From seabirds to polar bears, primates to orangutans, various animals appear to kiss. Now, scientists suggest that ancient hominins also engaged in this behavior – and might even have locked lips with modern humans.
This isn't the initial instance scientists have suggested ancient relatives and early modern humans were intimately acquainted. In 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 swapped saliva.
"Probably they were kissing," the researcher noted, explaining that the idea aligned with studies that has found humans of non-African ancestry contain Neanderthal DNA in their genetic makeup, demonstrating genetic mixing was at play.
"It certainly puts a more romantic spin on ancient interactions," Brindle said.
Writing in the publication a scientific periodical, Brindle and her team report how, to explore the historical roots of kissing, they first had to come up with a description that was not limited to how humans kiss.
"Previously there were some previous attempts to describe a intimate act, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Currently we understand that they likely engage, it may appear different from what our intimate contact resembles," said Brindle.
However, she said some actions that resembled intimate contact were distinct activities – such as the processing and transfer of food, or "kiss-fighting", observed in fish known as certain marine animals.
Consequently the research group developed a description of kissing centered around friendly interactions involving intentional oral interaction with a individual of the same species, with some motion of the mouth but absence of food.
The lead researcher said they focused on accounts of kissing in primates from Africa and Asia, including primates, apes and great apes, and employed online videos to confirm the observations.
The researchers then integrated this information with information on the evolutionary relationships between living and ancient types of such animals.
The team say the results suggest intimate contact evolved approximately 21.5 million and 16.9m years ago in the ancestors of the large apes.
The position of Neanderthals on this family tree means it is likely they, too, engaged in a intimate act, the researchers conclude. But the behavior may not have been limited to their specific group.
"The fact that modern people kiss, the fact that we currently have shown that ancient relatives probably kissed, indicates that the both groups are also likely to have kissed," the researcher added.
Although the evolutionary explanation is debated, the expert said intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it might help strengthen connections when used in a platonic way.
Another expert in the behavior of great apes commented that as kissing behavior was seen in a broad spectrum of primates it was logical its origins lie deep in our evolutionary past, and an examination of various types of kissing among a broader range of species might push its beginnings back further still.
"Behaviors that we think of as signatures of human life, like kissing, are not exclusive to us if we examine carefully at other animals," he said.
An archaeology expert said that kissing had a social component as it was not universal to all human groups.
"However, as people we thrive or fail on the strength of our relationships, and methods of encouraging trust and closeness will have been important for eons," the professor stated. "This could represent an concept that seems a bit contradictory to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be expected that Neanderthals – and even Neanderthals and our own species together – engaged intimately."
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