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Among Galápagos albatrosses to Arctic mammals, chimpanzees to great apes, certain species engage in mouth-to-mouth contact. Currently, researchers suggest that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance scientists have proposed Neanderthals and Homo sapiens were closely connected. Among earlier research, researchers have found modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, suggesting they exchanged oral fluids.

"Probably they were engaging in intimate contact," she said, adding that the idea aligned with research that has found humans of certain genetic backgrounds contain ancient genetic material in their genetic makeup, revealing interbreeding was at play.

Intimate Spin

"This offers a different spin on ancient interactions," Brindle commented.

Writing in the journal a scientific periodical, Brindle and her team detail how, to investigate the evolutionary origins of intimate contact, they first had to develop a description that was not limited to how humans kiss.

Describing Kissing

"Previously there were some efforts to describe a kiss, but it's very much been human-centric, which implies that basically non-human species don't kiss. Currently we understand that they probably do, it may appear different from what our intimate contact resembles," said Brindle.

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

Consequently the team developed a description of kissing centered around friendly interactions involving directed oral interaction with a member of the identical group, with some motion of the oral area but no transfer of food.

Study Approach

Brindle said they focused on reports of intimate behavior in non-human species from the African continent and Asian regions, including primates, apes and great apes, and used digital recordings to confirm the reports.

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

Historical Timeline

Researchers say the findings suggest intimate contact evolved somewhere between 21.5 million and 16.9 million years ago in the predecessors of the large apes.

Placement of Neanderthals on this family tree means it is likely they, too, indulged in a intimate act, the scientists say. But the behavior might not have been confined to their own species.

"The fact that modern people kiss, the reality that we now have demonstrated that Neanderthals probably engaged, suggests that the two [species] are also likely to have kissed," the researcher noted.

Biological Significance

Although the evolutionary explanation is debated, the expert said intimate contact could be used in sexual contexts to possibly increase mating outcomes or help choose between mates, while it might help reinforce bonding when practiced in a platonic way.

A separate researcher in the behavior of primates said that as kissing behavior was seen in a wide range of primates it was logical its origins lie deep in our evolutionary past, and an examination of various types of kissing among a wider variety of animals might extend its beginnings back further 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 Elements

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

"Nonetheless, as people we thrive or fail on the quality of our relationships, and methods of encouraging trust and closeness will have been important for millions of years," she said. "It might be an concept that seems a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that Neanderthals – and even Neanderthals and our own species together – kissed."