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Among seabirds to Arctic mammals, primates to orangutans, certain species appear to kiss. Currently, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Shared Oral Evidence

It is not the first time scientists have suggested ancient relatives and early modern humans were intimately acquainted. In earlier research, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the concept chimed with studies that has found people of non-African ancestry have bits of ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Interpretation

"This offers a different spin on ancient interactions," the lead researcher commented.

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which means that basically non-human species don't kiss. Currently we know that they probably do, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that resembled intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", seen in aquatic species known as certain marine animals.

As a result the team came up with a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some motion of the oral area but no transfer of food.

Research Approach

The lead researcher explained they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including primates, chimpanzees and great apes, and used online videos to verify the reports.

Scientists then integrated this information with information on the evolutionary relationships between living and extinct types of such primates.

Historical Timeline

The team say the results indicate intimate contact evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

The position of ancient hominins on this family tree means it is likely they, too, indulged in a kiss, the scientists conclude. But the activity may not have been confined to their own species.

"The fact that humans engage intimately, the fact that we currently have shown that ancient relatives probably engaged, indicates that the both groups are also likely to have engage," the researcher noted.

Evolutionary Importance

While the scientific reasoning is debated, the expert explained kissing could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when practiced in a non-sexual manner.

A separate researcher in the activities of primates commented that as intimate contact was seen in a broad spectrum of apes it was logical its roots extend far into our ancient history, and an analysis of various types of intimate behavior among a broader range of species might push its origins back even earlier still.

"Things that we think of as characteristics of our species, like intimate contact, are not unique to us if we look closely at different species," the expert noted.

Cultural Elements

An archaeology expert explained that kissing had a social component as it was not common to all human groups.

"However, as people we succeed or struggle on the quality of our relationships, and ways of encouraging trust and closeness will have been significant for eons," she said. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – kissed."