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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

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, researchers have discovered humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea aligned with studies that has revealed people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

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

Writing in the journal a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans smooch.

Defining Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been human-centric, which implies that basically other animals don't kiss. Now we know that they probably do, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like kissing were something rather different – such as the processing and food sharing, or "mouth contact", seen in fish called certain marine animals.

Consequently the team developed a description of kissing centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but absence of food.

Study Methods

Brindle explained they concentrated on accounts of kissing in non-human species from Africa and Asian regions, including bonobos, chimpanzees and great apes, and employed online videos to confirm the observations.

The researchers then integrated this information with information on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Origins

The team say the findings suggest kissing evolved approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of Neanderthals on this family tree means it is likely they, too, engaged in a intimate act, the researchers conclude. But the activity might not have been confined to their specific group.

"The fact that modern people kiss, the fact that we now have shown that Neanderthals probably engaged, indicates that the two [species] are also likely to have engage," the researcher added.

Biological Significance

Although the scientific reasoning is debated, Brindle said intimate contact could be employed in sexual contexts to potentially enhance reproductive success or help choose between mates, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was observed in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an examination of different forms of kissing among a wider variety of animals might push its origins back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we look closely at other animals," he said.

Cultural Elements

Another professor said that intimate contact had a cultural element as it was not universal to all societies.

"However, as humans we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been important for eons," the professor stated. "It might be an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it ought to be expected that Neanderthals – and even them and our own species together – engaged intimately."