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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Now, scientists propose that ancient hominins also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Oral Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with research that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, the researcher and her team detail how, to investigate the evolutionary origins 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 intimate act, but it's largely human-centric, which means that essentially other animals don't kiss. Now we know that they probably do, it might just not look from what human kissing looks like," explained Brindle.

However, she said some actions that looked like intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species called certain marine animals.

As a result the research group developed a description of kissing based on social behaviors involving directed oral interaction with a member of the identical group, with some motion of the mouth but no transfer of food.

Research Approach

The lead researcher explained they focused on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used online videos to verify the reports.

Scientists then integrated this data with information on the evolutionary relationships between living and extinct species of such primates.

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved approximately 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the researchers conclude. But the activity may not have been limited to their own species.

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

Biological Importance

While the scientific reasoning is discussed, Brindle said intimate contact could be employed in sexual contexts to potentially increase reproductive success or help choose between partners, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the activities of primates said that as intimate contact was observed in a wide range of apes it was logical its origins extend far into our ancient history, and an analysis of various types of intimate behavior among a wider variety of animals might push its origins back further still.

"Things that we think of as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

Another professor explained that kissing had a social component as it was not universal to all human groups.

"However, as humans we succeed or struggle on the strength of our relationships, and methods of promoting trust and closeness will have been significant for millions of years," she said. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and even Neanderthals and our own species collectively – kissed."