A new study using primate behaviour records and evolutionary models estimates that kissing originated 21.5–16.9 million years ago among primate ancestors of the great apes. Researchers combined field observations with phylogenetic data and ran millions of simulations to date the behaviour. They defined kissing as non-aggressive, mouth-to-mouth contact excluding food transfer, covering sexual and platonic forms. The study also suggests Neanderthals likely engaged in similar mouth-to-mouth contact, supported by evidence of interbreeding and a shared oral microbe.
First Kiss Predates Humans — Primate Ancestors 'Locked Lips' 21.5–16.9 Million Years Ago
A new study using primate behaviour records and evolutionary models estimates that kissing originated 21.5–16.9 million years ago among primate ancestors of the great apes. Researchers combined field observations with phylogenetic data and ran millions of simulations to date the behaviour. They defined kissing as non-aggressive, mouth-to-mouth contact excluding food transfer, covering sexual and platonic forms. The study also suggests Neanderthals likely engaged in similar mouth-to-mouth contact, supported by evidence of interbreeding and a shared oral microbe.
10:48 AM, 11/19/2025Science
By Toby Sterling
A new study suggests that mouth-to-mouth contact commonly recognized as kissing did not begin with modern humans but with the primate ancestors of the great apes roughly 21.5–16.9 million years ago. The research, led by scientists at Oxford University and the Florida Institute of Technology, reconstructs the evolutionary history of this behaviour using field observations and phylogenetic data.
The team combined published and field observations of primate behaviour with data on evolutionary relationships and used a modelling framework to simulate millions of evolutionary scenarios. Lead author Dr. Matilda Brindle of Oxford's Department of Biology said the simulations consistently placed the origin of non-aggressive mouth-to-mouth contact—what the authors define as kissing—between about 21.5 and 16.9 million years ago. The findings were published in the journal Evolution and Human Behavior.
"Using these two key pieces of information, we employed a modelling approach that allowed us to simulate different evolutionary scenarios," Dr. Brindle said, noting the model was run millions of times to estimate the timing.
For clarity, the researchers adopted an intentionally unromantic definition of kissing: "non-aggressive, mouth-to-mouth contact that did not involve food transfer." That broad definition includes sexual kissing as well as non-sexual mouth contact such as family pecks and friendly greetings.
Why kissing evolved and why it persists remain subjects of debate. Some scientists propose sexual kissing helps assess mate quality or suitability; others argue it functions as foreplay that raises arousal and could increase the chance of fertilization. Platonic mouth contact, by contrast, is thought to help manage complex social relationships and strengthen bonds among group members.
The paper also suggests that Neanderthals likely engaged in similar mouth-to-mouth contact. Evidence that Neanderthals and modern humans interbred and shared an oral microbe—indicating saliva exchange—long after the two lineages diverged 450,000–750,000 years ago supports the idea the behaviour existed in both groups.
While the adaptive value of kissing is not settled, this study places its origin deep in the ape lineage and highlights the role of social and sexual behaviours in primate evolution.