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From seabirds to Arctic mammals, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

It is not the first time experts have proposed ancient relatives and early modern humans were intimately acquainted. Among earlier research, researchers have discovered modern people and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the concept aligned with studies that has revealed people of non-African ancestry have bits of ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Romantic Interpretation

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

Writing in the journal Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of intimate contact, they first had to come up with a description that was not limited to how people kiss.

Defining Kissing

"There have been some efforts to define a kiss, but it's very much been human-centric, which means that basically other animals do not engage in this. Now we know that they probably do, it might just not look from what our intimate contact resembles," said Brindle.

However, she said some behaviors that looked like kissing were distinct activities – such as the processing and transfer of food, or "mouth contact", observed in aquatic species called French grunts.

Consequently the team came up with a description of kissing centered around friendly interactions involving intentional oral interaction with a individual of the same species, with some motion of the oral area but absence of food.

Study Approach

The lead researcher explained they concentrated on reports of intimate behavior in non-human species from the African continent and Asia, including bonobos, apes and great apes, and used digital recordings to confirm the reports.

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

Evolutionary Timeline

Researchers say the findings indicate kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of Neanderthals on this family tree suggests it is probable they, too, engaged in a intimate act, the researchers conclude. But the behavior might not have been confined to their specific group.

"Reality that humans engage intimately, the fact that we now have demonstrated that ancient relatives very likely engaged, indicates that the two [species] are also likely to have kissed," the researcher noted.

Biological Importance

While the scientific reasoning is debated, Brindle explained intimate contact could be used in reproductive situations to potentially increase reproductive success or help choose between mates, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was seen in a wide range of apes it made sense its origins extend far into our ancient history, and an examination of various types of kissing among a broader range of animals might push its origins back even earlier still.

"Behaviors that we think of as characteristics of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Elements

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

"Nonetheless, as people we succeed or struggle on the strength of our emotional bonds, and ways of promoting confidence and intimacy will have been important for eons," the professor stated. "It might be an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our own species collectively – kissed."