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From Galápagos albatrosses to polar bears, primates to great apes, various animals engage in mouth-to-mouth contact. Currently, researchers propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with modern humans.

Common Microbial Clues

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were closely connected. In earlier research, researchers have found humans and their thick-browed cousins shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, implying they exchanged oral fluids.

"Likely they were engaging in intimate contact," the researcher noted, adding that the idea chimed with research that has revealed people of non-African ancestry contain ancient genetic material in their genetic makeup, demonstrating genetic mixing was occurring.

Romantic Interpretation

"It certainly puts a different spin on ancient interactions," the lead researcher said.

Publishing in the journal a scientific periodical, the researcher and colleagues detail how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some previous attempts to describe a kiss, but it's very much been human-centric, which implies that essentially non-human species don't kiss. Currently we know that they probably do, it might just not look from what human kissing looks like," said Brindle.

Nonetheless, she said some behaviors that looked like kissing were something rather different – such as the chewing and transfer of food, or "kiss-fighting", seen in fish called French grunts.

Consequently the team came up with a definition of intimate contact centered around friendly interactions involving intentional oral interaction with a member of the same species, with some motion of the oral area but absence of food.

Research Methods

The lead researcher said they focused on reports of intimate behavior in primates from Africa and Asian regions, including bonobos, apes and great apes, and employed digital recordings to confirm the observations.

Scientists then integrated this data with details on the evolutionary relationships between living and ancient species of such primates.

Historical Timeline

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

Placement of ancient hominins on this family tree means it is probable they, too, indulged in a kiss, the scientists conclude. But the activity might not have been confined to their specific group.

"Reality that modern people kiss, the fact that we currently have shown that Neanderthals very likely kissed, suggests that the both groups are also likely to have kissed," the researcher noted.

Evolutionary Importance

Although the scientific reasoning is debated, Brindle explained intimate contact could be employed in sexual contexts to potentially enhance reproductive success or assist in selecting between mates, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of primates commented 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 examination of various types of intimate behavior among a broader range of species might push its origins back further still.

"Things that we think of as signatures of our species, like kissing, are not exclusive to us if we look closely at other animals," the expert noted.

Social Elements

An archaeology expert explained that intimate contact had a social component as it was not universal to all societies.

"Nonetheless, as humans we succeed or struggle on the quality of our relationships, and methods of encouraging confidence and intimacy will have been important for millions of years," the professor stated. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but actually it ought to be expected that Neanderthals – and including Neanderthals and our own species collectively – engaged intimately."