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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists propose that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.

Common Microbial Clues

This isn't the initial instance experts have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Likely they were kissing," she said, adding that the idea chimed with studies that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating interbreeding was occurring.

Romantic Spin

"It certainly puts a more romantic perspective on ancient interactions," the lead researcher commented.

Writing in the publication a scientific periodical, the researcher and colleagues 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.

Describing Kissing

"There have been some previous attempts to describe a intimate act, but it's very much been human-centric, which implies that basically non-human species do not engage in this. Now we understand that they likely engage, it may appear different from what our intimate contact looks like," said Brindle.

Nonetheless, she said some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", seen in aquatic species called certain marine animals.

Consequently the research group came up with a definition of kissing centered around social behaviors involving intentional oral interaction with a individual of the identical group, with some motion of the mouth but no transfer of food.

Research Approach

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

The researchers then integrated this information with details on the evolutionary relationships between extant and extinct species of such primates.

Evolutionary Origins

Researchers say the findings suggest kissing evolved approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of Neanderthals on this evolutionary lineage suggests it is probable they, too, indulged in a kiss, the scientists conclude. But the behavior may not have been limited to their specific group.

"The fact that humans kiss, the fact that we now have demonstrated that Neanderthals probably kissed, indicates that the both groups are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is debated, Brindle said intimate contact could be employed in sexual contexts to possibly enhance mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of primates it was logical its origins lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of species might extend its beginnings back even earlier still.

"Things that we consider as signatures of human life, like kissing, are not unique to us if we look closely 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 humans we thrive or fail on the quality of our emotional bonds, and methods of encouraging confidence and closeness will have been significant for millions of years," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be no surprise that Neanderthals – and even them and our own species together – kissed."