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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Now, 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 experts have proposed Neanderthals and Homo sapiens were closely connected. Among earlier research, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the concept aligned with research that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Publishing in the publication a scientific periodical, Brindle and colleagues detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how people kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's largely focused on humans, which means that basically other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some movement of the oral area but no transfer of food.

Study Methods

Brindle explained they concentrated on accounts of kissing in non-human species from Africa and Asia, including bonobos, apes and great apes, and used online videos to verify the observations.

The researchers then combined this data with information on the evolutionary relationships between living and extinct types of such animals.

Historical Timeline

Researchers say the results suggest intimate contact evolved somewhere between 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 likely they, too, indulged in a kiss, the scientists conclude. But the behavior might not have been confined to their own species.

"Reality that humans engage intimately, the fact that we currently have demonstrated that ancient relatives probably kissed, indicates that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, Brindle said intimate contact could be employed in sexual contexts to possibly increase reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was observed in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an analysis of various types of kissing among a broader range of species might push its origins back further still.

"Things that we think of as signatures of our species, like intimate contact, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert explained that kissing had a social component as it was not common to all societies.

"However, as people we thrive or fail on the quality of our relationships, and ways of promoting confidence and intimacy will have been significant for eons," the professor stated. "It might be an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but really it should be no surprise that ancient hominins – and including them and our human ancestors collectively – kissed."