Recent research from Oxford University suggests that kissing originated from a common ancestor of large apes approximately 17 to 21.5 million years ago, with Neanderthals likely engaging in the behavior as well. Their findings, which earned the 2026 Ig Nobel Prize in biomechanics, provide a precise definition of kissing applicable across species, including ants, birds, polar bears, and humans.
Defining a Kiss Across the Animal Kingdom
Evolutionary biologists Matilda Brindle, Stuart West of the University of Oxford, and Catherine Talbot of the Florida Institute of Technology published their analysis, “A Comparative Approach to Kissing,” in the journal Evolution and Human Behavior. The study established a standardized definition of kissing as non-aggressive mouth-to-mouth contact within a species, involving lip or mouthpart movement without food transfer, to exclude behaviors like mouth-to-mouth feeding or aggressive encounters.
“This is the first time anyone has taken a broad evolutionary lens to examine kissing,” Brindle noted in an Oxford announcement, emphasizing the importance of a shared definition for cross-species comparisons.
Tracing the First Kiss Through the Primate Tree
The researchers analyzed published observations of kissing in primates from Africa, Europe, and Asia, employing Bayesian phylogenetic analysis to map traits onto evolutionary family trees. Their model, run 10 million times, identified the common ancestor of large apes (living 21.5 to 16.9 million years ago) as the likely origin. Neanderthal kissing was inferred through evidence of shared oral microbes and interbreeding with modern humans.
Brindle described the Ig Nobel recognition as “one of my proudest life achievements,” while the team aims to explore why this behavior persists.
An Ancient Behavior with a Germ-Sharing Side
Kissing’s role in pathogen transmission is underscored by the World Health Organization’s estimate that 64% of individuals under 50 carry herpes simplex virus type 1 (HSV-1), primarily spread through oral contact. Historical records from 2500 BCE Mesopotamian texts also highlight kissing’s long-standing influence on microbial spread, according to historians.
Big Gaps in the Evidence
The study acknowledges limitations in data, particularly for non-ape species, and questions whether kissing is an inherited trait or cultural practice, noting its documentation in only 46% of human cultures. Practical advice includes avoiding close contact during active cold sores and following medical guidance for oral herpes management.
Key Questions Answered
When did kissing evolve? The Oxford analysis estimates its origin in the common ancestor of large apes between 21.5 million and 16.9 million years ago.
Did Neanderthals kiss? Yes, based on shared oral microbes and interbreeding evidence.
How did researchers define a kiss? As non-aggressive, species-specific mouth-to-mouth contact with lip movement and no food transfer.
Are all human cultures familiar with kissing? No, it is documented in only 46% of cultures.
Can kissing spread infections? Yes, particularly HSV-1, which affects 64% of people under 50 and spreads via oral contact.
What remains unknown? The evolutionary purpose of kissing and its prevalence in non-ape animals due to inconsistent behavioral records.
