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Journalism of Courage
The humble kiss, perhaps the most ubiquitous of human gestures, has long been a mystery to scientists. A new study argues that the origins of mouth-to-mouth kissing might predate not just recorded history but humanity itself. Moreover, the researchers argue that the kiss may not even have been a human invention.
The study, conducted jointly by researchers from the University of Oxford, University College London, and the Florida Institute of Technology, takes an evolutionary deep dive into the evolution of kissing across primates, using a comparative biological approach.
“Kissing poses an evolutionary problem, since it does not appear to aid survival or reproduction in an obvious way, while the potential costs of disease transfer are high,” they say.
What then is its benefit, or adaptive function, they ask.
Defining a kiss
To begin with, the researchers provided a definition of kissing that did not rely on human ideas of romance. They also filtered out countless behaviours that appear like kissing, such as aggressive mouth fights or a mother bird feeding its young.
In very surgical terms, they defined kissing as “non-agonostic interaction involving directed, intra-specific, oral-oral contact with some movement of the lips/mouthparts and no food transfer.”
According to this definition, kissing occurs across species as diverse as ants, birds, and polar bears, but most reports appear to be in primates. The researchers decided to focus on Afro-Eurasian monkeys and apes because sufficient data were available to conduct a comparative analysis.
They gathered observations on kissing across monkeys and apes — from scientific papers, wildlife descriptions, and even online video archives. The data, while likely not exhaustive, has been assembled as a starting point for future analysis.
Further, to trace the evolution of the behaviour, the researchers turned to Bayesian phylogenetic modelling — a statistical method that uses probability and prior information to infer evolutionary relationships and timelines from genetic data.
A kiss between two species
What they discovered challenges the idea that kissing is a human invention.
Kissing has been observed in most great apes — chimpanzees, orangutans, and humans — and in several species of Afro-Eurasian monkeys. Some use it in a sexual context to increase chances of reproduction, others in courtship, and some to reinforce emotional bonding in a non-platonic way.
Having analysed 10,000 possible primate phylogenies, they reached the stunning conclusion that kissing likely originated anywhere around 16 to 21 million years ago. From then, the act is believed to have persisted across almost all lineages, with the possible exception of Eastern gorillas, among whom the behaviour is yet to be documented.
They also found that it existed among Neanderthals, an extinct species of ancient humans who are known to have lived in Europe and parts of Asia around 40,000 years ago. Moreover, ancient DNA shows that Neanderthals and humans shared oral microbes, long after their evolutionary split, thereby suggesting that the two species kissed.
The researchers caution that, given limited data sets, their conclusions remain premature. Nonetheless, they argue that the paper is a starting point for future comparative research on the evolution of kissing.
For now, however, we know for sure that long before humans wrote poetry and love songs to express their affection, our ancestors sealed social and emotional bonds with a sweet and simple kiss.
