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Among Galápagos albatrosses to polar bears, primates to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists propose that ancient hominins did it too – and possibly exchanged kisses with modern humans.

Shared Oral Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In earlier research, scientists have found humans and their Neanderthal relatives possessed the same mouth microbe for hundreds of thousands of years after the two species split, implying they swapped saliva.

"Likely they were kissing," she said, adding that the idea chimed with studies that has revealed humans of non-African ancestry have bits of ancient genetic material in their genome, demonstrating genetic mixing was occurring.

Romantic Spin

"This offers a more romantic perspective on ancient interactions," the lead researcher commented.

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of kissing, they first had to develop a definition that was not restricted by how people smooch.

Describing Intimate Contact

"There have been some previous attempts to define a intimate act, but it's very much been human-centric, which means that essentially 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 resembles," explained Brindle.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in aquatic species known as certain marine animals.

As a result the research group developed a definition of kissing centered around friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the mouth but absence of nutrition.

Research Methods

Brindle explained they focused on reports of kissing in primates from the African continent and Asian regions, including bonobos, apes and great apes, and used digital recordings to confirm the reports.

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

Historical Origins

The team say the results indicate kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

Placement of ancient hominins on this evolutionary lineage suggests it is likely they, too, indulged in a kiss, the researchers conclude. But the activity may not have been confined to their specific group.

"The fact that modern people engage intimately, the fact that we currently have shown that Neanderthals very likely kissed, indicates that the two [species] are also likely to have kissed," the researcher added.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle explained intimate contact could be employed in reproductive situations to possibly increase mating outcomes or help choose between mates, while it could assist reinforce bonding when used in a platonic way.

A separate researcher in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of primates it made sense its roots extend far into our evolutionary past, and an examination of different forms of intimate behavior among a wider variety of animals might extend its origins back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we examine carefully at other animals," the expert noted.

Cultural Aspects

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

"However, as people we thrive or fail on the quality of our relationships, and ways of promoting trust and closeness will have been significant for millions of years," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it should be expected that Neanderthals – and even Neanderthals and our human ancestors together – engaged intimately."