A new investigation led by researchers at the University of Oxford suggests that kissing may have emerged in the common ancestor of humans and other great apes roughly 21 million years ago. The findings, published on November 19 in Evolution and Human Behavior, also imply that Neanderthals likely engaged in kissing.
Although many animal species exhibit behaviors resembling kissing, its evolutionary purpose has long been debated. The act can transmit pathogens and does not directly enhance survival or reproduction, yet for many human societies, kissing carries strong emotional and cultural significance. Despite its prevalence, its evolutionary origins have seldom been explored in depth.
Tracing the Roots of Kissing in Primates
In their study, the Oxford team undertook the first cross-species analysis aimed at reconstructing when kissing might have appeared along the primate evolutionary tree. Their results indicate that the behavior likely originated in an ancestor of great apes sometime between 21.5 and 16.9 million years ago. This form of affectionate contact appears to have been preserved throughout primate evolution and is still observed in most living species of large apes.
The researchers also concluded that Neanderthals likely kissed. This inference is supported by previous evidence showing that humans and Neanderthals exchanged oral microbes—suggesting saliva transfer— and interbred, pointing to intimate social behaviors such as kissing.
Lead author Dr. Matilda Brindle, an evolutionary biologist in Oxford’s Department of Biology, noted: “This is the first time anyone has applied a broad evolutionary perspective to investigate kissing. Our findings contribute to a growing body of research revealing the extraordinary diversity of sexual and social behaviors in our primate relatives.”
How Scientists Defined Kissing Across Species
To conduct their analysis, the team first needed a clear, universal definition of what counts as kissing, since many mouth-to-mouth interactions may look similar but serve different purposes. They defined kissing as non-aggressive, mouth-to-mouth contact that does not involve feeding or food exchange.
Using this definition, they surveyed scientific literature documenting instances of kissing in modern primates, focusing on species native to Africa, Europe, and Asia, including bonobos, chimpanzees, and orangutans.
Modelling the Evolutionary Path of Kissing
The next step involved a detailed phylogenetic analysis in which kissing was categorized as a behavioral trait and mapped onto the evolutionary relationships of primates. Using Bayesian modelling, the researchers simulated millions of potential evolutionary scenarios to estimate how likely various ancestral species were to have kissed. The model ran 10 million iterations, providing a strong statistical basis for their conclusions.
Co-author Professor Stuart West, a professor of evolutionary biology at Oxford, explained: “By combining evolutionary models with behavioral observations, we can draw informed conclusions about traits that leave no fossil record—such as kissing. This allows us to explore the social lives of both living and extinct species.”
Cultural Differences and Directions for Future Research
The authors note that current data remain limited, especially for primate species outside the great apes. However, their work establishes a foundation for future comparative studies and offers primatologists a standardized framework for reporting kissing behavior in nonhuman animals.
“Kissing may feel like a universal human behavior, but it has only been documented in 46% of human cultures,” said co-author Catherine Talbot, Assistant Professor at the Florida Institute of Technology. “Social norms and contexts vary widely, leaving open the question of whether kissing is an evolved behavior or a cultural creation. Our study represents an important first step in addressing that question.”
