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Among Galápagos albatrosses to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Evidence

It is not the first time scientists have proposed ancient relatives and early modern humans were closely connected. In previous studies, scientists have discovered modern people and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea chimed with research that has revealed people of non-African ancestry contain ancient genetic material in their genome, revealing interbreeding was occurring.

Intimate Spin

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

Writing in the journal a scientific periodical, Brindle and her team detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not restricted by how humans kiss.

Describing Intimate Contact

"There have been some previous attempts to describe a kiss, but it's very much been human-centric, which implies that basically other animals don't kiss. Now we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

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

As a result the team developed a definition of intimate contact based on social behaviors involving intentional oral interaction with a member of the identical group, with some movement of the mouth but absence of food.

Study Approach

Brindle said they focused on accounts of intimate behavior in primates from the African continent and Asian regions, including bonobos, apes and orangutans, and used online videos to verify the reports.

Scientists then integrated this information with information on the evolutionary relationships between extant and extinct species of such animals.

Historical Timeline

Researchers propose the results indicate intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the behavior might not have been confined to their own species.

"The fact that humans kiss, the reality that we currently have shown that Neanderthals very likely engaged, suggests that the both groups are also likely to have kissed," Brindle added.

Biological Importance

While the scientific reasoning is discussed, the expert said intimate contact could be employed in reproductive situations to possibly enhance mating outcomes or help choose between mates, while it might help strengthen connections when used in a non-sexual manner.

Another expert in the activities of great apes said that as kissing behavior was observed in a wide range of apes it made sense its origins 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 even earlier still.

"Behaviors that we consider as characteristics of human life, like intimate contact, are not unique to us if we examine carefully at other animals," he said.

Cultural Aspects

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

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and methods of promoting confidence and closeness will have been important for eons," she said. "This could represent an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be expected that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."