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From Galápagos albatrosses to Arctic mammals, primates to great apes, various animals appear to kiss. Currently, scientists suggest that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time scientists have suggested ancient relatives and Homo sapiens were closely connected. In previous studies, scientists have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, adding that the concept chimed with studies that has found humans of certain genetic backgrounds have bits of ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different perspective on human-Neanderthal relations," Brindle said.

Writing in the publication a scientific periodical, the researcher and her team report how, to investigate the evolutionary origins of kissing, they first had to develop a description that was not restricted by how people smooch.

Describing Kissing

"There have been some previous attempts to define a intimate act, but it's largely focused on humans, which means that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what our intimate contact looks like," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish called French grunts.

Consequently the research group came up with a definition of intimate contact based on friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but no transfer of food.

Study Approach

Brindle said they focused on reports of kissing in non-human species from the African continent and Asia, including bonobos, apes and orangutans, and used digital recordings to confirm the reports.

Scientists then combined this information with details on the genetic connections between extant and extinct species of such animals.

Evolutionary Timeline

The team propose the findings suggest kissing evolved approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the researchers conclude. But the activity may not have been limited to their own species.

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

Evolutionary Importance

While the scientific reasoning is debated, the expert explained intimate contact could be used in sexual contexts to potentially enhance reproductive success or help choose between mates, while it could assist reinforce bonding when practiced in a platonic way.

Another expert in the activities of great apes commented that as intimate contact was observed in a wide range of apes it was logical its origins lie deep in our ancient history, and an analysis of different forms of intimate behavior among a broader range of animals might push its beginnings back further still.

"Things that we consider as characteristics of human life, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not universal to all human groups.

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and ways of encouraging trust and closeness will have been important for millions of years," she said. "This could represent an image that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and including them and our own species collectively – kissed."