kissing came from, the team first had to define it — not easy, given how many mouth-to-mouth behaviors look alike. Some involve submission, food sharing, or grooming. To compare species consistently, the researchers defined kissing as non-aggressive, mouth-to-mouth contact without food transfer.
With that definition in place, the team turned to the scientific literature to determine which modern primates engage in this behavior. They focused on monkeys and apes that evolved in Africa, Europe, and Asia — a group where behavioral records are more complete. Chimpanzees, bonobos, and orangutans all met the criteria, each showing their own versions of mouth-to-mouth contact.
The researchers then treated kissing as a behavioral trait and mapped its presence or absence onto the primate family tree. Using a phylogenetic approach and Bayesian modeling, they simulated millions of possible evolutionary scenarios to estimate the likelihood that various primate ancestors also engaged in kissing. In total, the model was run 10 million times, producing estimates for when the behavior most likely emerged.
“By integrating evolutionary biology with behavioural data, we’re able to make informed inferences about traits that don’t fossilise — like kissing. This lets us study social behaviour in both modern and extinct species,” said Stuart West, co-author of the study.
A New Framework for Studying Primate Behavior
Even with gaps in behavioral data, especially beyond the great apes, the work gives researchers a clearer framework to build on and a standardized approach for documenting kissing in other primate species.
“This is the first time anyone has taken a broad evolutionary lens to examine kissing,” said lead author Dr. Matilda Brindle, in a statement. “Our findings add to a growing body of work highlighting the remarkable diversity of sexual behaviours exhibited by our primate cousins.”
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