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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists propose that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. In 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 swapped saliva.

"Likely they were kissing," she said, explaining that the idea chimed with studies that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genome, revealing interbreeding was at play.

Intimate Spin

"This offers a more romantic spin on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, Brindle and colleagues detail how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not restricted by how people smooch.

Defining Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which implies that essentially other animals don't kiss. Now we understand that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as French grunts.

As a result the team came up with a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some motion of the oral area but absence of nutrition.

Research Methods

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

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

Evolutionary Timeline

The team propose the results indicate intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior may not have been limited to their own species.

"Reality that humans kiss, the fact that we currently have shown that ancient relatives very likely engaged, indicates that the two [species] are also likely to have kissed," the researcher added.

Biological Importance

While the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a wide range of primates 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 animals might push its beginnings back even earlier still.

"Behaviors that we think of as signatures of our species, like kissing, are not unique to us if we look closely at different species," he said.

Cultural Elements

An archaeology expert said that intimate contact had a social component as it was not common to all societies.

"However, as people we thrive or fail on the strength of our relationships, and methods of encouraging confidence and intimacy will have been important for millions of years," the professor stated. "It might be an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and including them and our human ancestors collectively – engaged intimately."