A team of researchers from the University of Oxford, University College of London (UCL), and the Florida Institute of Technology have proposed that the act of kissing may date back to our ape ancestors, long before humans appeared.

The team was inspired by observing many modern primates, such as chimpanzees, macaques, baboons, and more, kissing others of their kind. As these species share the same ancestors, the researchers set out to ascertain when and at what stage the act originated, Discover Wildlife reports.

First, the team defined kissing in evolutionary terms as “non-aggressive mouth-to-mouth contact, without food exchange, within the same species.”

Using this definition, the researchers gathered a large amount of observational data from a number of modern primate studies and fed it to a model that allowed them to recreate the evolutionary history of kissing.

Their analysis suggests that this type of lip locking emerged in the last common relative of great apes between 21.5 and 16.9 million years ago. From these findings, the researchers also hypothesised that the Neanderthals, our closest ancestors who are now extinct, also engaged in the act of kissing each other.

In fact, this also seems to back up a study from 2017 that suggested that Homo sapiens and Neanderthals might have engaged in kissing one another, based on shared oral microbes between Neanderthals and modern humans.

Not all species, however, appear to practise this behaviour: Eastern gorillas, for instance, were found not to ‘kiss’ in the same way, while groups like macaques and baboons may have developed similar behaviours independently.

As for why kissing evolved, the scientists suggest that the act may have roots in premastication (the act of a parent mouth-feeding an infant), gradually transforming into affectionate lip contact.

Other researchers have theorised that it was to strengthen social bonds, facilitate mating, or even exchange beneficial microbes, though risks such as disease transmission remain.

Dr Matilda Brindle of Oxford, who led the study, noted that this is the first time anyone has attempted to reconstruct the evolutionary history of kissing using a broad comparative approach.