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From Galápagos albatrosses to polar bears, primates to orangutans, various animals appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Microbial Clues

It is not the first time scientists have proposed Neanderthals and early modern humans were intimately acquainted. In previous studies, scientists have found modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea aligned with studies that has found humans of certain genetic backgrounds have bits of Neanderthal DNA in their genome, revealing genetic mixing was at play.

Romantic Interpretation

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

Writing in the publication a scientific periodical, the researcher and her team report how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans smooch.

Defining Kissing

"There have been some previous attempts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact resembles," explained the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were something rather different – such as the processing and transfer of food, or "mouth contact", seen in aquatic species known as French grunts.

As a result the team came up with a definition of intimate contact centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the oral area but absence of food.

Study Approach

Brindle said they focused on accounts of intimate behavior in primates from the African continent and Asia, including primates, apes and orangutans, and used online videos to verify the observations.

Scientists then integrated this data with details on the evolutionary relationships between living and ancient species of such primates.

Historical Origins

Researchers propose the findings suggest intimate contact developed somewhere between 21.5 million and 16.9m years ago in the ancestors of the large apes.

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

"Reality that modern people engage intimately, the fact that we now have shown that Neanderthals probably kissed, indicates that the both groups are also likely to have engage," the researcher noted.

Biological Importance

Although the evolutionary explanation is debated, the expert said kissing could be used in sexual contexts to possibly enhance mating outcomes or assist in selecting between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes commented that as intimate contact was observed in a wide range of primates it made sense its roots extend far into our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might push its beginnings back even earlier still.

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

Social Aspects

An archaeology expert explained that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and ways of promoting confidence and closeness will have been significant for eons," she said. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it should be expected that Neanderthals – and even them and our own species together – kissed."