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Among Galápagos albatrosses to Arctic mammals, primates to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with early Homo sapiens.

Common Oral Evidence

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were intimately acquainted. In previous studies, researchers have found humans and their thick-browed cousins shared the identical oral bacteria for millions of years after the two species split, implying they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea aligned with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing interbreeding was occurring.

Intimate Spin

"This offers a more romantic perspective on human-Neanderthal relations," Brindle commented.

Publishing in the journal Evolution and Human Behavior, Brindle and colleagues detail how, to explore the evolutionary origins of kissing, they first had to come up with a definition that was not limited to how humans kiss.

Describing Intimate Contact

"There have been some previous attempts to define a kiss, but it's largely focused on humans, which implies that basically non-human species don't kiss. Now we know that they probably do, it may appear different from what human kissing resembles," explained the evolutionary biologist.

Nonetheless, she said some behaviors that resembled kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", seen in fish called French grunts.

Consequently the research group came up with a definition of intimate contact based on social behaviors involving intentional oral interaction with a individual of the identical group, with some motion of the oral area but absence of food.

Research Approach

Brindle said they concentrated on accounts of kissing in primates from the African continent and Asia, including bonobos, apes and great apes, and employed online videos to confirm the observations.

Scientists then integrated this data with information on the evolutionary relationships between extant and ancient types of such animals.

Evolutionary Timeline

The team say the results suggest kissing evolved somewhere between 21.5m and 16.9m years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree means it is likely they, too, indulged in a intimate act, the researchers say. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the fact that we now have demonstrated that ancient relatives probably kissed, indicates that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

While the evolutionary explanation is debated, Brindle said kissing could be used in reproductive situations to possibly increase reproductive success or assist in selecting between mates, while it could assist reinforce bonding when used in a platonic way.

Another expert in the activities of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its roots extend far into our evolutionary past, and an analysis of different forms of intimate behavior among a broader range of species might push its beginnings back further still.

"Behaviors that we think of as characteristics of human life, like kissing, are not unique to us if we look closely at other animals," the expert noted.

Social Aspects

An archaeology expert explained that intimate contact had a social component as it was not universal to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of promoting trust and closeness will have been significant for millions of years," the professor stated. "It might be an image that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it ought to be expected that Neanderthals – and even them and our human ancestors collectively – engaged intimately."