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From Galápagos albatrosses to polar bears, primates to orangutans, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins did it too – and might even have 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 previous studies, scientists have found modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the idea aligned with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genome, demonstrating interbreeding was at play.

Romantic Spin

"It certainly puts a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the publication a scientific periodical, the researcher and colleagues detail how, to explore the historical roots of kissing, they first had to develop a definition 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 human-centric, which implies that basically other animals do not engage in this. Currently we know that they probably do, it might just not look from what our intimate contact resembles," explained Brindle.

Nonetheless, she said some actions that looked like intimate contact were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in fish known as French grunts.

As a result the team developed a description of kissing centered around social behaviors involving directed oral interaction with a individual of the identical group, with some movement of the mouth but absence of food.

Research Methods

The lead researcher said they concentrated on accounts of intimate behavior in non-human species from Africa and Asia, including primates, chimpanzees and orangutans, and used online videos to confirm the observations.

The researchers then combined this information with details on the genetic connections between extant and ancient species of such primates.

Historical Timeline

Researchers propose the results indicate intimate contact developed somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the researchers say. But the activity may not have been limited to their specific group.

"Reality that humans kiss, the reality that we currently have shown that ancient relatives very likely kissed, indicates that the both groups are also likely to have kissed," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, the expert said kissing could be employed in reproductive situations to possibly enhance reproductive success or help choose between partners, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was observed in a broad spectrum of apes it was logical its origins extend far into our evolutionary past, and an examination of various types of intimate behavior among a wider variety of species might push its origins back even earlier still.

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

Social Elements

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

"However, as humans we succeed or struggle on the quality of our emotional bonds, and methods of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it ought to be no surprise that ancient hominins – and including them and our own species together – kissed."