The study in the Journal of Human Behavior and Evolution Society traces lip‑to‑lip contact back to about 21.5–16.9 million years ago, suggesting kissing predates Homo sapiens. Researchers used an operational definition of kissing and phylogenetic analyses of primate behavior to reconstruct its evolutionary history. The work indicates kissing occurs across most large apes, probably existed in Neanderthals, and may have served social or bonding functions. The authors note kissing‑like contact is observed in other animals as well, placing humans among many species with similar behaviors.
A 21‑Million‑Year‑Old Smooch: Study Suggests Kissing Predates Humans
The study in the Journal of Human Behavior and Evolution Society traces lip‑to‑lip contact back to about 21.5–16.9 million years ago, suggesting kissing predates Homo sapiens. Researchers used an operational definition of kissing and phylogenetic analyses of primate behavior to reconstruct its evolutionary history. The work indicates kissing occurs across most large apes, probably existed in Neanderthals, and may have served social or bonding functions. The authors note kissing‑like contact is observed in other animals as well, placing humans among many species with similar behaviors.
03:05 AM, 11/20/2025Science
Stories of awkward first kisses and fairy‑tale smooches are timeless, but new research suggests lip‑to‑lip contact likely began millions of years before modern humans. A study published in the Journal of Human Behavior and Evolution Society traces the origins of kissing to roughly 21.5–16.9 million years ago, placing the behavior deep in the primate family tree.
How the researchers approached it
To compare behaviors across species without projecting human traits onto animals, the authors defined kissing operationally as a non‑agonistic, directed, intraspecific oral–oral contact that involves some movement of the lips or mouthparts and does not involve transfer of food. Using observational data from living primates and phylogenetic methods to reconstruct ancestral traits, the team mapped where and when kissing‑like behaviors likely evolved.
Key findings
The analysis indicates that mouth‑to‑mouth contact appears in most large apes and likely existed in Neanderthals as well. The researchers argue the behavior has a strong phylogenetic signal across Afro‑Eurasian monkeys and apes, meaning it is plausibly an inherited trait from common ancestors rather than a recent cultural invention unique to Homo sapiens.
“These are deep‑rooted evolutionary behaviors,” said Matilda Brindle, an evolutionary biologist and lead author of the study.
What this implies about humans and Neanderthals
Previous studies documenting shared commensal oral microbes between humans and Neanderthals, together with evidence that both groups engaged in mouth‑to‑mouth contact, led the authors to suggest it is plausible humans and Neanderthals might have exchanged saliva through kissing. The team presents this as a cautious, more humanizing perspective on interactions between the two groups rather than definitive proof.
Broader context and open questions
The evolutionary function of kissing remains uncertain. Hypotheses range from social bonding and mate assessment to simple affiliative touch. The paper also notes that kissing‑like behaviors are observed beyond primates—instances of mouth contact have been reported in other mammals such as polar bears—emphasizing that humans are part of a wider spectrum of animals that engage in similar behaviors.
“A kiss is more than a lovely trick designed by nature to stop speech when words become superfluous,” the authors write, borrowing a line often attributed to Ingrid Bergman to highlight the cultural as well as biological interest in kissing.
Overall, the study provides a systematic evolutionary perspective on a behavior long regarded primarily as cultural, showing it likely has deep roots in primate evolution. The findings invite further research into the sensory, social and reproductive roles that mouth‑to‑mouth contact may have played across species.