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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, adding that the concept chimed with research that has revealed people of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing interbreeding was occurring.

Romantic Interpretation

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher commented.

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 definition that was not restricted by how humans smooch.

Describing Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely focused on humans, which implies that basically non-human species do not engage in this. Now we understand that they likely engage, it might just not look from what human kissing resembles," said Brindle.

However, she noted some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as certain marine animals.

Consequently the team developed a definition of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but absence of nutrition.

Research Approach

The lead researcher said they focused on reports of kissing in primates from Africa and Asia, including bonobos, chimpanzees and great apes, and used online videos to verify the observations.

Scientists then integrated this data with information on the genetic connections between extant and ancient species of such animals.

Evolutionary Origins

Researchers propose the findings suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior may not have been limited to their own species.

"The fact that humans kiss, the fact that we now have shown that Neanderthals probably kissed, indicates that the both groups are also likely to have kissed," Brindle added.

Evolutionary Importance

Although the evolutionary explanation is debated, Brindle explained kissing could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of primates it made sense its origins lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of species might push its origins back further still.

"Things that we consider as signatures of our species, like kissing, are not unique to us if we look closely at different species," he said.

Cultural Aspects

Another professor said that kissing had a social component as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and ways of encouraging trust and closeness will have been significant for eons," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but actually it should be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."