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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they swapped saliva.

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

Romantic Spin

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

Publishing in the journal a scientific periodical, the researcher and colleagues report how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how people kiss.

Defining Intimate Contact

"There have been some efforts to define 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 the evolutionary biologist.

However, she noted some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish known as certain marine animals.

Consequently the team came up with a definition of intimate contact centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but no transfer of nutrition.

Research Approach

The lead researcher explained they focused on accounts of intimate behavior in non-human species from the African continent and Asian regions, including bonobos, apes and orangutans, and employed online videos to verify the observations.

The researchers then combined this data with information on the genetic connections between living and extinct species of such animals.

Historical Origins

The team propose the results indicate intimate contact evolved approximately 21.5 million 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 confined to their own species.

"The fact that humans kiss, the fact that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is discussed, Brindle explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or assist in selecting between mates, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of great apes commented that as kissing behavior was seen in a broad spectrum of primates it made sense its roots extend far into our evolutionary past, and an analysis of various types of intimate behavior among a broader range of species might push its beginnings back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Elements

Another professor said that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the quality of our emotional bonds, and ways of promoting trust and intimacy will have been important for eons," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but really it should be no surprise that ancient hominins – and even them and our human ancestors collectively – kissed."