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Among Galápagos albatrosses to polar bears, primates to orangutans, certain species appear to kiss. Now, researchers suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have proposed ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, suggesting they swapped saliva.

"Probably 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 Neanderthal DNA in their genetic makeup, demonstrating interbreeding was occurring.

Intimate Interpretation

"It certainly puts a more romantic spin on ancient interactions," Brindle commented.

Writing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of intimate contact, they first had to come up with a description that was not limited to how humans smooch.

Describing Kissing

"Previously there were some efforts to describe a intimate act, but it's largely human-centric, which means that essentially non-human species don't kiss. Now we understand that they likely engage, it might just not look from what our intimate contact looks like," said the evolutionary biologist.

Nonetheless, she said some behaviors that resembled intimate contact were something rather different – such as the processing and food sharing, or "mouth contact", seen in aquatic species called French grunts.

As a result the team came up with a description of intimate contact centered around friendly interactions involving directed oral interaction with a member of the same species, with some motion of the mouth but absence of food.

Research Approach

Brindle explained they concentrated on accounts of kissing in primates from Africa and Asian regions, including primates, apes and great apes, and used online videos to verify the reports.

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

Evolutionary Timeline

Researchers propose the findings indicate kissing evolved somewhere between 21.5m and 16.9m years ago in the ancestors of the great primates.

Placement of ancient hominins on this family tree means it is likely they, too, engaged in a kiss, the researchers say. But the activity might not have been limited to their own species.

"Reality that humans engage intimately, the fact that we now have demonstrated that ancient relatives probably engaged, suggests that the both groups are also likely to have engage," Brindle noted.

Evolutionary Significance

While the evolutionary explanation is debated, the expert said kissing could be employed in sexual contexts to possibly enhance mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when practiced in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was seen in a broad spectrum of apes it was logical its origins extend far into our ancient history, and an examination of various types of kissing among a broader range of species might push its origins back even earlier still.

"Behaviors that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at different species," he said.

Social Elements

Another professor explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as people we thrive or fail on the quality of our relationships, and ways of encouraging trust and intimacy will have been significant for millions of years," she said. "It might be an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it should be no surprise that Neanderthals – and even them and our own species collectively – engaged intimately."