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Among seabirds to polar bears, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals did it too – and possibly locked lips with modern humans.

Common Oral Evidence

This isn't the initial instance experts have proposed ancient relatives and early modern humans were intimately acquainted. Among earlier research, scientists have found modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were kissing," she said, adding that the concept chimed with research that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genome, revealing interbreeding was at play.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and colleagues report how, to investigate the historical roots of intimate contact, they first had to come up with a definition 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 implies that basically non-human species don't kiss. Currently we understand that they likely engage, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some actions that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the team developed a description of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the oral area but no transfer of food.

Study Methods

The lead researcher said they concentrated on accounts of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and employed online videos to verify the reports.

Scientists then combined this data with details on the evolutionary relationships between extant and ancient types of such primates.

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is probable they, too, indulged in a intimate act, the scientists conclude. But the activity may not have been limited to their own species.

"Reality that humans engage intimately, the reality that we currently have demonstrated that Neanderthals probably engaged, indicates that the two [species] are also likely to have engage," Brindle added.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to possibly increase mating outcomes or help choose between partners, while it might help strengthen connections when used in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of apes it was logical its roots extend far into our evolutionary past, and an examination of different forms of kissing among a wider variety of animals 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," the expert noted.

Cultural Elements

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

"Nonetheless, as people we thrive or fail on the quality of our emotional bonds, and ways of promoting confidence and intimacy will have been significant for millions of years," the professor stated. "It might be an image that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."