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Among Galápagos albatrosses to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance experts have proposed Neanderthals and early modern humans were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the concept aligned with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, demonstrating interbreeding was at play.

Romantic Interpretation

"This offers a more romantic perspective on ancient interactions," Brindle commented.

Writing in the publication a scientific periodical, the researcher and colleagues report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how people kiss.

Defining Kissing

"Previously there were some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Now we know that they likely engage, it might just not look from what our intimate contact resembles," explained Brindle.

However, she said some behaviors that resembled intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in fish called certain marine animals.

Consequently the team developed a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of food.

Research Approach

Brindle explained they focused on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to verify the observations.

Scientists then combined this information with details on the genetic connections between living and ancient species of such animals.

Historical Timeline

The team propose the findings indicate kissing developed 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 suggests it is likely they, too, indulged in a kiss, the researchers say. But the activity might not have been limited to their specific group.

"Reality that humans engage intimately, the reality that we now have shown that ancient relatives very likely engaged, indicates that the both groups are also likely to have engage," the researcher added.

Biological Importance

While the scientific reasoning is debated, Brindle explained intimate contact could be used in reproductive situations to possibly enhance reproductive success or assist in selecting between mates, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a wide range of primates it was logical its origins extend far into our ancient history, and an analysis of different forms of kissing among a broader range of animals might extend its origins back further still.

"Things that we think of as signatures of human life, like intimate contact, are not exclusive to us if we examine carefully at different species," he said.

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the quality of our emotional bonds, and ways of encouraging confidence and closeness will have been significant for millions of years," she said. "This could represent an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it should be no surprise that ancient hominins – and even Neanderthals and our own species collectively – kissed."