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From seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Now, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have discovered humans and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the concept chimed with research that has revealed people of non-African ancestry contain ancient genetic material in their genome, revealing interbreeding was occurring.

Romantic Interpretation

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

Writing in the publication a scientific periodical, the researcher and her team report how, to investigate the historical roots of kissing, they first had to develop a description that was not limited to how humans smooch.

Defining Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which means that essentially other animals don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as certain marine animals.

As a result the research group developed a definition of intimate contact based on friendly interactions involving directed oral interaction with a individual of the identical group, with some motion of the mouth but no transfer of nutrition.

Research Methods

Brindle said they focused on reports of intimate behavior in non-human species from Africa and Asia, including bonobos, apes and great apes, and employed digital recordings to verify the observations.

The researchers then integrated this data with details on the genetic connections between extant and extinct species of such animals.

Historical Origins

The team propose the results suggest kissing evolved somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been limited to their own species.

"The fact that modern people kiss, the fact that we currently have demonstrated that ancient relatives probably kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, the expert said intimate contact could be used in sexual contexts to possibly increase reproductive success or help choose between partners, while it might help strengthen connections when used in a platonic way.

A separate researcher in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might extend its beginnings back further still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not unique to us if we look closely at other animals," he said.

Social Elements

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

"However, as people we succeed or struggle on the quality of our relationships, and methods of encouraging trust and closeness will have been important for millions of years," the professor stated. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be expected that Neanderthals – and including them and our human ancestors collectively – engaged intimately."