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Among seabirds to Arctic mammals, chimpanzees to great apes, certain species appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Clues

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have found humans and their Neanderthal relatives possessed the identical oral bacteria for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Likely they were engaging in intimate contact," the researcher noted, explaining that the idea aligned with studies that has revealed people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.

Intimate Spin

"This offers a more romantic spin on human-Neanderthal relations," Brindle commented.

Writing in the publication a scientific periodical, Brindle and her team detail how, to explore the historical roots of kissing, they first had to develop a description that was not restricted by how humans smooch.

Describing Intimate Contact

"There have been some efforts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species don't kiss. Now we know that they probably do, it may appear different from what human kissing looks like," said Brindle.

Nonetheless, she said some actions that looked like intimate contact were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish called certain marine animals.

As a result the research group developed a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

The lead researcher said 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 confirm the observations.

The researchers then integrated this data with details on the evolutionary relationships between living and extinct species of such animals.

Evolutionary Origins

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

Placement of ancient hominins on this family tree means it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been confined to their own species.

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

Evolutionary Significance

While the scientific reasoning is discussed, the expert said kissing could be used in sexual contexts to possibly enhance reproductive success or help choose between mates, while it might help reinforce bonding when practiced in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of species might push its beginnings back even earlier still.

"Behaviors that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Elements

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

"Nonetheless, as humans we succeed or struggle on the strength of our emotional bonds, and methods of encouraging trust and intimacy will have been significant for millions of years," the professor stated. "This could represent an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."