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Among Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals appear to kiss. Currently, scientists suggest that ancient hominins also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Shared Oral Clues

It is not the first time scientists have proposed Neanderthals and Homo sapiens were closely connected. In previous studies, scientists have found modern people and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were kissing," the researcher noted, adding that the concept aligned with research that has found humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, revealing interbreeding was at play.

Romantic Interpretation

"This offers a different spin on human-Neanderthal relations," Brindle commented.

Writing in the journal a scientific periodical, the researcher and her team report how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how people smooch.

Describing Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been human-centric, which means that basically other animals don't kiss. Currently we understand that they likely engage, it might just not look from what our intimate contact looks like," said Brindle.

However, she noted some actions that looked like kissing were something rather different – such as the chewing and food sharing, or "mouth contact", observed in fish called certain marine animals.

Consequently the research group developed a definition of kissing centered around social behaviors involving directed oral interaction with a member of the same species, with some movement of the mouth but absence of nutrition.

Study Approach

The lead researcher said they concentrated on reports of kissing in primates from the African continent and Asia, including bonobos, chimpanzees and orangutans, and used online videos to confirm the observations.

Scientists then combined this information with information on the evolutionary relationships between living and extinct types of such animals.

Historical Origins

The team propose the results indicate kissing developed approximately 21.5 million and 16.9 million years ago in the ancestors of the large apes.

Placement of Neanderthals on this family tree means it is probable they, too, indulged in a intimate act, the scientists conclude. But the behavior may not have been limited to their own species.

"The fact that humans kiss, the fact that we currently have shown that Neanderthals probably engaged, indicates that the two [species] are probably did engage," Brindle added.

Biological Importance

While the evolutionary explanation is debated, the expert explained intimate contact could be employed in sexual contexts to possibly increase mating outcomes or assist in selecting between mates, while it could assist strengthen connections when used in a platonic way.

Another expert in the behavior of great apes said that as kissing behavior was seen in a broad spectrum of apes it was logical its origins lie deep in our ancient history, and an analysis of different forms of intimate behavior among a wider variety of species might extend its beginnings back even earlier still.

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

Cultural Aspects

Another professor said that kissing had a social component as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the quality of our emotional bonds, and methods of promoting confidence and intimacy will have been important for eons," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but really it ought to be no surprise that ancient hominins – and including Neanderthals and our human ancestors collectively – engaged intimately."