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From seabirds to polar bears, primates to orangutans, certain species appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. In previous studies, scientists have discovered humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the concept aligned with research that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Intimate Spin

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

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to come up with a description that was not limited to how humans kiss.

Defining Kissing

"There have been some efforts to describe a kiss, but it's largely human-centric, which implies that essentially non-human species do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing resembles," said Brindle.

However, she noted some actions that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "kiss-fighting", observed in aquatic species known as certain marine animals.

As a result the team came up with a description of kissing based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but absence of food.

Research Approach

Brindle explained they focused on accounts of intimate behavior in primates from the African continent and Asia, including primates, apes and great apes, and used digital recordings to confirm the reports.

The researchers then combined this information with information on the genetic connections between living and ancient types of such animals.

Evolutionary Timeline

The team say the results suggest intimate contact evolved approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

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

"Reality that humans engage intimately, the reality that we now have shown that Neanderthals very likely engaged, indicates that the two [species] are probably did kissed," Brindle added.

Biological Significance

While the evolutionary explanation is debated, Brindle said kissing could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between mates, while it could assist strengthen connections when used in a platonic way.

Another expert in the behavior of great apes commented that as intimate contact was seen in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of animals might extend its beginnings back even earlier still.

"Things that we consider as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," he said.

Social Elements

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

"However, as humans we thrive or fail on the quality of our relationships, and methods of encouraging confidence and intimacy will have been significant for eons," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be no surprise that ancient hominins – and even them and our own species together – engaged intimately."