A new study reveals that kissing on the lips may have evolved over 21 million years ago in the common ancestors of apes and humans, challenging long-held assumptions about the origins of this intimate behavior. Published on November 19, 2025, in the journal Evolution and Human Behavior, the research led by Matilda Brindle, an evolutionary biologist at Oxford University, used advanced phylogenetic analysis to trace the evolutionary history of kissing.
The study defined kissing as non-aggressive mouth-to-mouth contact that doesn’t involve food, a behavior that cannot be directly observed in the fossil record. To overcome this limitation, Brindle and her team compiled data from existing scientific literature on kissing observed in modern primates, including chimpanzees, bonobos, orangutans and one species of gorilla. This approach allowed them to infer behaviors in extinct species based on related living animals, providing a novel way to explore evolutionary traits without physical evidence.
Using phylogenetic analysis, which reconstructs evolutionary trees from genetic data, the researchers simulated millions of scenarios to estimate the probability of kissing in ape ancestors. The results strongly suggest that kissing evolved in the lineage of large apes between 21.5 million and 16.9 million years ago. This implies that extinct human relatives, such as Neanderthals, were likely to have engaged in kissing, too. It’s also possible — since scientists know that our species, Homo sapiens, interbred with Neanderthals — that humans and Neanderthals kissed one another, the study noted.
Kissing presents an ‘evolutionary conundrum,’ as Brindle described it, because it carries significant risks such as disease transmission while offering no obvious reproductive or survival advantages. Despite this, kissing is pervasive across many animal species, indicating it may be an evolved trait. The research highlights the mystery of why such a risky behavior would persist over millions of years, prompting questions about its potential roles in social bonding, mate assessment, or other functions that are not yet fully understood.
The behavior is not universal among humans; it is documented in only 46% of cultures worldwide, according to a 2015 study. Brindle explained that primates are highly adaptable, so kissing might be beneficial in some social contexts but discarded in others where the risks outweigh the benefits. This flexibility accounts for its variable presence across different societies, showing how cultural and environmental factors can shape even deeply rooted biological behaviors.
Experts not involved in the study provided additional insights. Adriano Reis e Lameira, an evolutionary psychologist and primatologist at the University of Warwick, noted that the majority of human kisses are not mouth-to-mouth, emphasizing cultural influences and the diversity of kissing practices. Justin Garcia, an evolutionary biologist and executive director of The Kinsey Institute, described kissing as a blend of biological evolution and cultural nurture, illustrating the complex interplay in intimate behaviors and how they vary across individuals and populations.
The study opens avenues for future research to explore the specific reasons why kissing evolved and how it functions in different contexts. Brindle called for more data on kissing in various species to better understand its developmental trajectory and adaptive significance, noting that much current information comes from animals in captivity, which may not fully represent natural behaviors. This could lead to discoveries about the evolutionary drivers behind other social behaviors.
This groundbreaking work provides a deeper understanding of the evolutionary roots of human intimacy, connecting biological heritage with cultural practices. It underscores how behaviors we often take for granted have deep historical origins, inviting further exploration into the nature of human connection and the ways evolution and culture intertwine to shape our daily lives.
