Kissing likely evolved in the common ancestor of large apes between 21.5 and 16.9 million years ago, according to a recent study.
In a comparative analysis published in Evolution and Human Behavior, researchers concluded that kissing emerged once in the common ancestor of large apes after divergence from small apes and was largely retained across hominid lineages, with a possible loss in eastern gorillas. Using Bayesian phylogenetic modeling, the analyses also estimated a high probability that Neanderthals engaged in kissing, with a posterior probability of 0.843.
To examine the evolutionary history of kissing, the researchers developed a non-anthropocentric definition to allow consistent cross-species comparison. Kissing was defined as a non-agonistic, directed, intraspecific oral-to-oral contact involving movement of the lips or mouthparts without food transfer. This definition excluded behaviors such as premastication, trophallaxis, and agonistic mouth contact. Using a targeted scoping review of the literature and supplementary observational sources, the researchers documented whether kissing had been observed across Afro-Eurasian monkeys and apes. Species were coded as having kissing present if at least one observation existed, acknowledging that absence of evidence did not indicate true absence.
Phylogenetic analyses were conducted across a posterior distribution of 10,000 molecular phylogenies from the 10kTrees Project to account for uncertainty in primate evolutionary relationships. Ancestral state reconstructions were performed using a Bayesian Markov chain Monte Carlo framework implemented in BayesTraits. These analyses demonstrated a strong phylogenetic signal, indicating that the distribution of kissing across species was better explained by shared ancestry than by independent evolution. As the lead author, Matilda Brindle of the Department of Biology, University of Oxford, UK, and Department of Genetics, Evolution and Environment, University College London, UK, and colleagues stated, “kissing is an ancient and phylogenetically conserved trait within the large apes, including extinct human species.”
Within the large apes, ancestral reconstructions placed the emergence of kissing between approximately 21.5 and 16.9 million years ago. Outside this clade, reconstructions suggested multiple independent evolutionary origins among Afro-Eurasian monkeys, although the researchers highlighted that sparse data likely limited the reliability of these estimates. To evaluate kissing in Neanderthals, the team used Bayesian tip-state interpolation, treating Neanderthals as missing data during model fitting and subsequently estimating their most likely behavioral state.
The researchers also explored potential life history correlates. Kissing tended to co-occur with multi-male mating systems and non-folivorous diets among apes, while food sharing showed no consistent association. Premastication was observed in all species in which kissing was reported, although insufficient comparative data prevented formal testing of this relationship.
Several limitations were noted. Available data was sparse, uneven across taxa, and frequently derived from captive populations. The binary classification of kissing ignored variation in frequency, context, and social function within species, including documented cultural variation in humans. Limited data outside the large apes reduced phylogenetic power and constrained hypothesis testing.
The researchers reported no conflicts of interest, and the study was supported by funding from the European Research Council and the Gruter Institute.
Source: Evolution and Human Behavior
