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From seabirds to polar bears, primates to great apes, certain species appear to kiss. Now, researchers suggest that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. In earlier research, scientists have discovered modern people and their thick-browed cousins shared 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 studies that has revealed humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Spin

"It certainly puts a different perspective on ancient interactions," Brindle said.

Writing in the journal Evolution and Human Behavior, Brindle and her team detail how, to explore the historical roots of kissing, they first had to come up with a definition that was not limited to how people kiss.

Describing Intimate Contact

"There have been some efforts to define a kiss, but it's largely human-centric, which implies that basically non-human species do not engage in this. Now we know that they likely engage, it might just not look from what human kissing looks like," explained Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish known as French grunts.

As a result the team came up with a description of intimate contact based on friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some movement of the oral area but absence of food.

Research Methods

Brindle said they concentrated on reports of intimate behavior in primates from Africa and Asian regions, including primates, apes and great apes, and employed digital recordings to verify the reports.

The researchers then combined this information with details on the genetic connections between extant and extinct types of such primates.

Evolutionary Timeline

The team say the findings indicate intimate contact developed approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is probable they, too, engaged in a kiss, the researchers say. But the behavior may not have been confined to their specific group.

"The fact that humans engage intimately, the reality that we now have shown that ancient relatives probably kissed, suggests that the both groups are probably did engage," the researcher noted.

Evolutionary Importance

While the evolutionary explanation is discussed, Brindle said kissing could be used in sexual contexts to possibly increase reproductive success or assist in selecting between mates, while it could assist strengthen connections when used in a non-sexual manner.

Another expert in the behavior of primates said that as intimate contact was seen in a wide range of primates it was logical its roots extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of species might push its origins back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Social Elements

Another professor said that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and ways of encouraging trust and intimacy will have been important for millions of years," she said. "It might be an image that seems a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be expected that Neanderthals – and including them and our human ancestors together – engaged intimately."