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Among seabirds to polar bears, chimpanzees to orangutans, various animals appear to kiss. Currently, scientists propose that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance scientists have suggested Neanderthals and early modern humans were intimately acquainted. Among earlier research, researchers have discovered humans and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were kissing," she said, explaining that the concept chimed with studies that has found humans of non-African ancestry contain ancient genetic material in their genetic makeup, revealing genetic mixing was occurring.

Intimate Spin

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

Writing in the journal Evolution and Human Behavior, the researcher and her team report how, to explore the historical roots of kissing, they first had to come up with a definition that was not limited to how people smooch.

Describing Intimate Contact

"There have been some efforts to describe a intimate act, but it's very much been human-centric, which means that basically other animals do not engage in this. Currently we know that they likely engage, it may appear different from what human kissing looks like," explained Brindle.

However, she noted some behaviors that resembled intimate contact were distinct activities – such as the chewing and food sharing, or "kiss-fighting", observed in fish called French grunts.

Consequently the research group came up with a description of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a member of the same species, with some movement of the oral area but absence of food.

Research Methods

The lead researcher explained they focused on accounts of intimate behavior in non-human species from the African continent and Asian regions, including primates, chimpanzees and great apes, and used digital recordings to confirm the reports.

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

Historical Origins

Researchers say the results suggest intimate contact developed approximately 21.5 million and 16.9m years ago in the predecessors of the great primates.

Placement of ancient hominins on this family tree means it is likely they, too, indulged in a intimate act, the researchers say. But the activity may not have been limited to their own species.

"The fact that modern people kiss, the reality that we now have shown that ancient relatives very likely kissed, suggests that the both groups are probably did kissed," the researcher noted.

Evolutionary Significance

While the scientific reasoning is discussed, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or assist in selecting between mates, while it might help reinforce bonding when used in a platonic way.

Another expert in the behavior of great apes said that as intimate contact was seen in a broad spectrum of primates it made sense its origins extend far into our ancient history, and an analysis of various types of intimate behavior among a broader range of animals might extend its beginnings back even earlier still.

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

Cultural Aspects

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

"Nonetheless, as people we thrive or fail on the strength of our relationships, and methods of encouraging confidence and closeness will have been significant for eons," the professor stated. "It might be an image that appears a bit contradictory to our misplaced ideas of a supposedly aggressive and aggressive past, but actually it ought to be expected that ancient hominins – and including them and our own species collectively – engaged intimately."