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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, scientists have discovered modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the idea chimed with studies that has revealed people of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Spin

"It certainly puts a different perspective on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to investigate the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how people kiss.

Describing Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely focused on humans, which implies that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she noted some actions that resembled kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in aquatic species called certain marine animals.

Consequently the team developed a description of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of food.

Study Approach

Brindle explained they focused on reports of kissing in primates from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to verify the observations.

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

Historical Origins

Researchers say the findings suggest intimate contact evolved approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of Neanderthals on this evolutionary lineage suggests it is likely they, too, engaged in a intimate act, the scientists say. But the activity might not have been limited to their specific group.

"The fact that modern people engage intimately, the reality that we now have shown that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," Brindle added.

Evolutionary Significance

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

Another expert in the activities of primates said that as kissing behavior was observed in a wide range of primates it was logical its roots extend far into our ancient history, and an examination of various types of intimate behavior among a wider variety of animals might extend its beginnings back even earlier still.

"Behaviors that we consider as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

Another professor said that kissing had a social component as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting confidence and closeness will have been significant for millions of years," the professor stated. "It might be an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be expected that ancient hominins – and even Neanderthals and our own species collectively – kissed."