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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, various animals appear to kiss. Now, researchers suggest that ancient hominins did it too – and possibly locked lips with early Homo sapiens.

Common Microbial Evidence

It is not the first time experts have proposed ancient relatives and Homo sapiens were closely connected. In earlier research, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Probably they were kissing," she said, adding that the concept aligned with research that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"It certainly puts a more romantic perspective on human-Neanderthal relations," Brindle said.

Publishing in the journal a scientific periodical, the researcher and her team report how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how people kiss.

Describing Kissing

"There have been some efforts to define a kiss, but it's very much been focused on humans, which implies that basically non-human species don't kiss. Now we know that they probably do, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

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

Consequently the research group developed a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Methods

Brindle explained they focused on accounts of kissing in non-human species from Africa and Asia, including bonobos, apes and orangutans, and employed online videos to verify the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and ancient species of such primates.

Historical Origins

The team propose the findings indicate intimate contact evolved somewhere between 21.5m 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, engaged in a kiss, the scientists say. But the behavior may not have been limited to their own species.

"Reality that humans kiss, the fact that we now have demonstrated that ancient relatives probably kissed, suggests that the two [species] are also likely to have kissed," Brindle added.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert said kissing could be employed in sexual contexts to potentially increase reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates said 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 analysis of different forms of kissing among a wider variety of animals might extend its origins back further still.

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

Cultural Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been important for eons," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a rather ruthless and ancient history, but actually it should be no surprise that Neanderthals – and even them and our own species collectively – engaged intimately."