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Among seabirds to polar bears, primates to orangutans, various animals appear to kiss. Now, scientists propose that Neanderthals also engaged in this behavior – and possibly exchanged kisses with early Homo sapiens.

Common Microbial Evidence

This isn't the initial instance scientists have proposed ancient relatives and early modern humans were intimately acquainted. Among previous studies, scientists have found 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, adding that the concept chimed with research that has revealed humans of certain genetic backgrounds contain Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Intimate Spin

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

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

Defining Kissing

"There have been some efforts to define a intimate act, but it's largely focused on humans, which means that essentially non-human species don't kiss. Now we understand that they probably do, it may appear different from what human kissing looks like," said the evolutionary biologist.

However, she said some behaviors that resembled intimate contact were something rather different – such as the processing and transfer of food, or "kiss-fighting", seen in fish called French grunts.

Consequently the team developed a definition of kissing centered around social behaviors involving intentional mouth-to-mouth contact with a individual of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Methods

The lead researcher explained they focused on accounts of intimate behavior in primates from the African continent and Asia, including primates, chimpanzees and great apes, and used online videos to confirm the observations.

The researchers then integrated this information with information on the evolutionary relationships between extant and ancient species of such primates.

Evolutionary Timeline

Researchers propose the results indicate intimate contact developed approximately 21.5m and 16.9m years ago in the ancestors of the large apes.

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

"The fact that humans kiss, the fact that we now have shown that Neanderthals probably kissed, suggests that the two [species] are probably did kissed," the researcher noted.

Evolutionary Importance

While the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to possibly enhance reproductive success or help choose between mates, while it might help reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of primates said that as intimate contact was seen in a wide range of primates it made sense its roots lie deep in our ancient history, and an analysis of various types of kissing among a wider variety of species might push its origins back further still.

"Behaviors that we think of as characteristics of our species, like kissing, are not unique to us if we look closely at different species," the expert noted.

Social Elements

An archaeology expert explained that intimate contact had a social component as it was not common to all human groups.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and methods of encouraging confidence and closeness will have been significant for eons," she said. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but actually it ought to be expected that ancient hominins – and including them and our human ancestors together – engaged intimately."