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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species appear to kiss. Now, researchers suggest that ancient hominins did it too – and might even have exchanged kisses with modern humans.

Shared Microbial Clues

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, researchers have found modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, explaining that the idea chimed with research that has found humans of non-African ancestry have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different spin on human-Neanderthal relations," Brindle commented.

Publishing in the publication a scientific periodical, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how people kiss.

Defining Kissing

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which means that basically non-human species do not engage in this. Now we understand that they likely engage, it may appear different from what human kissing looks like," explained Brindle.

However, she said some behaviors that resembled intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", observed in fish known as certain marine animals.

Consequently the research group developed a definition of intimate contact centered around social behaviors involving intentional oral interaction with a member of the identical group, with some motion of the oral area but no transfer of food.

Research Approach

Brindle explained they focused on accounts of kissing in non-human species from Africa and Asian regions, including bonobos, chimpanzees and orangutans, and employed digital recordings to verify the observations.

The researchers then combined this data with information on the evolutionary relationships between living and ancient species of such primates.

Historical Timeline

Researchers say the findings indicate kissing evolved somewhere between 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree suggests it is probable they, too, engaged in a kiss, the scientists say. But the behavior might not have been limited to their own species.

"Reality that modern people kiss, the fact that we currently have demonstrated that ancient relatives probably kissed, indicates that the two [species] are probably did kissed," the researcher noted.

Biological Importance

While the scientific reasoning is debated, Brindle said kissing could be employed in reproductive situations to potentially enhance mating outcomes or assist in selecting between mates, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the activities of primates commented that as intimate contact was seen in a wide range of primates it was logical its origins lie deep in our ancient history, and an examination of various types of kissing among a broader range of animals might push its beginnings back even earlier still.

"Things that we think of as characteristics of our species, like kissing, are not exclusive to us if we examine carefully at other animals," the expert noted.

Social Elements

An archaeology expert said that kissing had a cultural element as it was not common to all human groups.

"Nonetheless, as people we thrive or fail on the strength of our relationships, and methods of promoting confidence and closeness will have been significant for millions of years," she said. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be expected that Neanderthals – and including Neanderthals and our own species together – kissed."