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From seabirds to Arctic mammals, primates to great apes, various animals engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance experts have suggested Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept chimed with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating genetic mixing was at play.

Romantic Spin

"It certainly puts a different spin on human-Neanderthal relations," the lead researcher said.

Writing in the journal Evolution and Human Behavior, Brindle and colleagues detail how, to explore the historical roots of intimate contact, they first had to develop a definition that was not limited to how humans smooch.

Defining Intimate Contact

"There have been 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. Now we know that they probably do, it may appear different from what human kissing resembles," said Brindle.

Nonetheless, she said some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the team came up with a definition of intimate contact centered around social behaviors involving intentional oral interaction with a member of the same species, with some motion of the oral area but no transfer of food.

Study Approach

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

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

Evolutionary Origins

Researchers propose the results indicate kissing developed approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a intimate act, the researchers conclude. But the activity may not have been limited to their specific group.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives very likely kissed, indicates that the two [species] are probably did kissed," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert said intimate contact could be used in reproductive situations to possibly enhance mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates said that as kissing behavior was seen in a wide range of primates it was logical 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 consider as signatures of our species, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

Another professor said that intimate contact had a social component as it was not universal 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 important for eons," the professor stated. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors together – kissed."