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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time scientists have suggested ancient relatives and early modern humans were closely connected. In earlier research, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the idea aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Intimate Interpretation

"It certainly puts a more romantic spin on ancient interactions," the lead researcher commented.

Publishing in the journal Evolution and Human Behavior, the researcher and colleagues report how, to investigate the historical roots of intimate contact, they first had to develop a definition that was not limited to how people smooch.

Describing Intimate Contact

"Previously there were some efforts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we know that they probably do, it may appear different from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in aquatic species known as certain marine animals.

Consequently the team developed a definition of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some motion of the mouth but absence of food.

Research Approach

Brindle said they focused on accounts of kissing in non-human species from Africa and Asian regions, including bonobos, apes and great apes, and used online videos to verify the reports.

Scientists then integrated this data with information on the genetic connections between extant and extinct types of such primates.

Evolutionary Timeline

The team propose the results indicate intimate contact developed somewhere between 21.5m and 16.9m years ago in the ancestors of the large apes.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a intimate act, the scientists say. But the behavior may not have been confined to their own species.

"Reality that modern people kiss, the fact that we now have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are also likely to have kissed," the researcher added.

Biological Significance

While the scientific reasoning is discussed, Brindle said intimate contact could be employed in sexual contexts to possibly enhance reproductive success or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates said that as intimate contact was seen in a wide range of primates it made sense its roots extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might push its origins back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

An archaeology expert explained that intimate contact had a social component as it was not common to all human groups.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and ways of promoting confidence and closeness will have been important for millions of years," the professor stated. "It might be an concept that seems a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and including Neanderthals and our own species collectively – kissed."