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From Galápagos albatrosses to Arctic mammals, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Shared Microbial Evidence

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were closely connected. In previous studies, researchers have found modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea chimed with studies that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

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

Publishing in the publication a scientific periodical, the researcher and her team report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's largely human-centric, which implies that basically other animals do not engage in this. Currently we know that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", observed in fish called French grunts.

As a result the research group developed a description of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

Brindle explained they focused on reports of intimate behavior in primates from Africa and Asia, including primates, chimpanzees and great apes, and employed digital recordings to confirm the observations.

Scientists then combined this information with information on the genetic connections between extant and ancient types of such primates.

Evolutionary Origins

The team say the findings suggest kissing developed approximately 21.5 million and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage means it is likely they, too, indulged in a intimate act, the researchers conclude. But the activity might not have been limited to their specific group.

"The fact that modern people engage intimately, the fact that we now have demonstrated that ancient relatives probably kissed, suggests that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is discussed, the expert said kissing could be employed in reproductive situations to potentially enhance reproductive success or assist in selecting between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of primates said that as kissing behavior was observed in a wide range of apes it was logical its roots extend far into our evolutionary past, and an examination of various types of intimate behavior among a broader range of species might extend its origins back even earlier still.

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

Social Aspects

Another professor explained that intimate contact had a social component as it was not common to all societies.

"However, as people we succeed or struggle on the strength of our relationships, and ways of promoting confidence and intimacy will have been significant for millions of years," she said. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors together – kissed."