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From seabirds to polar bears, primates to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were closely connected. In previous studies, researchers have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Likely they were kissing," she said, adding that the idea chimed with research that has revealed humans of non-African ancestry contain Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Intimate Spin

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to develop a definition that was not restricted by how humans smooch.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's very much been focused on humans, which implies that basically other animals do not engage in this. Currently we know that they probably do, it may appear different from what our intimate contact looks like," said Brindle.

Nonetheless, she noted some behaviors that looked like intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", observed in fish called French grunts.

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

Study Methods

Brindle explained they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and great apes, and used digital recordings to verify the observations.

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

Historical Timeline

The team say the results indicate intimate contact developed somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

The position of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the behavior might not have been confined to their specific group.

"Reality that modern people kiss, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the both groups are probably did kissed," Brindle noted.

Biological Importance

Although the scientific reasoning is discussed, Brindle said intimate contact could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

Another expert in the activities of primates commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins lie deep in our ancient history, and an analysis of various types of kissing among a wider variety of species might extend its origins back further still.

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

Cultural Aspects

An archaeology expert explained that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and ways of encouraging trust and closeness will have been significant for eons," the professor stated. "It might be an image that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be no surprise that Neanderthals – and including Neanderthals and our human ancestors collectively – engaged intimately."