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Among seabirds to polar bears, primates to great apes, certain species appear to kiss. Currently, scientists suggest that Neanderthals also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time scientists have proposed Neanderthals and Homo sapiens were closely connected. Among earlier research, scientists 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 exchanged oral fluids.

"Probably they were engaging in intimate contact," the researcher noted, explaining that the concept chimed with studies that has found people of non-African ancestry have bits of Neanderthal DNA in their genome, demonstrating genetic mixing was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher said.

Publishing in the publication Evolution and Human Behavior, Brindle and her team report how, to explore the historical roots of kissing, they first had to develop a description that was not limited to how people smooch.

Defining Intimate Contact

"Previously there were some efforts to describe a kiss, but it's largely focused on humans, which means that basically other animals don't kiss. Currently we know that they likely engage, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she said some actions that looked like kissing were something rather different – such as the chewing and transfer of food, or "mouth contact", observed in aquatic species known as certain marine animals.

As a result the team developed a definition of kissing based on friendly interactions involving directed oral interaction with a individual of the same species, with some movement of the mouth but no transfer of nutrition.

Study Methods

Brindle said they concentrated on reports of intimate behavior in primates from the African continent and Asian regions, including bonobos, chimpanzees and orangutans, and employed online videos to verify the observations.

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

Evolutionary Timeline

The team propose the results suggest kissing evolved approximately 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of ancient hominins on this family tree suggests it is probable they, too, indulged in a intimate act, the scientists conclude. But the activity may not have been limited to their own species.

"The fact that humans kiss, the fact that we now have shown that Neanderthals probably engaged, suggests that the both groups are also likely to have engage," the researcher noted.

Evolutionary Importance

Although the evolutionary explanation is discussed, Brindle explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between mates, while it might help strengthen connections when used in a platonic way.

A separate researcher in the behavior of great apes commented that as intimate contact was seen in a wide range of primates it was logical its origins extend far into our evolutionary past, and an analysis of various types of intimate behavior among a wider variety of animals might push its beginnings back further still.

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

Cultural Elements

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and ways of promoting trust and intimacy will have been important for eons," the professor stated. "It might be an concept 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 own species together – kissed."