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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, certain species appear to kiss. Now, scientists propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Shared Oral Evidence

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. Among earlier research, scientists have discovered humans and their Neanderthal relatives shared the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the idea chimed with research that has found humans of non-African ancestry contain ancient genetic material in their genome, revealing interbreeding was at play.

Romantic Spin

"It certainly puts a different spin on ancient interactions," the lead researcher commented.

Writing in the publication 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 description that was not restricted by how people kiss.

Describing Kissing

"Previously there were some efforts to define a kiss, but it's very much been focused on humans, which means that basically non-human species don't kiss. Currently we know that they likely engage, it may appear different from what our intimate contact resembles," explained the evolutionary biologist.

Nonetheless, she said some actions that resembled kissing were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called certain marine animals.

Consequently the team developed a description of intimate contact centered around friendly interactions involving intentional oral interaction with a member of the identical group, with some motion of the mouth but no transfer of nutrition.

Study Methods

The lead researcher explained they concentrated on reports of kissing in primates from Africa and Asia, including bonobos, apes and great apes, and used online videos to confirm the reports.

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

Historical Timeline

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

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a kiss, the researchers say. But the activity might not have been limited to their own species.

"The fact that humans engage intimately, the fact that we currently have shown that ancient relatives very likely kissed, indicates that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is debated, the expert said kissing could be used in sexual contexts to potentially enhance reproductive success or help choose between partners, while it might help 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 wide range of apes it made sense its roots lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might push its beginnings back further still.

"Behaviors that we think of as signatures of our species, like kissing, are not unique to us if we examine carefully at different species," he said.

Cultural Aspects

An archaeology expert said that kissing had a social component as it was not universal to all societies.

"However, as people we succeed or struggle on the quality of our relationships, and methods of promoting trust and intimacy will have been significant for millions of years," the professor stated. "This could represent an concept that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but really it ought to be no surprise that ancient hominins – and including them and our own species collectively – engaged intimately."