Kissing in humans is a nearly universal behavior that signals affection, serves as a greeting, and acts as a precursor to intimacy. To understand its origins, researchers study whether our primate relatives also engage in this behavior. By comparing the mouth-to-mouth contact seen in other primates with the human kiss, scientists can trace how a simple physical action transformed into a complex social ritual, defining the biological roots of human affectionate behavior.
Observing Primate Orofacial Contact
While the romantic, prolonged lip-lock of humans is unique, a form of mouth-to-mouth or mouth-to-face contact, known as orofacial contact, exists in great apes. This behavior is most frequently observed in chimpanzees and bonobos, the two species most closely related to humans. In chimpanzees, the contact is generally a quick, soft press of the lips, often accompanied by a distinct sound. This mechanism involves brief contact rather than the complex muscle movements of a human kiss.
Bonobos, often called the “make love, not war” ape, engage in this type of contact much more frequently and openly than chimpanzees. Their interactions can involve prolonged mouth-to-mouth contact, sometimes with tongue use, which is outwardly similar to human kissing. However, this contact is often integrated into their broader sociosexual repertoire, which is used for many purposes beyond reproduction. Recent phylogenetic analysis suggests that this kiss-like behavior is an ancient trait, likely evolving in a common ancestor of large apes between 21.5 and 16.9 million years ago.
The Social Role of Mouth-to-Mouth Contact
The function of orofacial contact in primates diverges significantly from the human concept of romantic kissing. For great apes, the behavior is primarily affiliative, serving to strengthen social bonds and manage group dynamics. In chimpanzees, for example, the brief lip contact is most often seen immediately following a conflict as a form of reconciliation. This post-conflict resolution helps reduce tension and repair the social fabric of the group.
Bonobos use their varied mouth-to-mouth contact in a wider range of social contexts, including during greetings and before food sharing. This behavior is considered a stress-reducing mechanism that promotes social tolerance within their highly cohesive groups. Another function observed in both species is mouth-to-mouth food transfer, where a mother may pre-masticate food and pass it directly to an infant. This high-trust interaction is a form of parental care that establishes a close physical and nutritional bond.
Primate orofacial contact is rarely linked to courtship or sexual attraction in the way it is for humans. Instead, it functions as a social lubricant, a gesture of reassurance, or a means of maintaining group harmony. By using the mouth area for these non-aggressive interactions, the apes establish communication focused on survival and cooperation rather than romance.
The Evolutionary Divergence of Human Kissing
The human kiss likely evolved by co-opting these ancient primate behaviors and adding a layer of complex psychological and hormonal signaling. One compelling hypothesis, the “groomer’s final kiss,” suggests that the act originated as the final, vestigial stage of grooming. As early humans lost most of their body hair, the need for extensive grooming decreased, but the final lip-to-skin touch persisted as a symbolic gesture of comfort and bonding.
The mother-infant food transfer, or “kiss-feeding,” may also have provided the foundation for romantic kissing, linking a high-trust, mouth-based interaction with intense bonding and nutrient transfer. Over time, this evolved into a mechanism for mate assessment, allowing partners to exchange chemical signals. Kissing triggers the release of neurochemicals like oxytocin and dopamine, which create feelings of affection, pleasure, and attachment.
This neurochemical response, which also lowers stress hormones like cortisol, transformed the physical act into a powerful pair-bonding tool. The exchange of saliva during a kiss allows for the subconscious assessment of a potential mate’s health and genetic compatibility, particularly through the major histocompatibility complex (MHC) genes. While primates use orofacial contact to maintain social order, humans refined it into a complex psychological process that sustains long-term romantic relationships.