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Among seabirds to polar bears, primates to great apes, certain species appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Evidence

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were closely connected. In previous studies, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the idea chimed with research that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher commented.

Publishing in the publication a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of kissing, they first had to develop a definition that was not restricted by how people smooch.

Describing Kissing

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which means that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing looks like," said Brindle.

However, she said some behaviors that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", 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 member of the identical group, with some movement of the oral area but absence of nutrition.

Study Methods

Brindle said they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including primates, apes and great apes, and used digital recordings to confirm the reports.

Scientists then combined this information with details on the genetic connections between living and ancient types of such animals.

Historical Timeline

Researchers say the results suggest kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

The position of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the researchers say. But the activity may not have been confined to their own species.

"The fact that humans kiss, the fact that we now have shown that ancient relatives very likely kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was observed in a wide range of primates it made sense its origins extend far into our ancient history, and an analysis of different forms of kissing among a wider variety of animals might push its origins back further still.

"Behaviors that we consider as signatures of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Social Aspects

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

"However, as people we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for millions of years," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and including them and our human ancestors collectively – kissed."