Kissing May Trace Back 21 Million Years to a Common Great‑Ape Ancestor, Study Finds

A new comparative study suggests that non‑aggressive, mouth‑to‑mouth contact — the behaviour many of us call kissing — likely existed in a common ancestor of the great apes between about 21.5 and 16.9 million years ago. The analysis draws on observations of living primates and statistical reconstructions of ancestral traits to estimate when the behaviour first appeared.

Matilda Brindle, lead author and an evolutionary biologist at Oxford’s Department of Biology, describes kissing as an “evolutionary conundrum” because it appears to carry clear risks, such as disease transmission, while offering no obvious direct reproductive or survival advantage.

How the researchers worked

Because kissing leaves no trace in fossils, the team used a comparative, phylogenetic approach. They compiled published observations of mouth‑to‑mouth contact (excluding food‑related mouth contact) from living primate species — including chimpanzees, bonobos, orangutans and one gorilla species — and mapped those behaviours onto a genetic tree of primate relationships.

Using statistical modelling, the researchers simulated evolutionary scenarios along that tree to estimate the probability that ancestral species engaged in kissing. To ensure robust results, the model was run 10 million times. The findings were published in the journal Evolution and Human Behavior.

Main findings and implications

The results indicate a strong probability that mouth‑to‑mouth contact is an ancient trait in the great‑ape lineage, arising in a common ancestor roughly 21.5–16.9 million years ago. By extension, closely related extinct human species — such as Neanderthals — may have engaged in similar behaviour, and recurring interbreeding between Homo sapiens and Neanderthals raises the possibility they sometimes kissed one another.

However, the model does not identify why the behaviour evolved. The authors note several plausible functions: assessing potential mates, acting as foreplay, strengthening social bonds, reducing social tension, or even food transfer to offspring. The analysis cannot distinguish between these hypotheses.

Limitations and next steps

The researchers acknowledge important data gaps: observations of kissing are sparse outside the great apes, and many available reports come from animals in captivity or sanctuaries rather than from wild populations. More field data across diverse species and contexts are needed to reconstruct the behaviour’s deeper history and variation.

“What we’ve done, which is a really important first step, is showing it’s an evolved trait. It’s really ancient. But why? And that’s the amazing next step,” Brindle said.

Other experts noted caveats. Adriano Reis e Lameira, an evolutionary psychologist and primatologist, pointed out that most human kisses do not fit the narrow, mouth‑to‑mouth definition used in the analysis, so human kissing practices are more diverse than the study captures. Justin Garcia, an evolutionary biologist and director of The Kinsey Institute, called the work a valuable starting point for investigating how biological and cultural factors interact to shape intimate behaviours.

Notably, kissing is not universal across human societies: previous research has documented mouth‑to‑mouth kissing in about 46% of cultures. The authors emphasize that a strong evolutionary origin does not require the behaviour to be retained everywhere; primates are behaviourally flexible, and kissing may be adaptive in some social environments but unnecessary or risky in others.

Overall, the study frames kissing as an ancient, likely evolved trait in the great‑ape lineage while leaving open the central question of its original function — an invitation for further comparative and field research.