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From Galápagos albatrosses to polar bears, chimpanzees to great apes, various animals engage in mouth-to-mouth contact. Now, researchers propose that ancient hominins also engaged in this behavior – and possibly locked lips with modern humans.

Common Microbial Clues

It is not the first time experts have proposed ancient relatives and Homo sapiens were intimately acquainted. In earlier research, researchers have discovered humans and their thick-browed cousins possessed the identical oral bacteria for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Probably they were kissing," the researcher noted, adding that the idea aligned with studies that has revealed humans of non-African ancestry contain ancient genetic material in their genome, revealing genetic mixing was at play.

Intimate Interpretation

"It certainly puts a different perspective on ancient interactions," the lead researcher commented.

Publishing in the publication Evolution and Human Behavior, the researcher and her team report how, to investigate the historical roots of kissing, they first had to develop a description that was not restricted by how humans kiss.

Describing Kissing

"There have been some efforts to describe a intimate act, but it's largely focused on humans, which means that essentially other animals do not engage in this. Currently we understand that they probably do, it might just not look from what human kissing resembles," explained the evolutionary biologist.

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

Consequently the team came up with a definition of kissing centered around social behaviors involving directed oral interaction with a individual of the same species, with some motion of the oral area but absence of nutrition.

Research Approach

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

Scientists then integrated this information with information on the genetic connections between extant and extinct types of such animals.

Historical Timeline

The team propose the findings suggest intimate contact developed somewhere between 21.5m and 16.9m years ago in the predecessors of the large apes.

The position of Neanderthals on this family tree means it is likely they, too, engaged in a kiss, the scientists conclude. But the activity might not have been limited to their specific group.

"Reality that modern people kiss, the reality that we now have demonstrated that Neanderthals probably engaged, indicates that the two [species] are also likely to have engage," Brindle noted.

Evolutionary Significance

Although the evolutionary explanation is discussed, Brindle said intimate contact could be used in sexual contexts to possibly increase reproductive success or assist in selecting between partners, while it could assist reinforce bonding when used in a non-sexual manner.

Another expert in the activities of primates said that as kissing behavior was observed in a wide range of primates it made sense its origins extend far into our evolutionary past, and an examination of various types of kissing among a broader range of animals might extend its origins back further still.

"Things that we think of as characteristics of human life, like kissing, are not exclusive to us if we look closely at other animals," he said.

Cultural Elements

Another professor explained that kissing 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 promoting trust and closeness will have been significant for millions of years," she said. "It might be an concept that seems a bit contradictory to our incorrect assumptions of a supposedly aggressive and aggressive past, but really it ought to be expected that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."