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From seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, researchers propose that ancient hominins did it too – and possibly exchanged kisses with early Homo sapiens.

Common Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, explaining that the concept aligned with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Romantic Interpretation

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

Writing in the journal a scientific periodical, Brindle and colleagues report 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 kiss.

Defining Intimate Contact

"Previously there were some previous attempts to describe a kiss, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Currently we know that they likely engage, it might just not look from what our intimate contact resembles," said Brindle.

However, she said some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in fish called certain marine animals.

As a result the team came up with a description of intimate contact based on friendly interactions involving intentional oral interaction with a individual of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Approach

Brindle explained they concentrated on reports of kissing in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and used online videos to confirm the observations.

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

Historical Timeline

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

The position of ancient hominins on this family tree suggests it is likely they, too, indulged in a intimate act, the researchers say. But the activity might not have been confined to their own species.

"The fact that modern people kiss, the fact that we now have demonstrated that Neanderthals very likely kissed, suggests that the two [species] are probably did engage," Brindle noted.

Biological Significance

Although the scientific reasoning is discussed, the expert explained kissing could be used in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help strengthen connections when practiced in a platonic way.

Another expert in the activities of great apes said that as kissing behavior was seen in a wide range of primates it made sense its roots extend far into our evolutionary past, and an examination of different forms of intimate behavior among a broader range of animals might push its beginnings back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Social Elements

An archaeology expert said that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging confidence and closeness will have been important for millions of years," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and including them and our own species together – engaged intimately."