Ancient Intimacy

A new study from the University of Oxford suggests that Neanderthals and early modern humans likely engaged in kissing, adding a romantic dimension to our understanding of the interactions between the two species. Published on Tuesday, November 18, 2025, in the journal *Evolution and Human Behavior*, the research traces the evolutionary roots of kissing back millions of years, long before *Homo sapiens* emerged. The findings propose that this intimate act was not only common among our ancestors but also likely shared between species.

Dr. Matilda Brindle, an evolutionary biologist at Oxford's Department of Biology and the study's lead author, argues that kissing is a probable explanation for existing evidence of saliva transfer between Neanderthals and humans. “Probably they were kissing,” she stated, noting this aligns with genetic findings that humans of non-African ancestry have Neanderthal DNA, a clear sign of interbreeding. This conclusion is supported by previous research which found that the two species shared the same mouth microbes for hundreds of thousands of years after their evolutionary split.

The Origins of a Kiss

To investigate the origins of kissing, the Oxford team first had to establish a broad definition of the behaviour that wasn't limited to human customs. They defined a kiss as “non-aggressive, mouth-to-mouth contact that did not involve food transfer.” By mapping observations of this behaviour in modern primates—such as chimpanzees, bonobos, and orangutans—onto the primate family tree, the researchers used phylogenetic analysis to trace its origins. The results suggest that kissing is an ancient trait that likely evolved in a common ancestor of the great apes between 16.9 and 21.5 million years ago.

“The fact that humans kiss, the fact that we now have shown that Neanderthals very likely kissed, indicates that the two [species] are also likely to have kissed,” Dr. Brindle explained. While the precise evolutionary advantage is still debated, scientists suggest kissing may help in assessing a mate's suitability, increasing sexual arousal to improve chances of fertilization, or reinforcing social bonds in a platonic context.

Relevance to the Cradle of Humankind

While Neanderthal fossils are primarily found in Europe and Asia, their story is deeply intertwined with that of *Homo sapiens*, who originated in Africa. Genetic studies confirm that interbreeding between the two species occurred after modern humans began migrating out of Africa. This is why Neanderthal DNA makes up 1-4% of the genomes of modern non-African populations.

For many years, it was believed that sub-Saharan African populations had no Neanderthal ancestry due to this geographical separation. However, a 2020 study published in the journal *Cell* by researchers at Princeton University revealed that all modern African populations also possess Neanderthal DNA. The study proposed that this genetic footprint is the result of ancient migrations of early Europeans back into Africa, reintroducing the Neanderthal sequences. This complex migratory history underscores the interconnectedness of all human populations.

Furthermore, analysis of ancient oral microbiomes has revealed that Neanderthals and early humans shared specific strains of Streptococcus bacteria adapted to consume starch. This suggests that starchy foods like roots and tubers became a crucial part of the hominin diet long before the advent of agriculture, potentially fueling the brain growth that characterizes our species—a key chapter in the human story that began in East Africa.

This latest research from Oxford adds another layer to the complex relationship between our direct ancestors and their closest extinct relatives. It paints a picture not just of competition or replacement, but of coexistence and intimacy, the echoes of which remain in our DNA and behaviours today.