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From seabirds to Arctic mammals, primates to orangutans, various animals appear to kiss. Currently, scientists suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the concept chimed with studies that has found people of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

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

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's very much been human-centric, which means that essentially non-human species do not engage in this. Currently we know that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some actions that looked like intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the research group developed a definition 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 no transfer of food.

Study Methods

The lead researcher explained they focused on reports of intimate behavior in primates from the African continent and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the reports.

The researchers then combined this information with details on the genetic connections between extant and extinct species of such animals.

Historical Origins

The team say the findings suggest intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of Neanderthals on this family tree means it is probable they, too, engaged in a intimate act, the scientists say. But the activity might not have been confined to their specific group.

"Reality that humans kiss, the reality that we now have demonstrated that ancient relatives probably kissed, suggests that the two [species] are probably did kissed," the researcher added.

Biological Significance

Although the scientific reasoning is debated, Brindle explained intimate contact could be employed in reproductive situations to potentially increase mating outcomes or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of great apes commented that as kissing behavior was observed in a wide range of primates it made sense its origins extend far into our ancient history, and an examination of different forms of intimate behavior among a wider variety of species might push its origins back even earlier still.

"Behaviors that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," the expert noted.

Cultural Aspects

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and ways of promoting confidence and intimacy will have been significant for eons," she said. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and including them and our own species together – engaged intimately."