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From seabirds to polar bears, chimpanzees to great apes, various animals appear to kiss. Now, researchers propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance experts have proposed ancient relatives and early modern humans were closely connected. In earlier research, scientists have discovered humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the idea chimed with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating interbreeding was occurring.

Intimate Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," the lead researcher said.

Publishing in the publication Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not limited to how humans kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's largely human-centric, which implies that basically non-human species don't kiss. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

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 called certain marine animals.

As a result the research group developed a definition of kissing based on friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the oral area but absence of nutrition.

Study Approach

Brindle explained they concentrated on accounts of intimate behavior in primates from the African continent and Asian regions, including primates, apes and great apes, and used digital recordings to verify the reports.

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

Historical Timeline

The team propose the findings suggest kissing developed approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a kiss, the researchers conclude. But the activity might not have been confined to their own species.

"The fact that modern people kiss, the fact that we currently have shown that Neanderthals probably engaged, suggests that the both groups are probably did engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is discussed, Brindle explained intimate contact could be used in sexual contexts to possibly enhance reproductive success or assist in selecting between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as intimate contact was observed in a wide range of primates it was logical its roots lie deep in our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might extend its origins back further still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Elements

An archaeology expert said that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of encouraging confidence and closeness will have been significant for eons," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it ought to be expected that ancient hominins – and even Neanderthals and our human ancestors together – engaged intimately."