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Among seabirds to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have suggested ancient relatives and early modern humans were closely connected. Among previous studies, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Probably they were kissing," she said, adding that the concept chimed with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Intimate Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and colleagues detail how, to explore the historical roots of intimate contact, they first had to develop a description that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some efforts to define a intimate act, but it's very much been focused on humans, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing looks like," explained Brindle.

Nonetheless, she said some behaviors that looked like kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in fish known as certain marine animals.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Research Methods

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used online videos to confirm the reports.

Scientists then combined this information with details on the evolutionary relationships between extant and extinct species of such primates.

Historical Timeline

The team propose the results suggest intimate contact developed somewhere between 21.5m and 16.9 million years ago in the ancestors of the great primates.

The position of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the scientists say. But the behavior may not have been confined to their own species.

"The fact that modern people kiss, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are also likely to have engage," the researcher noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert explained intimate contact could be used in reproductive situations to possibly increase mating outcomes or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of great apes said that as intimate contact was observed in a broad spectrum of primates it was logical its roots extend far into our evolutionary past, and an examination of different forms of kissing among a broader range of animals might push its beginnings back even earlier still.

"Things that we think of as signatures of human life, like kissing, are not unique to us if we look closely at other animals," he said.

Cultural Aspects

An archaeology expert said that intimate contact had a cultural element as it was not common to all societies.

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and methods of promoting trust and closeness will have been important for eons," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be expected that Neanderthals – and even them and our human ancestors together – kissed."