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From seabirds to polar bears, primates to great apes, certain species engage in mouth-to-mouth contact. Now, researchers suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Shared Microbial Evidence

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among earlier research, scientists have found modern people and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the two species split, suggesting they swapped saliva.

"Likely they were kissing," the researcher noted, adding that the concept chimed with research that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Interpretation

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

Publishing in the publication Evolution and Human Behavior, the researcher and her team detail how, to explore the historical roots of kissing, they first had to come up with a description that was not restricted by how people smooch.

Describing Kissing

"Previously there were some efforts to define a intimate act, but it's largely focused on humans, which means that essentially other animals don't kiss. Now we understand that they likely engage, it may appear different from what human kissing resembles," said Brindle.

However, she noted some behaviors that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in fish called French grunts.

Consequently the team came up with a description of intimate contact centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the oral area but no transfer of nutrition.

Research Methods

Brindle said they concentrated on reports of kissing in non-human species from the African continent and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to verify the observations.

Scientists then combined this data with details on the genetic connections between living and extinct species of such primates.

Evolutionary Origins

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

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

"The fact that humans engage intimately, the fact that we currently have shown that ancient relatives very likely kissed, suggests that the two [species] are probably did engage," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is discussed, Brindle said kissing could be used in sexual contexts to possibly enhance mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was seen in a broad spectrum of primates it was logical its origins lie deep in our evolutionary past, and an analysis of various types of intimate behavior among a broader range of animals might push its origins back further still.

"Things that we think of as characteristics of our species, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Social Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of encouraging confidence and closeness will have been significant for eons," she said. "This could represent an concept that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be no surprise that Neanderthals – and including Neanderthals and our own species together – kissed."