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Among seabirds to Arctic mammals, chimpanzees to orangutans, various animals appear to kiss. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly locked lips with modern humans.

Common Oral Evidence

It is not the first time experts have proposed Neanderthals and Homo sapiens were intimately acquainted. In earlier research, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the idea aligned with studies that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was occurring.

Intimate Interpretation

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

Publishing in the journal a scientific periodical, Brindle and her team report how, to explore the historical roots of kissing, they first had to develop a definition that was not restricted by how humans smooch.

Defining Intimate Contact

"There have been some efforts to describe a kiss, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Now we know that they probably do, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

However, she noted some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the team came up with a definition of intimate contact based on social behaviors involving intentional mouth-to-mouth contact with a member of the identical group, with some movement of the mouth but no transfer of nutrition.

Research Methods

Brindle explained they focused on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and used online videos to confirm the reports.

Scientists then integrated this information with information on the evolutionary relationships between living and extinct types of such primates.

Historical Timeline

The team say the results suggest kissing evolved approximately 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this family tree suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the activity might not have been confined to their specific group.

"The fact that modern people engage intimately, the fact that we currently have demonstrated that ancient relatives probably engaged, indicates that the both groups are probably did engage," Brindle added.

Evolutionary Importance

Although the evolutionary explanation is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a non-sexual manner.

A separate researcher in the behavior of great apes said that as intimate contact was seen in a wide range of primates it was logical its roots lie deep in our evolutionary past, and an analysis of different forms of intimate behavior among a wider variety of animals might extend its origins back further still.

"Things that we think of as signatures of human life, like intimate contact, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

An archaeology expert said that intimate contact had a social component as it was not universal to all human groups.

"However, as humans we succeed or struggle on the quality of our relationships, and methods of promoting trust and closeness will have been significant for millions of years," the professor stated. "This could represent an image that seems a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that ancient hominins – and including Neanderthals and our own species collectively – engaged intimately."