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From Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

This isn't the initial instance experts have suggested Neanderthals and early modern humans were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the concept aligned with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher commented.

Writing in the publication a scientific periodical, Brindle and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some efforts to define a kiss, but it's largely focused on humans, which implies that basically other animals do not engage in this. Currently we understand that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some actions that resembled kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called French grunts.

Consequently the research group came up with a definition of kissing based on social behaviors involving directed oral interaction with a member of the same species, with some motion of the mouth but absence of nutrition.

Research Methods

Brindle said they concentrated on reports of kissing in primates from Africa and Asian regions, including bonobos, chimpanzees and great apes, and employed digital recordings to verify the observations.

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

Evolutionary Timeline

The team say the results suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the large apes.

The position of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a kiss, the scientists say. But the behavior may not have been limited to their own species.

"Reality that modern people engage intimately, the reality that we now have shown that ancient relatives very likely engaged, indicates that the two [species] are probably did kissed," the researcher noted.

Evolutionary Importance

While the evolutionary explanation is debated, the expert explained kissing could be employed in reproductive situations to possibly increase mating outcomes or help choose between partners, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was observed in a wide range of primates it made sense its origins lie deep in our ancient history, and an analysis of different forms of kissing among a wider variety of species might extend its origins back even earlier still.

"Behaviors that we consider as signatures of human life, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Social Elements

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

"However, as humans we thrive or fail on the strength of our emotional bonds, and methods of promoting trust and intimacy will have been important for millions of years," she said. "It might be an image that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and even them and our human ancestors collectively – engaged intimately."