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From seabirds to Arctic mammals, primates to great apes, various animals appear to kiss. Currently, scientists propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance experts have proposed Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have found humans and their thick-browed cousins possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," she said, adding that the idea aligned with research that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genome, revealing interbreeding was at play.

Intimate Interpretation

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

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the historical roots of intimate contact, 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 means that basically non-human species do not engage in this. Currently we know that they likely engage, it might just not look from what our intimate contact looks like," explained Brindle.

However, she said some actions that resembled kissing were distinct activities – such as the processing and food sharing, or "mouth contact", observed in aquatic species known as French grunts.

As a result the research group came up with a description of intimate contact based on social behaviors involving directed mouth-to-mouth contact with a member of the same species, with some movement of the mouth but absence of food.

Study Approach

Brindle said they concentrated on accounts of kissing in non-human species from the African continent and Asia, including bonobos, apes and great apes, and used online videos to confirm the reports.

Scientists then combined this data with details on the evolutionary relationships between extant and extinct species of such primates.

Evolutionary Origins

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

Placement of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the researchers say. But the activity may not have been limited to their own species.

"Reality that modern people kiss, the reality that we now have demonstrated that ancient relatives very likely engaged, indicates that the two [species] are also likely to have engage," Brindle added.

Evolutionary Significance

While the scientific reasoning is debated, the expert explained intimate contact could be used in reproductive situations to potentially enhance mating outcomes or help choose between partners, while it could assist reinforce bonding when practiced in a non-sexual manner.

Another expert in the behavior of primates commented that as intimate contact was seen in a broad spectrum of primates it was logical its roots lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might extend its origins back further still.

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

Cultural Elements

Another professor explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and methods of promoting confidence and closeness will have been important for millions of years," the professor stated. "This could represent an image that seems a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even Neanderthals and our human ancestors collectively – engaged intimately."