For millennia, humans have been captivated by the act of kissing. From stolen first kisses at 17 to cinematic portrayals of love and longing, the kiss holds a unique cultural and biological significance. But how far back does this behavior truly go? A recent study by evolutionary biologist Matilda Brindle at the University of Oxford suggests kissing isn’t just a human invention — it’s an ancient primate behavior dating back 21.5 million years.
The Evolutionary Roots of Lip-to-Lip Contact
Brindle’s research, detailed in a 2025 paper, examines kissing across primate species. The study defines kissing not as a romantic gesture, but as “non-agonistic interaction involving directed, intraspecific oral-oral contact.” This means no aggressive behavior and no food transfer (excluding behaviors like mothers pre-chewing food for offspring). Surprisingly, most large ape species, excluding one gorilla variety, engage in kissing. This indicates that the common ancestor of these apes, living roughly 21.5 million years ago, likely kissed as well.
The persistence of kissing across primates raises a key question: why? The act carries significant health risks, including the spread of diseases like mononucleosis (“the kissing disease”). However, Brindle argues that kissing serves two primary evolutionary purposes. First, it acts as a form of “mate assessment,” allowing individuals to subtly evaluate potential partners before committing to reproduction. This is especially crucial for females, who invest heavily in pregnancy and offspring care.
Second, kissing functions as a form of “precopulatory arousal,” or foreplay. By changing vaginal pH levels, arousal increases the likelihood of successful fertilization. This suggests that kissing may have evolved as a biological mechanism to ensure reproductive success.
Beyond Romance: Platonic Kissing and Social Bonding
Kissing isn’t limited to romantic or sexual contexts. Platonic kissing between parents and offspring, or even among friends, serves as a bonding mechanism, releasing oxytocin and reducing social tension. Chimpanzees, known for their aggression, have been observed kissing to reconcile after conflicts, highlighting its role in maintaining social harmony.
An Unexpected Revelation: Neandertal Kisses?
Perhaps the most startling finding is the evidence suggesting that Neandertals also kissed. By reconstructing ancestral primate behaviors, Brindle’s team determined that Neandertals likely engaged in kissing as well. This is supported by genetic evidence showing that humans and Neandertals interbred, sharing saliva for hundreds of thousands of years. This raises the intriguing possibility that early humans and Neandertals not only coexisted but also shared intimate moments, challenging our modern perceptions of these ancient hominids.
Ultimately, kissing is not a uniquely human behavior but a deeply rooted evolutionary trait, refined and exaggerated by our species. While other animals may not sculpt monuments to it, the fact remains: the first kiss happened millions of years ago, long before Valentine’s Day, Hollywood, or even modern humans.
