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Among seabirds to polar bears, chimpanzees to great apes, various animals appear to kiss. Currently, scientists propose that ancient hominins did it too – and might even have locked lips with modern humans.

Common Oral Clues

It is not the first time experts have suggested Neanderthals and Homo sapiens were intimately acquainted. Among earlier research, researchers have discovered modern people and their Neanderthal relatives shared 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 studies that has revealed people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to explore the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"Previously there were some previous attempts to define a kiss, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained Brindle.

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

Consequently the research group developed a description of kissing centered around social behaviors involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of food.

Research Methods

Brindle explained they focused on accounts of kissing in primates from Africa and Asian regions, including primates, chimpanzees and great apes, and employed online videos to verify the observations.

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

Evolutionary Timeline

Researchers propose the results suggest intimate contact developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

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

"The fact that modern people kiss, the reality that we currently have demonstrated that ancient relatives very likely engaged, suggests that the two [species] are probably did engage," the researcher added.

Biological Significance

Although the evolutionary explanation is discussed, the expert said intimate contact could be used in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help strengthen connections when practiced in a non-sexual manner.

Another expert in the behavior of great apes said that as intimate contact was observed in a wide range of primates it made sense its roots lie deep in our evolutionary past, and an examination of different forms of intimate behavior among a wider variety of species might push its origins back further still.

"Things that we think of as signatures of our species, like kissing, are not exclusive to us if we examine carefully at other animals," he said.

Social Elements

An archaeology expert said that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as people we thrive or fail on the strength of our relationships, and methods of promoting trust and intimacy will have been important for millions of years," the professor stated. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be expected that ancient hominins – and including them and our human ancestors collectively – kissed."