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From seabirds to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Currently, researchers suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Oral Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the idea aligned with research that has found humans of non-African ancestry have bits of ancient genetic material in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Writing in the publication Evolution and Human Behavior, the researcher and her team detail how, to explore the evolutionary origins of intimate contact, they first had to develop a description that was not restricted by how people kiss.

Describing Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's largely focused on humans, which implies that basically non-human species do not engage in this. Currently we understand that they likely engage, it may appear different from what human kissing resembles," explained Brindle.

However, she noted some behaviors that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in aquatic species known as certain marine animals.

Consequently the research group developed a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of food.

Research Methods

Brindle explained they concentrated on reports of kissing in primates from Africa and Asian regions, including bonobos, chimpanzees and great apes, and employed digital recordings to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Timeline

The team propose the findings suggest kissing evolved approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a kiss, the scientists say. But the activity might not have been confined to their specific group.

"The fact that humans kiss, the reality that we currently have shown that Neanderthals probably engaged, suggests that the two [species] are also likely to have kissed," the researcher added.

Biological Importance

While the scientific reasoning is debated, Brindle explained kissing could be employed in reproductive situations to possibly enhance reproductive success or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was observed in a broad spectrum of primates it made sense its origins extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might push its beginnings back further still.

"Behaviors that we consider as characteristics of human life, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Elements

Another professor explained that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of promoting trust and intimacy will have been significant for eons," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it should be no surprise that Neanderthals – and including Neanderthals and our human ancestors together – kissed."