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From Galápagos albatrosses to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

Common Microbial Clues

It is not the first time experts have proposed Neanderthals and early modern humans were intimately acquainted. Among previous studies, researchers have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with research that has revealed people of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was at play.

Romantic Spin

"It certainly puts a different spin on ancient interactions," Brindle commented.

Writing in the journal a scientific periodical, Brindle and her team detail how, to investigate the historical roots of intimate contact, they first had to develop a description that was not restricted by how people kiss.

Describing Intimate Contact

"Previously there were some previous attempts to describe a intimate act, but it's very much been focused on humans, which implies that essentially other animals don't kiss. Now we know that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she noted some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "kiss-fighting", observed in fish called certain marine animals.

As a result the team developed a definition of kissing based on social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

The lead researcher said they focused on reports of kissing in non-human species from Africa and Asia, including primates, apes and orangutans, and used digital recordings to confirm the observations.

The researchers then integrated this information with information on the evolutionary relationships between extant and extinct species of such animals.

Evolutionary Timeline

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

The position of ancient hominins on this family tree suggests it is probable they, too, engaged in a intimate act, the researchers conclude. But the activity might not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we currently have shown that Neanderthals probably engaged, indicates that the two [species] are probably did kissed," the researcher added.

Biological Significance

While the scientific reasoning is debated, the expert explained intimate contact could be employed in sexual contexts to possibly increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a broad spectrum of apes it was logical its origins lie deep in our ancient history, and an examination of different forms of intimate behavior among a broader range of species might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Elements

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

"However, as people we thrive or fail on the strength of our emotional bonds, and ways of encouraging trust and closeness will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our human ancestors together – kissed."