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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Shared Microbial Clues

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were closely connected. In earlier research, researchers have found humans and their thick-browed cousins possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," she said, explaining that the idea aligned with research that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Intimate Interpretation

"This offers a different perspective on ancient interactions," Brindle said.

Writing in the journal a scientific periodical, Brindle and colleagues detail how, to investigate the historical roots of kissing, they first had to come up with a description that was not limited to how humans kiss.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's largely human-centric, which implies that basically other animals don't kiss. Currently we know that they probably do, it might just not look from what human kissing looks like," explained the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the processing and transfer of food, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the team came up with a description of kissing centered around friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but absence of nutrition.

Study Approach

The lead researcher said they focused on reports of kissing in primates from Africa and Asian regions, including primates, apes and great apes, and employed digital recordings to verify the reports.

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

Historical Timeline

Researchers propose the findings suggest kissing developed approximately 21.5 million and 16.9 million years ago in the predecessors of the large apes.

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

"The fact that humans engage intimately, the reality that we currently have demonstrated that Neanderthals very likely engaged, indicates that the two [species] are probably did kissed," the researcher noted.

Biological Importance

Although the evolutionary explanation is debated, Brindle said intimate contact could be used in sexual contexts to potentially enhance reproductive success or assist in selecting between partners, while it could assist strengthen connections when used in a non-sexual manner.

A separate researcher in the activities of great apes commented that as kissing behavior was observed in a wide range of apes it made sense its origins lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of species might push its origins back even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Aspects

Another professor explained that intimate contact had a social component as it was not common to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been significant for millions of years," she said. "This could represent an image that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and even Neanderthals and our human ancestors collectively – engaged intimately."