New phylogenetic research indicates that non-aggressive, mouth-to-mouth contact without food—what the team defines as kissing—likely emerged in the common ancestors of great apes roughly 21.5–16.9 million years ago. The study, published in Evolution and Human Behavior and reported by CNN on November 19, 2025, used behavioral records from living primates and statistical simulations run across the evolutionary tree to estimate when the trait arose. Lead author Matilda Brindle, an evolutionary biologist at Oxford’s Department of Biology, described the finding as evidence that kissing is an ancient, evolved behavior, while also noting major gaps in why the trait persisted in some lineages and not others. The analysis implies extinct relatives such as Neanderthals probably engaged in kissing and, given interbreeding with Homo sapiens, that some cross-species kissing is possible.
Key Takeaways
- The team estimates kissing evolved in a great-ape ancestor between 21.5 million and 16.9 million years ago, based on phylogenetic reconstruction.
- Researchers compiled observations of kissing in chimpanzees, bonobos, orangutans and one gorilla species, then modelled trait evolution across the primate tree.
- The statistical model was sampled robustly—the authors report running the simulation 10 million times to generate probability estimates.
- The behavior is defined specifically as non-aggressive mouth-to-mouth contact excluding feeding, and cannot be read from fossils.
- Historic human records of kissing appear about 4,500 years ago in Mesopotamia and ancient Egypt, but the study treats that as cultural documentation, not origin.
- Kissing is not universal: prior cross-cultural research finds it documented in about 46% of the world’s cultures (2015 paper).
- Much of the animal observation data comes from captive or sanctuary populations, which the authors say limits confidence in wild incidence rates.
Background
Scientists have long debated the origins and functions of human kissing because direct evidence does not survive in the fossil record. Anthropological traces of kissing in human societies date back roughly 4,500 years in Mesopotamia and ancient Egypt, but those records say little about much older primate behavior. To overcome the absence of fossils that preserve social acts, researchers increasingly rely on comparative behavioral data from living species and on phylogenetic methods that estimate ancestral traits.
Matilda Brindle and colleagues framed kissing as an evolutionary puzzle: it poses obvious risks such as pathogen transfer yet has no single obvious reproductive or survival payoff. The behavior—observed across several large-ape species—could serve multiple functions, from mate assessment and sexual foreplay to social bonding or conflict mitigation. Understanding when it first appeared can narrow hypotheses about proximate causes and selective pressures.
Main Event
The research team compiled published observations noting mouth-to-mouth contact in extant primates, identifying instances in chimpanzees, bonobos, orangutans and one gorilla species. They then used genetic trees that describe how primate species are related and ran phylogenetic reconstructions to infer whether ancestral nodes likely displayed the trait. Because living species that share a behavior increase the probability their common ancestor also had it, the presence of kissing in chimpanzees, bonobos and humans boosted the inferred ancestral probability.
To quantify uncertainty and robustness, the authors deployed statistical models that simulate trait changes along tree branches and repeated those simulations millions of times—the paper reports 10 million model iterations. The resulting posterior distributions placed the origin of kissing in the large-ape lineage between 21.5 and 16.9 million years ago. The authors present this as evidence the trait is ancient rather than a recent human innovation.
The paper also notes important caveats: the coded data come largely from captive or semi-captive animals, observational effort is uneven across species, and kissing outside apes is sparsely documented. Because the models infer presence or absence but not function, they cannot resolve whether early kissing evolved primarily for mate assessment, grooming-like bonding, stress relief, or another use.
Analysis & Implications
The inferred age pushes the behavior well back into the Miocene, before the Homo lineage split from other great apes. If correct, this implies kissing was available to any lineage descending from that ancestor, including Neanderthals, and could help explain shared social repertoires among hominins. The finding reframes kissing as a deep-rooted social tool rather than solely a culturally recent human invention.
Functionally, the result narrows hypotheses but does not select one. Because kissing appears in species with diverse social systems—egalitarian bonobos and more hierarchical chimpanzees, for example—its adaptive value may be context-dependent and multifunctional. That would align with Brindle’s point that primates are behaviorally flexible: a trait can be adaptive in some ecological or social settings and costly in others.
Public-health and cultural implications follow: a behavior that facilitates close oral contact carries infection risk, which may partly explain why some human populations do not practice it (documented in roughly 54% of cultures). Evolutionary persistence despite risk suggests benefits—social bonding, mate sampling or parental care—that outweighed costs in ancestral environments, at least for some groups.
Comparison & Data
| Item | Value |
|---|---|
| Estimated origin | 21.5–16.9 million years ago |
| Model iterations | 10,000,000 runs |
| Documented in extant apes | Chimpanzee, bonobo, orangutan, one gorilla species |
| Human historical record | ≈4,500 years ago (Mesopotamia, Egypt) |
| Cultures reporting kissing | 46% (2015 cross-cultural study) |
The table summarizes key quantitative claims from the study and prior literature. While the phylogenetic range gives a clear temporal window, it is a statistical estimate hinging on input data quality and model assumptions. The heavy reliance on captive observations and uneven species sampling means the numerical bounds should be treated as best current estimates, not precise timestamps.
Reactions & Quotes
Journalistic coverage highlighted both the novelty and the limits of the finding, placing it within ongoing debates about cultural versus biological origins of intimate behaviors.
“Kissing is one of these things that we were just really interested in understanding… It’s pervasive across animals, which gives you a hint that it might be an evolved trait.”
Matilda Brindle, Oxford Department of Biology (lead author)
Brindle emphasized that demonstrating an evolved history is a first step, and that follow-up work must test competing functional hypotheses and collect more wild observations.
“The large majority of kisses humans give are not mouth-to-mouth,”
Adriano Reis e Lameira, University of Warwick (evolutionary psychologist, not involved)
Lameira’s comment underscores a distinction between the narrow behavioral definition used in the study and the varied kissing behaviors humans display, a difference that complicates extrapolation from apes to modern human practice.
Unconfirmed
- Whether the trait had a single original function (mate assessment, bonding, food sharing, etc.) remains unresolved and is speculative.
- Incidence of kissing in wild populations for some ape species is poorly documented; captive records may not reflect wild behavior.
- The suggestion that humans and Neanderthals kissed one another is an inference based on co-occurrence and interbreeding, not direct evidence.
Bottom Line
The study provides a well-documented phylogenetic case that mouth-to-mouth kissing is likely an ancient trait in great apes, originating roughly 21.5–16.9 million years ago. That shifts the question from whether kissing is shared ancestry to why it persisted or disappeared across lineages and human cultures.
Answering the why will require more field data from wild primates, clearer behavioral definitions across contexts, and integrative work testing disease risk, social bonding, sexual signaling and parental behaviors as possible drivers. For now, kissing appears less a recent cultural oddity and more a long-standing element in the social toolkit of some primates.
Sources
- CNN (news report) — media coverage of the study published November 19, 2025.
- Evolution and Human Behavior (academic journal) — peer-reviewed outlet where the study appeared.
- Oxford Department of Biology (academic department) — institutional affiliation of lead author Matilda Brindle.