The kiss, a universally recognized but culturally variable gesture, has long raised questions about its origins. Long considered a specifically human behavior, it is now the subject of re-examination in the light of data from evolutionary biology and ethology. A study published in the journal Evolution and Human Behavior by a team of researchers from the University of Oxford, in collaboration with the Florida Institute of Technology, offers a new reading of this behavior.
Far from being a recent cultural invention, kissing appears to have emerged more than 20 million years ago in a common ancestor of the great apes. Drawing on behavioral, genetic, and microbial data, the researchers also argue that Neanderthals, and perhaps even their interactions with Homo sapiens, included this type of intimate contact. This finding calls into question our perceptions of social bonds among human and pre-human species.
A practice rooted in the evolutionary history of great apes
The idea that kissing is a simple human cultural construct no longer stands up to examination of the behavior of great apes. In their study, biologists from the University of Oxford conducted an in-depth phylogenetic analysis. They combined it with Bayesian statistical modeling to trace the appearance of kissing in primates. The results point to an origin dating back between 21.5 and 16.9 million years, in the common ancestor of the great apes: gorillas, chimpanzees, orangutans, bonobos and humans.
The study was based on behavioral observations from the scientific literature. But also video sources, such as YouTube, to validate the presence of non-aggressive mouth-to-mouth contact between individuals of the same species. These behaviors are particularly documented in bonobos and orangutans. And often in contexts of play, appeasement or social reinforcement.
This methodology makes it possible to consider kissing as an evolutionary trait in the biological sense. In other words, a characteristic inherited from a common ancestor and retained across different species. By treating the kiss as observable and modelable data, researchers were able to project its probability of appearance in current species, but also in extinct species.
The importance of this approach lies in the possibility of explaining behavior which, like other social interactions, leaves no fossil trace. However, as Professor Stuart West, co-author of the study, explains in a press release, these behavioral traits play a crucial role in social organization. They can be as stable as morphological characters in evolution.
Neanderthals probably kissed too
If today's great apes exhibit behaviors comparable to kissing, researchers have also studied the case of extinct human species. And in particular the
Neanderthalsby combining behavioral, genetic and microbiological data. Their conclusion is clear. It appears highly likely that Neanderthals also practiced kissing.
This hypothesis is primarily based on indirect but solid evidence. Genetic analyzes have shown that non-African humans have on average 1-2% Neanderthal DNA. This indicates inter-species sexual relations after Africa's exit fromHomo sapiensabout 50,000 years ago. But beyond reproduction, other clues suggest frequent oral contact. In particular, the presence of common oral microorganisms in both species. Microorganisms revealed by the analysis of fossilized dental calculus.
The transfer of these microbes, such as Methanobrevibacter oraliscannot be explained without a direct exchange of saliva. Which implies, as you will have understood, mouth-to-mouth type behavior. For Dr Matilda Brindle, the most parsimonious conclusion remains the following: “ They were probably kissing “.
The study is not limited to speculation. It is also based on behavioral modeling, positioning Neanderthals in the evolutionary tree of great apes. It shows that all nearby branches exhibit this behavior. By behavioral analogy, it would be surprising if Neanderthals lost this practice while retaining other similar social traits.
This conclusion reshuffles the cards of our perception of interactions between extinct human species. It highlights a social and emotional intimacy, well beyond just reproductive functions, in the relationships between Homo sapiens and its extinct cousins.
A universal definition to overcome anthropocentric limits
To compare kissing between species, we still need to agree on what this behavior covers. The authors of the study therefore had to establish a rigorous definition, but applicable to different species. This represented a major methodological challenge.
Unlike traditional definitions focused on human culture, the team defined kissing as non-aggressive, mouth-to-mouth contact without food transfer. This approach makes it possible to distinguish gestures of tenderness or social interaction from similar, but fundamentally different, behaviors. For example, the food kiss, common among certain primates, where the mother chews the food before passing it to her young.
This distinction made it possible to exclude ambiguous or misinterpreted behaviors in other studies. Namely, the “
kiss-fighting » observed in certain species of fish. The definition adopted therefore focused on the emotional and relational context of the gesture, observable in bonobos or chimpanzees in calming situations after a conflict, or during social games.
The scientific issue is also intended to be ethological. This functional definition allows researchers to research kissing in other animal species, particularly within social mammals. It also serves as a behavioral database for future studies, guaranteeing homogeneity of analysis.
As psychologist Dr Catherine Talbot, co-author of the study, points out, this approach opens the way to a rereading of numerous social behaviors that are often neglected, because they are too linked to an anthropocentric reading. The kiss here becomes an observable and comparable phenomenon, not just a human cultural manifestation.
The human kiss is not universal, but biologically anchored
Despite its symbolic charge in many societies, the kiss is not universal, far from it. According to anthropological data cited by the authors, only 46% of documented human cultures practice romantic or emotional kissing. This disparity poses the following question. Is kissing a biological behavior or a cultural invention?
The study shows that it is not simple social learning. The fact that such behavior exists independently in different species of great apes, not sharing human culture, argues for a biological origin. Kissing could thus constitute a latent behavior, activated or not depending on the cultural context.
On a functional level, researchers suggest several possible roles of kissing in evolution. It could be used to strengthen social bonds, ease tensions or even assess the immune compatibility of a potential partner. These functions, observed in human and non-human contexts, reinforce the idea of multi-use behavior, relevant for group cohesion or reproduction.
Dr Jake Brooker, a specialist in great ape behavior at Durham University, not involved in the study, confirms this reading at Guardian. “ What we thought was specific to the human species is often not, if we carefully observe other social animals “.
Finally, Penny Spikins, professor of archeology at the University of York, recalls that these gestures may have played a crucial role in establishing trust. A key concept essential to the survival of prehistoric groups. “ It is not surprising that Neanderthals and Homo sapiens shared gestures of tenderness “.
This observation challenges the idea of a primitive past dominated by aggressiveness. Intimacy and affection have also shaped human history.
Source: Matilda Brindle et al., “A comparative approach to the evolution of kissing”. Evolution and Human Behavior. Available online 19 November 2025, 106788. In Press, Corrected Proof
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