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Among Galápagos albatrosses to Arctic mammals, primates to great apes, certain species appear to kiss. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with modern humans.

Common Oral Evidence

It is not the first time experts have proposed Neanderthals and Homo sapiens were closely connected. In earlier research, scientists have discovered humans and their thick-browed cousins shared the identical oral bacteria for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the idea chimed with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Romantic Spin

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

Writing in the journal a scientific periodical, the researcher and her team report how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not limited to how people smooch.

Describing Intimate Contact

"There have been some previous attempts to describe a intimate act, but it's largely human-centric, which means that essentially other animals don't kiss. Now we know that they probably do, it might just not look from what our intimate contact resembles," said the evolutionary biologist.

Nonetheless, she noted some actions that resembled kissing were something rather different – such as the processing and transfer of food, or "mouth contact", seen in fish called French grunts.

Consequently the research group came up with a description of intimate contact based on friendly interactions involving directed oral interaction with a individual of the identical group, with some motion of the oral area but absence of food.

Research Approach

Brindle explained they focused on accounts of intimate behavior in non-human species from Africa and Asia, including bonobos, chimpanzees and orangutans, and used digital recordings to verify the reports.

The researchers then integrated this data with details on the genetic connections between living and ancient types of such animals.

Evolutionary Timeline

Researchers say the results suggest intimate contact developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

The position of ancient hominins on this family tree suggests it is probable they, too, indulged in a kiss, the researchers conclude. But the activity might not have been confined to their own species.

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

Biological Significance

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

A separate researcher in the activities of great apes commented that as intimate contact was seen in a wide range of apes it made sense its origins extend far into our evolutionary past, and an analysis of different forms of kissing among a wider variety of animals might push its beginnings back even earlier still.

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

Social Aspects

Another professor said that intimate contact had a social component as it was not universal to all societies.

"However, as humans we succeed or struggle on the quality of our emotional bonds, and ways of encouraging trust and intimacy will have been important for millions of years," she said. "This could represent an concept that seems a bit incongruous to our misplaced ideas of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including Neanderthals and our own species together – engaged intimately."