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From seabirds to polar bears, primates 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.

Shared Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. Among previous studies, researchers have found 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 kissing," she said, explaining that the concept chimed with studies that has revealed humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different perspective on ancient interactions," the lead researcher commented.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how people kiss.

Describing Kissing

"There have been some previous attempts to define a kiss, but it's largely focused on humans, which implies that basically other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

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

Consequently the research group came up with a definition of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but absence of food.

Study Methods

The lead researcher explained they focused on reports of intimate behavior in primates from Africa and Asia, including bonobos, apes and orangutans, and used online videos to confirm the observations.

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

Evolutionary Timeline

Researchers say the results indicate intimate contact evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, engaged in a kiss, the researchers conclude. But the activity may not have been confined to their specific group.

"Reality that humans engage intimately, the reality that we currently have shown that ancient relatives very likely engaged, indicates that the both groups are probably did kissed," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, the expert said kissing could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes said that as intimate contact was observed in a wide range of apes it was logical its origins lie deep in our ancient history, and an examination of various types of intimate behavior among a broader range of species might push its origins back further still.

"Things that we consider as characteristics of our species, like kissing, are not exclusive to us if we look closely at different species," the expert noted.

Cultural Aspects

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

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," she said. "This could represent an image that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be no surprise that Neanderthals – and including them and our human ancestors together – engaged intimately."