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From Galápagos albatrosses to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals did it too – and possibly locked lips with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance experts have suggested ancient relatives and Homo sapiens were closely connected. Among previous studies, researchers have found humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the two species split, implying they exchanged oral fluids.

"Likely they were kissing," the researcher noted, adding that the idea aligned with research that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Romantic Interpretation

"This offers a different perspective on ancient interactions," the lead researcher commented.

Writing in the publication 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 definition that was not restricted by how humans kiss.

Defining Intimate Contact

"Previously there were some previous attempts to define a intimate act, but it's very much been focused on humans, which implies that essentially non-human species do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing looks like," said the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were something rather different – such as the chewing and food sharing, or "mouth contact", observed 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 individual of the identical group, with some movement of the oral area but absence of food.

Study Approach

Brindle said they focused on accounts of intimate behavior in non-human species from the African continent and Asia, including primates, apes and orangutans, and employed digital recordings to confirm 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 suggest intimate contact developed approximately 21.5m and 16.9 million years ago in the ancestors of the large apes.

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

"Reality that modern people kiss, the reality that we currently have demonstrated that Neanderthals probably kissed, indicates that the two [species] are also likely to have kissed," Brindle noted.

Evolutionary Importance

Although the evolutionary explanation is debated, the expert explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or help choose between mates, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of great apes commented that as kissing behavior was seen in a broad spectrum of apes it was logical its origins extend far into our evolutionary past, and an examination of different forms of intimate behavior among a wider variety of species might extend its origins back even earlier still.

"Things that we think of as signatures of human life, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Cultural Aspects

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

"Nonetheless, as humans we succeed or struggle on the strength of our relationships, and ways of encouraging trust and intimacy will have been important for eons," she said. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and aggressive past, but really it ought to be no surprise that Neanderthals – and including them and our own species collectively – engaged intimately."