Not only a human thing

Mouth-to-mouth contact is not exclusive to our species. Chimpanzees touch mouths when they reconcile after a fight, bonobos kiss in both social and sexual contexts, and similar behaviour has been described in orangutans and gorillas. A recent phylogenetic study (Brindle et al., 2025) traced kissing back to the common ancestor of the great apes, somewhere between 21 and 17 million years ago, and concluded that Neanderthals probably kissed too.

Calling these behaviours "kisses", however, requires some caution. In other primates the same gesture can work as a greeting, a way to calm tension or a request for food. The form looks alike; the function is not necessarily the same.

What is a kiss for?

Several hypotheses try to explain its origin. One links it to premastication, when mothers passed chewed food to their infants mouth to mouth. Another connects it to grooming and physical contact as a way of maintaining relationships. A third sees it as a tool for assessing a potential partner at very close range, through smell and taste. These explanations are not mutually exclusive: a behaviour can arise for one reason and later be maintained for others.

"When two chimpanzees touch mouths after a conflict, the question isn't whether it looks like our kiss, but what that contact achieves: it calms things down and repairs the relationship. Our kiss has probably inherited part of that function."
— Miquel Llorente, Comparative Minds Research Group

Biology, but also culture

In humans, kissing is shaped as much by culture as by biology. Romantic-sexual kissing has been documented in fewer than half of the cultures studied (Jankowiak et al., 2015), and yet written records from Mesopotamia already mention it more than 4,000 years ago. Understanding why we kiss therefore means looking both at our evolutionary history and at the ways each society gives meaning to physical closeness.

Listen to the full conversation: Las edades de Millás – A vivir que son dos días (Cadena SER)

References: Brindle, M. et al. (2025). Evolution and Human Behavior. · Jankowiak, W. R., Volsche, S. L. & Garcia, J. R. (2015). American Anthropologist, 117(3), 535–539.