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Among seabirds to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals did it too – and possibly locked lips with modern humans.

Shared Microbial Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have discovered modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, explaining that the idea aligned with studies that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing interbreeding was at play.

Romantic Spin

"This offers a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of kissing, they first had to develop a description that was not restricted by how humans smooch.

Describing Intimate Contact

"Previously there were some previous attempts to define a kiss, but it's largely human-centric, which implies that essentially non-human species do not engage in this. Currently we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

However, she noted some actions that looked like intimate contact were distinct activities – such as the processing and food sharing, or "mouth contact", observed in fish known as French grunts.

Consequently the research group developed a description of kissing based on social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but absence of nutrition.

Research Approach

The lead researcher explained they concentrated on accounts of intimate behavior in non-human species from the African continent and Asia, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the reports.

The researchers then combined this data with details on the genetic connections between extant and extinct types of such animals.

Historical Origins

Researchers propose the results indicate intimate contact developed approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, engaged in a kiss, the scientists conclude. But the behavior might not have been limited to their specific group.

"The fact that humans kiss, the reality that we now have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is discussed, the expert explained kissing could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of primates commented that as kissing behavior was seen in a wide range of primates it was logical its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a broader range of animals might push its origins back further still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Social Aspects

Another professor explained that kissing had a cultural element as it was not universal to all human groups.

"However, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and intimacy will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be expected that Neanderthals – and including Neanderthals and our own species together – engaged intimately."