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Among seabirds to polar bears, chimpanzees to great apes, certain species appear to kiss. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance experts have proposed Neanderthals and early modern humans were closely connected. Among previous studies, researchers have discovered modern people and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, explaining that the idea chimed with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Spin

"This offers a different spin on ancient interactions," Brindle commented.

Publishing 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 description that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's largely human-centric, which means that essentially other animals do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing looks like," explained Brindle.

However, she said some actions that resembled intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", observed in fish known as certain marine animals.

As a result the research group came up with a definition of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some movement of the oral area but absence of food.

Study Approach

Brindle explained they focused on reports of kissing in primates from Africa and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to confirm the reports.

Scientists then integrated this data with information on the evolutionary relationships between extant and extinct types of such animals.

Evolutionary Origins

Researchers propose the findings suggest kissing evolved approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree means it is likely they, too, indulged in a kiss, the scientists say. But the behavior might not have been limited to their own species.

"The fact that humans engage intimately, the reality that we currently have demonstrated that ancient relatives very likely kissed, indicates that the two [species] are probably did engage," Brindle added.

Evolutionary Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to potentially increase mating outcomes or help choose between mates, while it might help strengthen connections when practiced in a platonic way.

Another expert in the behavior of great apes commented that as kissing behavior was observed in a broad spectrum of primates it made sense its origins extend far into our evolutionary past, and an examination of various types of kissing among a wider variety of animals might push its origins back even earlier still.

"Behaviors that we think of as signatures of our species, like kissing, are not unique to us if we look closely at other animals," the expert noted.

Cultural Elements

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

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of promoting confidence and closeness will have been significant for eons," the professor stated. "It might be an image that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."