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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals appear to kiss. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Shared Oral Clues

It is not the first time experts have suggested ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, researchers have found humans and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, implying they exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, adding that the concept aligned with studies that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Spin

"This offers a different spin on human-Neanderthal relations," Brindle said.

Writing in the publication Evolution and Human Behavior, the researcher and her team detail how, to explore the evolutionary origins of intimate contact, they first had to develop a definition that was not limited to how humans kiss.

Defining Intimate Contact

"Previously there were some efforts to describe a intimate act, but it's largely human-centric, which means that basically other animals do not engage in this. Now we know that they likely engage, it might just not look from what our intimate contact looks like," said Brindle.

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

As a result the team came up with a description of kissing centered around friendly interactions involving directed mouth-to-mouth contact with a individual of the identical group, with some movement of the mouth but no transfer of nutrition.

Study Approach

Brindle said they focused on reports of intimate behavior in non-human species from Africa and Asian regions, including primates, apes and great apes, and used online videos to verify the reports.

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

Evolutionary Timeline

Researchers say the results suggest intimate contact evolved approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this family tree suggests it is likely they, too, indulged in a kiss, the scientists say. But the activity may 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 both groups are also likely to have engage," Brindle noted.

Biological Significance

While the evolutionary explanation is discussed, the expert explained kissing could be employed in reproductive situations to possibly increase mating outcomes or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates commented that as kissing behavior was observed in a broad spectrum of apes it made sense its origins extend far into our ancient history, and an analysis of various types of kissing among a broader range of animals might push its origins back even earlier still.

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

Cultural Aspects

Another professor said that intimate contact had a social component as it was not common to all human groups.

"However, as humans we thrive or fail on the quality of our relationships, and methods of promoting trust 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 really it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."