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From seabirds to Arctic mammals, primates to great apes, certain species appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, researchers have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the concept aligned with studies that has revealed humans of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on ancient interactions," Brindle said.

Writing in the publication Evolution and Human Behavior, the researcher and her team report how, to investigate the historical roots of intimate contact, they first had to develop a description that was not limited to how humans smooch.

Describing Kissing

"There have been some efforts to describe a kiss, but it's largely focused on humans, which implies that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing looks like," said the evolutionary biologist.

Nonetheless, she noted some behaviors that looked like intimate contact were distinct activities – such as the chewing and transfer of food, or "mouth contact", observed in aquatic species known as French grunts.

Consequently the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a individual of the same species, with some motion of the mouth but absence of nutrition.

Research Approach

Brindle explained they concentrated on reports of kissing in primates from the African continent and Asian regions, including primates, apes and orangutans, and used digital recordings to confirm the observations.

The researchers then integrated this data with information on the evolutionary relationships between living and extinct types of such primates.

Historical Timeline

The team say the results indicate intimate contact evolved somewhere between 21.5 million and 16.9m years ago in the predecessors of the large apes.

The position of ancient hominins on this evolutionary lineage suggests it is probable they, too, engaged in a kiss, the scientists say. But the behavior might not have been confined to their specific group.

"The fact that modern people kiss, the fact that we currently have shown that ancient relatives probably kissed, suggests that the both groups are also likely to have engage," the researcher added.

Biological Importance

Although the scientific reasoning is debated, the expert said kissing could be employed in sexual contexts to potentially increase reproductive success or help choose between mates, while it might help strengthen connections when practiced in a platonic way.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a wide range of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of animals might push its beginnings back even earlier still.

"Things that we think of as signatures of our species, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Aspects

An archaeology expert said that intimate contact had a cultural element as it was not common to all human groups.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging trust and intimacy will have been important for millions of years," she said. "This could represent an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and including them and our own species collectively – kissed."