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Among seabirds to polar bears, primates to orangutans, certain species appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly locked lips with early Homo sapiens.

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 modern people and their thick-browed cousins possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept chimed with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

"This offers a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the journal a scientific periodical, Brindle and colleagues detail how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how people kiss.

Describing Intimate Contact

"Previously there were some efforts to define a kiss, but it's largely human-centric, which implies that basically other animals don't kiss. Currently we know that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in aquatic species called certain marine animals.

Consequently the team developed a definition of kissing based on 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.

Study Methods

Brindle said they concentrated on reports of kissing in non-human species from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to verify the observations.

Scientists then combined this information with information on the evolutionary relationships between living and extinct types of such animals.

Evolutionary Origins

Researchers say the results suggest kissing developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the researchers say. But the behavior may not have been confined to their own species.

"The fact that modern people kiss, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the two [species] are also likely to have kissed," the researcher noted.

Evolutionary Importance

While the scientific reasoning is debated, the expert said kissing could be used in sexual contexts to potentially increase reproductive success or assist in selecting between partners, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes commented that as intimate contact was seen in a broad spectrum of primates it was logical its origins extend far into our evolutionary past, and an analysis of various types of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Things that we think of as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

Another professor said 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 confidence and intimacy will have been important for millions of years," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it should be no surprise that ancient hominins – and even them and our human ancestors collectively – kissed."