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Among seabirds to polar bears, chimpanzees to orangutans, certain species appear to kiss. Now, researchers suggest that ancient hominins did it too – and possibly locked lips with modern humans.

Shared Microbial Clues

This isn't the initial instance experts have proposed ancient relatives and Homo sapiens were closely connected. Among earlier research, researchers have discovered modern people and their Neanderthal relatives possessed the identical oral bacteria for millions of years after the two species split, suggesting they swapped saliva.

"Likely they were engaging in intimate contact," she said, adding that the idea chimed with research that has revealed humans of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating interbreeding was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on human-Neanderthal relations," the lead researcher said.

Publishing in the journal a scientific periodical, Brindle and colleagues detail how, to explore the historical roots of kissing, they first had to develop a definition that was not limited to how people smooch.

Describing Intimate Contact

"There have been some previous attempts to define a intimate act, but it's largely focused on humans, which implies that essentially non-human species do not engage in this. Now we know that they likely engage, it might just not look from what human kissing resembles," explained Brindle.

Nonetheless, she said some actions that looked like kissing were distinct activities – such as the chewing and transfer of food, or "mouth contact", seen in aquatic species called French grunts.

Consequently the research group came up with a definition of intimate contact centered around social behaviors involving directed oral interaction with a member of the identical group, with some motion of the oral area but no transfer of nutrition.

Research Approach

The lead researcher said they concentrated on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and used digital recordings to confirm the observations.

Scientists then combined this information with information on the genetic connections between extant and extinct species of such animals.

Evolutionary Timeline

The team say the findings indicate kissing developed somewhere between 21.5 million and 16.9 million years ago in the predecessors of the great primates.

Placement of Neanderthals on this evolutionary lineage means it is probable they, too, indulged in a intimate act, the researchers say. But the behavior may not have been limited to their own species.

"The fact that humans kiss, the fact that we now have shown that ancient relatives very likely engaged, suggests that the both groups are probably did engage," the researcher noted.

Biological Significance

Although the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance mating outcomes or help choose between partners, while it could assist strengthen connections when practiced in a platonic way.

A separate researcher in the activities of great apes commented that as kissing behavior was seen in a wide range of apes it was logical its origins extend far into our ancient history, and an analysis of different forms of intimate behavior among a broader range of species might push its origins back even earlier still.

"Things that we think of as characteristics of human life, like kissing, are not unique to us if we examine carefully at different species," the expert noted.

Cultural Elements

An archaeology expert said that intimate contact had a cultural element as it was not universal to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our relationships, and methods of encouraging confidence and closeness will have been significant for millions of years," she said. "This could represent an image that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be expected that ancient hominins – and including them and our human ancestors together – engaged intimately."