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From Galápagos albatrosses to Arctic mammals, primates to great apes, certain species engage in mouth-to-mouth contact. Currently, scientists suggest that Neanderthals did it too – and might even have exchanged kisses with early Homo sapiens.

Shared Oral Clues

This isn't the initial instance scientists have proposed Neanderthals and early modern humans were closely connected. In previous studies, researchers have discovered modern people and their thick-browed cousins shared the same mouth microbe for millions of years after the evolutionary divergence, implying they exchanged oral fluids.

"Probably they were kissing," she said, explaining that the idea chimed with studies that has revealed humans of certain genetic backgrounds contain ancient genetic material in their genome, revealing genetic mixing was occurring.

Intimate Interpretation

"It certainly puts a different spin on ancient interactions," Brindle commented.

Writing in the publication a scientific periodical, the researcher and her team report how, to investigate the historical roots of intimate contact, they first had to come up with a definition that was not restricted by how people smooch.

Defining Kissing

"Previously there were some efforts to define a kiss, but it's very much been human-centric, which means that basically other animals do not engage in this. Currently we understand that they probably do, it may appear different from what human kissing resembles," said the evolutionary biologist.

However, she noted some behaviors that resembled kissing were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in aquatic species known as French grunts.

As a result the research group came up with a definition of kissing based on social behaviors involving intentional oral interaction with a member of the same species, with some motion of the oral area but no transfer of nutrition.

Research Approach

The lead researcher said they focused on accounts of intimate behavior in non-human species from Africa and Asian regions, including bonobos, apes and orangutans, and used online videos to confirm the reports.

Scientists then combined this data with details on the evolutionary relationships between extant and extinct species of such animals.

Historical Timeline

Researchers propose the results suggest intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree means it is likely they, too, indulged in a kiss, the researchers conclude. But the behavior might not have been confined to their specific group.

"Reality that modern people kiss, the reality that we now have shown that Neanderthals probably kissed, suggests that the two [species] are probably did kissed," the researcher added.

Biological Significance

While the scientific reasoning is debated, the expert said kissing could be employed in reproductive situations to potentially increase mating outcomes or help choose between mates, while it might help reinforce bonding when used in a platonic way.

A separate researcher in the activities of great apes said that as intimate contact was observed in a broad spectrum of apes it made sense its origins extend far into our evolutionary past, and an analysis of various types of kissing among a wider variety of animals might extend its beginnings back further still.

"Behaviors that we consider as signatures of human life, like intimate contact, are not exclusive to us if we look closely at different species," he said.

Social Aspects

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

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and ways of promoting confidence and closeness will have been significant for millions of years," the professor stated. "It might be an concept that appears a bit incongruous to our misplaced ideas of a rather ruthless and ancient history, but actually it should be no surprise that ancient hominins – and even Neanderthals and our own species together – kissed."