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Among seabirds to polar bears, chimpanzees to great apes, various animals appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Shared Oral Evidence

This isn't the initial instance scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the idea chimed with research that has revealed people of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Romantic Spin

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

Publishing 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 come up with a definition that was not limited to how people smooch.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's very much been focused on humans, which implies that basically other animals do not engage in this. Now we know that they probably do, it may appear different from what our intimate contact looks like," explained Brindle.

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

As a result 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 motion of the oral area but no transfer of nutrition.

Research Approach

Brindle said they concentrated on accounts of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the observations.

The researchers then integrated this data with details on the genetic connections between living and ancient types of such primates.

Evolutionary Origins

Researchers propose the results suggest kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree means it is likely they, too, engaged in a intimate act, the scientists say. But the behavior may not have been confined to their own species.

"Reality that modern people engage intimately, the reality that we currently have demonstrated that Neanderthals very likely engaged, suggests that the both groups are probably did kissed," the researcher noted.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle said intimate contact could be employed in reproductive situations to possibly increase reproductive success or assist in selecting between partners, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was observed in a wide range of primates it made sense its origins lie deep in our ancient history, and an analysis of various types of kissing among a broader range of animals might push its beginnings back further still.

"Behaviors that we think of as signatures of our species, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"However, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting trust and closeness will have been significant for eons," the professor stated. "This could represent an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but actually it ought to be no surprise that ancient hominins – and even them and our own species collectively – kissed."