Kissing did not begin with humans but with primate ancestors of the great apes around 20 million years ago, according to a study published in Evolution and Human Behavior. Researchers from the University of Oxford and the Florida Institute of Technology sought to trace the origins of kissing, a behavior that carries no obvious evolutionary advantage and may even transmit disease.

The study highlights that humans, chimpanzees, orangutans, bonobos, and gorillas all engage in kissing, suggesting the practice was inherited from a common ancestor. To pinpoint its origins, the scientists combined behavioral observations of primates with data on evolutionary relationships, effectively rewinding the clock to estimate when kissing first appeared.

Kissing was defined in the study as “non-aggressive mouth-to-mouth contact not involving the transfer of food,” encompassing both sexual and platonic kisses, including those shared among family members or friends. By reviewing published research on primates across Africa, Asia, and Europe, the team found that some monkeys and most apes exhibit kissing behavior.

Using a phylogenetic comparative analysis, the researchers modeled the evolutionary history of kissing across species. Through millions of simulations, they concluded that the first kiss likely occurred between 21.5 and 16.9 million years ago. While the origins remain debated, the study offers insights into its persistence. Sexual kissing may serve to assess a partner’s suitability or quality, while platonic kisses appear to strengthen social bonds.

The findings also shed light on Neanderthal behavior. Previous research indicates that modern humans and Neanderthals shared an oral bacterium long after diverging as species, implying saliva—and potentially kisses—was exchanged between them.
This work not only traces the deep evolutionary roots of a familiar human ritual but also illuminates the complex social and reproductive functions of kissing across primate species, showing that the act may be far older and more widespread than previously imagined.