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From Galápagos albatrosses to polar bears, chimpanzees to orangutans, certain species appear to kiss. Currently, researchers propose that ancient hominins also engaged in this behavior – and might even have exchanged kisses with early Homo sapiens.

Common Oral Clues

It is not the first time scientists have suggested Neanderthals and Homo sapiens were intimately acquainted. Among previous studies, scientists have found modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the evolutionary divergence, implying they swapped saliva.

"Probably they were engaging in intimate contact," the researcher noted, adding that the idea aligned with research that has found people of non-African ancestry have bits of ancient genetic material in their genome, demonstrating interbreeding was occurring.

Intimate Spin

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

Publishing in the journal a scientific periodical, the researcher and her team detail how, to investigate the evolutionary origins of kissing, they first had to come up with a description that was not restricted by how humans smooch.

Defining Kissing

"Previously there were some previous attempts to define a intimate act, but it's largely human-centric, which implies that basically non-human species 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 actions that resembled intimate contact were distinct activities – such as the processing and food sharing, or "mouth contact", seen in aquatic species known as French grunts.

Consequently the research group came up with a definition of kissing based on friendly interactions involving directed oral interaction with a member of the identical group, with some movement of the oral area but no transfer of nutrition.

Study Approach

The lead researcher explained they concentrated on accounts of kissing in non-human species from the African continent and Asia, including primates, chimpanzees and great apes, and employed digital recordings to verify the observations.

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

Historical Timeline

The team say the findings suggest kissing developed approximately 21.5m and 16.9m years ago in the ancestors of the great primates.

The position of ancient hominins on this evolutionary lineage means it is likely they, too, indulged in a intimate act, the researchers conclude. But the activity might not have been limited to their specific group.

"The fact that humans engage intimately, the reality that we currently have shown that ancient relatives probably engaged, indicates that the both groups are probably did engage," the researcher added.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle explained kissing could be employed in reproductive situations to potentially increase mating outcomes or assist in selecting between partners, while it could assist reinforce bonding when used in a non-sexual manner.

A separate researcher in the behavior of great apes said that as kissing behavior was observed in a broad spectrum of primates it made sense its roots lie deep in our evolutionary past, and an examination of various types of intimate behavior among a broader range of species might push its beginnings back further still.

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

Social Aspects

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

"Nonetheless, as humans we thrive or fail on the quality of our relationships, and methods of encouraging trust and intimacy will have been significant for eons," she said. "It might be an concept that appears a bit contradictory to our incorrect assumptions of a rather ruthless and aggressive past, but really it should be no surprise that Neanderthals – and including Neanderthals and our human ancestors together – engaged intimately."