Researchers have traced the evolutionary origins of kissing, suggesting that mouth-to-mouth contact likely evolved over 21 million years ago and was practiced by the common ancestor of humans and other great apes.

The study also indicates that Neanderthals may have kissed, and that humans and Neanderthals could have exchanged kisses as well. Scientists explored kissing because, despite its widespread occurrence across human societies and the animal kingdom, it has no obvious survival or reproductive advantage.

To build their evolutionary family tree, researchers first defined a kiss precisely: non-aggressive, directed oral-oral contact involving movement of lips or mouthparts, without food transfer. Using this definition, kissing behavior was observed not only in humans, chimpanzees, and bonobos, but also in wolves, prairie dogs, polar bears, and even albatrosses.

Lead researcher Dr. Matilda Brindle from the University of Oxford explained that because humans, chimps, and bonobos all engage in kissing, it is likely that their most recent common ancestor did so as well. She estimates that kissing probably evolved around 21.5 million years ago in large apes.

The study also highlights shared oral microbes between modern humans and Neanderthals, indicating that saliva exchange may have occurred for hundreds of thousands of years after the two species diverged.

While the research establishes when kissing emerged, it does not explain why. Theories include its evolution from grooming behaviors or as a method for assessing partner health and compatibility. Dr. Brindle emphasizes the importance of studying kissing as a shared behavior with non-human relatives, rather than dismissing it due to its romantic associations.