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Among seabirds to Arctic mammals, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Clues

This isn't the initial instance scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. In earlier research, scientists have found humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genome, revealing interbreeding was occurring.

Romantic Interpretation

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

Writing in the publication a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not restricted by how humans smooch.

Defining Intimate Contact

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

However, she noted some actions that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called certain marine animals.

Consequently the team came up with a description of kissing based on social behaviors involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of nutrition.

Research Methods

The lead researcher explained they focused on reports of intimate behavior in non-human species from the African continent and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the reports.

Scientists then integrated this information with details on the genetic connections between extant and ancient species of such animals.

Evolutionary Origins

Researchers propose the findings indicate intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree means it is probable they, too, engaged in a kiss, the scientists conclude. But the activity may not have been confined to their specific group.

"Reality that humans kiss, the fact that we currently have demonstrated that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to potentially increase mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates commented that as intimate contact was observed in a wide range of primates it made sense its origins extend far into our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might extend its origins back even earlier still.

"Things that we consider as signatures of our species, like kissing, are not unique to us if we look closely at different species," he said.

Cultural Aspects

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

"However, as humans we succeed or struggle on the strength of our relationships, and methods of promoting trust and closeness will have been important for eons," the professor stated. "It might be an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it ought to be no surprise that ancient hominins – and even Neanderthals and our human ancestors collectively – kissed."