💋 Kissing was already practiced 20 million years ago

The act of bringing one's lips to another's seems a universal expression of human affection. Yet, this practice raises a persistent biological enigma, as its potential risks appear disproportionate to its immediate benefits. A team from the University of Oxford now proposes a radically new perspective, indicating that the origin of this behavior is lost in the mists of time, far beyond the emergence of our own species.

This research, published in the journal Evolution and Human Behavior, does not rely on historical or anthropological records, but on the principles of phylogenetics. The scientists adopted a novel comparative approach, searching for traces of this behavior across the evolutionary tree of primates. Their goal was to determine whether kissing constitutes a recent cultural invention or an ancient biological heritage shared with our closest cousins.

The biological foundations of an intimate gesture

To conduct this study, the first step was to establish a precise definition that could be applied to different species. The researchers thus defined kissing as a non-aggressive, directed mouth-to-mouth contact without food transfer. This definition allows distinguishing this behavior from premastication gestures or simple accidental contact. It serves as a filter to analyze decades of primatological observations.

The team then compiled data concerning monkey and great ape species from Africa, Europe, and Asia. They found that chimpanzees, bonobos, and orangutans regularly engage in gentle oral contact, in contexts of appeasement, reconciliation, or courtship. This behavior thus appears as a stable component of their social repertoire, and not as an anecdotal curiosity.

By integrating these observations into a Bayesian statistical model, the researchers were able to reconstruct the evolutionary history of this trait. Their analysis, repeated millions of times to ensure its robustness, indicates that the highest probability places the emergence of kissing in the common ancestor of great apes, between 21.5 and 16.9 million years ago. This dating makes it a characteristic far older than the genus Homo itself.

A heritage shared with our extinct cousins

The implications of this model go beyond the world of current primates. They shed light on the behavior of extinct human species, such as Neanderthals. Statistical reconstructions strongly suggest that they also practiced kissing. This inference is supported by indirect but converging evidence from other research fields.

Previous studies on the oral microbiome, notably published in Nature, revealed a remarkably similar bacterial community in Neanderthals and modern humans from the same period. The transfer of specific microbes through saliva necessarily implies a form of close and repeated contact between the two groups, beyond simple food sharing.

Furthermore, the genetic evidence for hybridization between Homo sapiens and Neanderthals is now undeniable. The combination of these elements — a behavior inherited from great apes, a shared microbiome, and proven interbreeding — forms a coherent set of clues. It paints a picture where intimate interactions, potentially including kissing, punctuated the encounters between our lineages.

This research paves the way for a more nuanced understanding of intimacy. It places an apparently commonplace gesture into an immense evolutionary perspective, linking it to a biological history shared with other species. Kissing thus appears as an ancient practice, whose expression and meaning were subsequently widely reshaped by the diversity of human cultures.