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From Galápagos albatrosses to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genome, demonstrating genetic mixing was at play.

Romantic Interpretation

"This offers a more romantic perspective on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, the researcher and her team detail how, to investigate the historical roots of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Defining Kissing

"There have been some previous attempts to define a kiss, but it's very much been human-centric, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it might just not look from what human kissing resembles," explained Brindle.

However, she said some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", seen in fish known as certain marine animals.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the mouth but no transfer of food.

Study Methods

Brindle said they focused on accounts of kissing in primates from Africa and Asia, including primates, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then combined this information with details on the evolutionary relationships between extant and extinct types of such primates.

Evolutionary Origins

The team say the results suggest kissing evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a kiss, the scientists say. But the activity might not have been confined to their specific group.

"The fact that humans engage intimately, the fact that we now have shown that ancient relatives very likely kissed, suggests that the two [species] are probably did kissed," the researcher noted.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert explained kissing could be used in reproductive situations to possibly increase reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it made sense its origins lie deep in our evolutionary past, and an examination of different forms of kissing among a broader range of animals might extend its beginnings back even earlier still.

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

Social Aspects

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of promoting trust and intimacy will have been significant for eons," the professor stated. "This could represent an concept that appears a bit contradictory to our misplaced ideas of a rather ruthless and aggressive past, but really it should be expected that Neanderthals – and including them and our own species together – kissed."