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From Galápagos albatrosses to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have proposed Neanderthals and early modern humans were closely connected. In earlier research, scientists have found humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, explaining that the idea aligned with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and her team report how, to investigate the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how humans kiss.

Defining Kissing

"There have been some efforts to describe a kiss, but it's very much been human-centric, which means that basically non-human species don't kiss. Currently we know that they likely engage, it might just not look from what human kissing resembles," explained Brindle.

Nonetheless, she said some actions that resembled intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", seen in fish known as French grunts.

As a result the research group developed a description of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Research Approach

Brindle explained they concentrated on accounts of intimate behavior in primates from the African continent and Asian regions, including bonobos, chimpanzees and great apes, and used digital recordings to confirm the observations.

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

Historical Origins

The team propose the findings indicate kissing developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the researchers say. But the activity might not have been limited to their own species.

"Reality that modern people kiss, the fact that we now have shown that Neanderthals probably engaged, suggests that the two [species] are probably did kissed," Brindle added.

Evolutionary Significance

Although the evolutionary explanation is debated, the expert said kissing could be used in sexual contexts to possibly increase mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was observed in a broad spectrum 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 push its beginnings back further still.

"Things that we consider as signatures of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Cultural Aspects

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

"However, as people we thrive or fail on the quality of our relationships, and methods 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 misplaced ideas of a supposedly aggressive and ancient history, but really it should be no surprise that ancient hominins – and including them and our own species collectively – engaged intimately."