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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWe do not yet know why kissing evolved. A 2025 comparative study estimates that kissing was present in the common ancestor of the large apes around 21.5–16.9 million years ago, and that Neanderthals probably kissed. Those are evolutionary reconstructions, not proof of kissing’s original purpose. To investigate that purpose, researchers first have to decide which animal behaviors count as kisses.
What counts as a kiss?
There is no single definition that captures every behavior people might call a kiss. A 2025 comparative study uses a definition designed to compare species: a non-aggressive interaction between members of the same species involving directed oral-to-oral contact, some movement of the lips or mouthparts, and no transfer of food.
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That definition excludes several lookalikes: food passed from mouth to mouth, aggressive mouth contact, accidental contact, and a dog licking a person’s mouth. It also differs from the narrower human benchmark used in a 2024 review, which emphasizes protruded lips and a suction movement. A behavior can qualify under one definition and not the other.
This choice matters. A definition restricted to human-style lip movements may rule out many animal interactions before they can be compared. A broader definition makes cross-species analysis possible, but risks grouping behaviors that do not share the same history or purpose. Sexual and platonic kissing may also need to be studied separately.
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How old might kissing be?
In the 2025 study, researchers collated observational reports on kissing among Afro-Eurasian monkeys and apes, then used Bayesian phylogenetic models to estimate where the behavior appeared in their evolutionary history. They report kissing in most living large apes and some monkeys.
The model places kissing in the common ancestor of the large apes, after that lineage split from small apes, roughly 21.5–16.9 million years ago. The authors infer one origin within large apes and a possible loss in ancestral gorillas. The monkey pattern may reflect independent origins, but the observations are too sparse to support a confident account of its history outside large apes.
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These dates describe a model’s reconstruction, not a fossilized kiss or a directly observed ancestral behavior. The study is a targeted scoping review with preliminary comparative analyses, and its species records often distinguish “present” from “not observed.” A behavior that has not been reported is not necessarily one that never occurred.
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Did Neanderthals kiss?
The model estimates a probability of 0.843 that Neanderthals kissed, with a reported standard error of 0.002. This is a statistical inference from comparative evidence, not direct observation of Neanderthals.
Shared oral microbes between Neanderthals and modern humans are consistent with mouth-to-mouth contact, but do not establish it: food sharing and parental care can also transfer microbes. The microbial evidence therefore supports a possibility rather than confirming a specific behavior.
Why might kissing have evolved?
The proposed explanations are hypotheses, not established causes. They differ in how well they account for the form of a human kiss, the social situations in which it occurs, and the evidence available to test them.
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| Candidate explanation | What it could explain | What the evidence establishes |
|---|---|---|
| Grooming and social bonding | A 2024 review proposes that kissing may have retained grooming’s affiliative role. In its “groomer’s final kiss” hypothesis, a grooming bout might have ended with the groomer using protruded lips and suction to remove a parasite or debris; the final contact could then persist as grooming declined. | The review argues this account fits the form, social context, and affiliative function better than alternatives, but it remains a proposal. Direct observation of a grooming bout ending this way would be an important test. |
| Premastication and food transfer | Premastication—chewing food before passing it to another individual—offers a possible behavioral precursor because it involves mouth contact and occurs in great apes. | The 2025 study reports premastication in each great-ape species where kissing has been observed, but comparable data for species without reported kissing are missing. That prevents an adequate test of the proposed relationship. Food transfer itself is excluded from the study’s definition of kissing. |
| Mate assessment or reproductive advantage | Kissing might help assess a potential mate or be related to sexual selection. | The study notes an association between kissing and multi-male mating systems among apes, with exceptions. The available data do not establish that kissing improves mate choice or reproductive success, and do not support a formal test of adaptive function. |
The grooming explanation is distinctive because it proposes a specific route from an existing behavior to the form of a human kiss, along with an observable prediction. The other accounts point to plausible connections, but the current comparative evidence does not show that any one of them caused kissing to evolve.
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Is kissing universal among humans?
No: romantic mouth-to-mouth kissing is not common to all human societies. That variation is a reminder that the behavior’s evolutionary history, its present-day social uses, and its cultural prevalence are separate questions. An evolutionary origin would not by itself explain why some communities practice a form of kissing and others do not.
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The comparative evidence also has limits beyond cultural variation. Many animal observations come from captive or sanctuary settings, and reports are often coarse records of whether kissing was seen. Sparse observations make it difficult to distinguish a true absence from a behavior researchers simply have not documented.
What can we conclude about kissing’s evolution?
The strongest current answer is about possible timing, not purpose: a 2025 comparative model places kissing in the large apes’ common ancestor roughly 21.5–16.9 million years ago and estimates that Neanderthals probably kissed. The underlying observations are limited, and both conclusions depend on definitions and modeling choices.
Whether kissing began as grooming, emerged from food-related behavior, provided an advantage in mate assessment, or arose through another path remains unresolved. Better observations across species—and clearer distinctions between sexual, affiliative, and food-related mouth contact—are needed to test those possibilities.
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