Researchers used phylogenetic mapping to trace mouth‑to‑mouth kissing back through primate evolutionary history and conclude the behavior likely existed in the common ancestor of all apes about 21 million years ago. The finding, published in the journal Evolution and Human Behavior, treats kissing as nonaggressive mouth‑to‑mouth contact that does not involve passing food. Lead author Matilda Brindle of the University of Oxford and her international team combined literature review, video evidence and trait‑mapping across primate lineages to reach their conclusion. The analysis also suggests Neanderthals engaged in kissing, adding a new angle to long‑standing evidence of contact between archaic and modern humans.
Key Takeaways
- The study used phylogenetic analysis to infer that kissing was present in the ancestor of all apes roughly 21 million years ago, based on trait mapping across primates.
- Researchers defined kissing as nonaggressive mouth‑to‑mouth contact excluding food transfer, a boundary the paper applies consistently across species observations.
- Evidence draws on published literature plus direct video observations; the team screened dozens of primate species for kissing behaviors.
- Analyses indicate Neanderthals likely practiced mouth‑to‑mouth contact, aligning with genomic evidence that most non‑African humans carry about 1–2% Neanderthal DNA.
- The paper appears in Evolution and Human Behavior and was led by Matilda Brindle (University of Oxford), with an international group of evolutionary biologists and primatologists contributing data.
- Primate taxa that commonly exhibit kissing‑like contact include humans, bonobos and chimpanzees, which informed ancestral state reconstructions.
Background
Anthropologists and primatologists have long debated the origins and functions of intimate mouth‑to‑mouth contact among animals and humans. Different hypotheses link kissing to mate assessment, social bonding, parenting behaviors or pathogen signalling avoidance; identifying a deep evolutionary origin helps narrow plausible functional explanations. Prior observational studies documented kissing or mouth contact in a range of primates, most notably in bonobos and chimpanzees, but a systematic evolutionary mapping across the whole primate tree was lacking. Brindle and colleagues set out to treat kissing as a discrete behavioral trait and use phylogenetic methods to estimate its ancestral states.
Phylogenetic comparative methods allow researchers to infer whether a trait observed in several modern species was present in their common ancestor by mapping presence/absence onto a dated evolutionary tree. For primates, well‑resolved phylogenies and a large observational literature make this approach feasible. The team compiled published reports and supplemental video evidence to code species for kissing behavior, then ran ancestral state reconstructions to estimate the trait’s history. The result places the origin of the behavior at the ape ancestor node about 21 million years ago, predating the human lineage split from other apes.
Main Event
Brindle’s group first operationalized a workable definition of kissing: nonaggressive mouth‑to‑mouth contact that does not include active food exchange. Using that definition, they surveyed primate species records and scored each taxon as exhibiting or not exhibiting the behavior. The authors then applied phylogenetic models to a primate evolutionary tree and evaluated the likelihood that kissing was present at key ancestral nodes, including the ape ancestor. The models returned a high probability that the trait existed in the ape ancestor approximately 21 million years ago.
In addition to the deep‑time inference, the researchers explored hominin implications. The mapping suggests Neanderthals—part of the broader hominin lineage—likely engaged in kissing, a claim the authors link to other lines of evidence showing occasional reproductive contact between Neanderthals and modern human ancestors. The study does not claim direct fossil evidence of kissing but interprets behavioral continuity across closely related taxa as supporting inference for archaic humans.
The team supplemented literature coding with modern footage, noting particularly elaborate mouth contact among bonobos and chimpanzees that strengthened the scoring for apes. Brindle publicly noted the abundance of tongue involvement in bonobo interactions, an observation recorded in the study’s supplementary material. The authors emphasize that their conclusion rests on comparative inference rather than a single direct observation of ancestral hominins.
