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Among Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Now, researchers propose that Neanderthals did it too – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Clues

It is not the first time scientists have suggested ancient relatives and early modern humans were closely connected. Among earlier research, researchers have found modern people and their Neanderthal relatives shared the identical oral bacteria for millions of years after the evolutionary divergence, implying 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 non-African ancestry contain ancient genetic material in their genome, revealing genetic mixing was occurring.

Romantic Interpretation

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

Writing in the journal a scientific periodical, the researcher and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a description that was not limited to how humans smooch.

Describing Kissing

"There have been some previous attempts to describe a intimate act, but it's very much been focused on humans, which means that basically other animals don't kiss. Now we understand that they likely engage, it might just not look from what human kissing looks like," explained the evolutionary biologist.

However, she said some actions that looked like intimate contact were something rather different – such as the chewing and food sharing, or "kiss-fighting", seen in aquatic species called French grunts.

Consequently the research group came up with a description of intimate contact centered around friendly interactions involving intentional mouth-to-mouth contact with a individual of the same species, with some motion of the mouth but absence of food.

Study Approach

Brindle explained they focused on accounts of kissing in primates from Africa and Asia, including bonobos, chimpanzees and great apes, and employed online videos to verify the observations.

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

Evolutionary Origins

The team say the findings suggest intimate contact evolved approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

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

"The fact that humans kiss, the reality that we currently have shown that ancient relatives very likely engaged, suggests that the both groups are also likely to have engage," the researcher added.

Evolutionary Importance

While the scientific reasoning is debated, the expert explained kissing could be used in reproductive situations to potentially enhance reproductive success or assist in selecting between partners, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of primates commented that as kissing behavior was observed in a broad spectrum of apes it made sense its roots lie deep in our evolutionary past, and an examination of different forms of kissing among a wider variety of species might extend its origins back even earlier still.

"Things that we consider as characteristics of our species, like kissing, are not unique to us if we look closely at different species," he said.

Social Aspects

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

"Nonetheless, as people we thrive or fail on the quality of our relationships, and ways of promoting trust and intimacy will have been significant for eons," she said. "It might be an concept that appears a bit incongruous to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be expected that ancient hominins – and even them and our human ancestors collectively – engaged intimately."