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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals appear to kiss. Currently, scientists suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Shared Oral Evidence

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were closely connected. Among earlier research, researchers have discovered modern people and their Neanderthal relatives shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the idea chimed with studies that has revealed people of certain genetic backgrounds contain ancient genetic material in their genome, revealing genetic mixing was occurring.

Intimate Interpretation

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

Publishing in the journal Evolution and Human Behavior, Brindle and her team report how, to investigate the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans smooch.

Defining Kissing

"Previously there were 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 probably do, it might just not look from what human kissing resembles," explained Brindle.

However, she noted some behaviors that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish known as certain marine animals.

As a result the research group came up with a description of kissing centered around friendly interactions involving intentional oral interaction with a member of the identical group, with some movement of the oral area but no transfer of food.

Research Approach

Brindle explained they focused on reports of kissing in primates from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the reports.

The researchers then combined this data with details on the evolutionary relationships between extant and extinct species of such primates.

Evolutionary Origins

The team propose the findings suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the researchers say. But the activity may not have been limited to their own species.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that Neanderthals very likely kissed, suggests that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Significance

While the scientific reasoning is debated, Brindle explained intimate contact could be used in sexual contexts to potentially increase mating outcomes or help choose between partners, while it might help strengthen connections when used in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was seen in a wide range of apes it made sense its roots lie deep in our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its beginnings back even earlier still.

"Things that we think of as signatures of human life, like intimate contact, are not unique to us if we examine carefully at different species," he said.

Social Elements

Another professor said that intimate contact had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of encouraging confidence and closeness will have been significant for millions of years," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including them and our own species together – engaged intimately."