Analysis & Implications
Locating kissing at the ape ancestor node reframes several hypotheses about its function. If kissing predates the human lineage by tens of millions of years, explanations tied solely to uniquely human pair‑bonding or romantic courtship become less convincing. Instead, a deeper origin points toward functions shared across apes—such as social bonding, affiliative grooming substitution, or close affiliative contact tied to group living and infant care.
The Neanderthal inference carries implications for interpreting archaic human social life. Genetic evidence that most non‑African humans retain around 1–2% Neanderthal DNA documents biological contact; behavioral continuity (such as kissing) would suggest shared social repertoires that facilitated intimate contact. However, behavioral inference from phylogenetic reconstruction is probabilistic: it indicates plausibility and likely continuity, not direct archaeological proof.
There are also public‑health and cultural dimensions. If mouth‑to‑mouth contact has long evolutionary roots, its persistence in human societies may reflect deeply embedded social uses that are resilient to changing norms. That helps explain why kissing serves multiple social roles today—pair bonding, parental affection, greeting rituals—in different cultures, even when explicit functions vary.
Comparison & Data
| Node / Group | Inferred Kissing Presence | Key Evidence |
|---|---|---|
| Ape ancestor (~21 million years ago) | High likelihood | Phylogenetic trait mapping across apes |
| Modern great apes (humans, chimpanzees, bonobos) | Observed | Literature and video observations |
| Neanderthals (hominins) | Probable (inferred) | Comparative inference + genomic contact evidence (1–2% Neanderthal DNA in most non‑Africans) |
The table summarizes the study’s principal inferences and the types of evidence used. While the ape ancestor inference rests on cross‑taxon similarities and phylogenetic modeling, the Neanderthal claim relies on reconstructed behavioral continuity combined with independent genomic evidence of interbreeding.
Reactions & Quotes
“If you think about the fact that humans and our closest living relatives, chimpanzees and bonobos all kiss, it makes sense that the common ancestor of those three species kisses as well.”
Matilda Brindle, University of Oxford (lead author)
“Kissing was treated as a nonaggressive, non‑food mouth‑to‑mouth contact and then mapped across primate phylogeny to estimate ancestral states.”
Evolution and Human Behavior (study summary)
Public response to the study has ranged from fascination to skepticism. Some primatologists welcomed a systematic comparative approach to a topic often covered anecdotally, while others cautioned that behavior reconstruction is probabilistic and dependent on the completeness and accuracy of observational records. Science communicators noted the study’s appeal for illustrating how behavioral traits can be traced over deep evolutionary time.
Unconfirmed
- The study does not present direct fossil or archaeological evidence that Neanderthals kissed; the claim is an inference from comparative methods and should be treated as probable but not proven.
- The specific evolutionary function of kissing (mate assessment, social bonding, infant care, pathogen avoidance, or combination thereof) remains unresolved; multiple functions may have coexisted.
- The completeness of behavioral records for many primate species is uneven, so absence of observed kissing in some taxa may reflect limited observation rather than true absence.
Bottom Line
The study offers a carefully argued, phylogenetically grounded case that mouth‑to‑mouth kissing likely predates the human lineage and was present in the ape ancestor about 21 million years ago. That deep timing shifts explanations for kissing away from purely human‑centric accounts and toward functions shared among apes, such as social bonding and affiliative contact. The authors’ Neanderthal inference is plausible within the comparative framework, but it remains an inferred behavior rather than directly evidenced in the fossil record.
For readers, the paper highlights how modern behaviors can be traced and contextualized using evolutionary trees and cross‑species observation. Future work—more systematic field observations, behavioural databases, and integration with archaeological and genomic data—can refine these inferences and probe the functional reasons why mouth‑to‑mouth contact has been conserved across millions of years of primate evolution.
Sources
- NPR Short Wave — “What’s In A Kiss? 21 Million Years Of Evolution” (media: news/transcript)
- Evolution and Human Behavior (academic journal: publisher site for the study)
- University of Oxford (academic institution: lead author affiliation)