Why Kissing Matters Across Primates: Evolution, Bonding, and Neanderthals

In this Science Quickly episode, host Kendra Pierre Lewis sits down with Matilda Brindle, an evolutionary biologist at the University of Oxford, to investigate why kissing exists across humans and other primates. The conversation covers how scientists define kissing, its prevalence among large apes, and two leading evolutionary hypotheses: kiss as a tool for mate assessment and kiss as pre-copulatory foreplay. Brindle also discusses platonic kissing and bonding, the role of oxytocin, and how Neanderthals may have kissed, potentially sharing saliva and microbes with early modern humans. The episode closes with a call for more data to test these ideas and a playful nod to Valentine’s Day.

Introduction and listener stories

The episode opens with a lighthearted prompt about memorable kisses from listeners, including Carrie's tale from New Orleans about a long-ago kiss that felt just right. This framing leads into a scientific investigation of kissing as a behavior with deep evolutionary roots rather than a purely human cultural pastime. "kissing is one of these things that we seem to do all the time" - Matilda Brindle, Evolutionary Biologist, University of Oxford.

Defining kissing and scope

Brindle clarifies how researchers define kissing. It is a non-aggressive interaction involving intraspecific oral contact with movement of lips or mouth parts and explicitly not about food transfer. The distinction from pre-mastication, which can resemble kissing but serves a different purpose, anchors the discussion in evolutionary analysis rather than mere mimicry. "we define kissing as a non-agonistic interaction... and no food transfer" - Matilda Brindle.

Kissing across the primate lineage

The conversation surveys kissing across primates, noting that large apes kiss in many contexts, with one gorilla species as a possible exception. Brindle stresses that absence of evidence is not evidence of absence for species lacking data, and that broad comparative work helps illuminate ancestral traits. "we found that all of the large apes, except for one species of gorilla, seemed to kiss one another" - Matilda Brindle.

Why humans kiss: two leading hypotheses

The discussion outlines two main hypotheses for romantic or sexual kissing: mate assessment, where kissing helps gauge genetic compatibility and mate-worthiness, and pre-copulatory arousal, which may prepare for subsequent mating by signaling arousal. Brindle explains how these ideas connect to broader reproductive biology, including how female physiology shifts during arousal to favor fertilization. "One thing that could be said for human kissing is that we've taken this kind of way above and beyond kissing in the animal kingdom" - Matilda Brindle.

In the platonic realm, kissing may serve bonding and social smoothing, releasing oxytocin and aiding social cohesion within families and peer groups, a pattern also seen in chimpanzee reconciliation.

Neanderthals, saliva, and ancient microbes

A striking part of Brindle’s work is reconstructing ancestral states to show that Neanderthals likely kissed one another. This finding, coupled with evidence of long-standing saliva exchange between humans and Neanderthals, suggests behavioral overlap that could have shaped microbiomes and even genetic exchange. Brindle notes how non-African human populations carry a small percentage of Neanderthal DNA, hinting at close contact beyond mere coexistence. "we reconstructed the Neanderthal tip of the tree... they were probably kissing one another" - Matilda Brindle. "they were sharing saliva" - Matilda Brindle.

Conclusion and future directions

The episode ends with Brindle emphasizing the need for more data across diverse species to test the proposed theories and better understand the deep history of kissing. The conversation links a playful Valentine’s Day sentiment with serious questions about behavior, evolution, and microbiomes, underscoring how a simple kiss can reveal complex biological and social dynamics.