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Humans do it, primates practice it, even polar bears participate in it. And now, scientists have reconstructed the evolutionary origins of mouth-to-mouth contact.

This study indicates that the oral-oral kiss developed over twenty-one thousand-thousand years in the past, and was something that the common ancestor of humans and additional great apes likely indulged in.

The same study determined that ancient humans could have kissed too – and that humans and Neanderthals may even have exchanged kisses one another.

By finding proof of other animals practicing kissing, researchers were able to construct an "evolutionary lineage" to determine at what time it was probably to have evolved.

To make certain that they were comparing the same behaviour across different species, the scientists had to give a very precise – rather clinical – definition to a "kiss".

In their study, released in a academic publication, they defined kissing as non aggressive, directed oral-oral interaction "involving movement of mouthparts or lips and without nourishment transfer".

"People, chimpanzees, and pygmy chimpanzees all engage in kissing," stated principal investigator Dr Matilda Brindle. From that, she concluded, "it's probable that their closest common ancestor kissed."

"Researchers believe mouth contact likely evolved approximately 21.5 million years in the past in the great primates."

In this research, scientists discovered behavior that fit their scientific description of kissing in wolves, prairie dogs, polar bears (quite messy – much lingual contact), and even albatrosses.

The researchers focused on monkeys – and large primates in specific – in order to construct an developmental picture of the origin of the people's smooch.

This study also found that Neanderthals – our nearest prehistoric human relatives that became extinct around forty thousand years before present – too engaged in kissing.

Earlier study on Neanderthal DNA additionally revealed that modern humans and ancient relatives shared an mouth bacteria – a type of microorganism present in our mouth fluid.

"That means that they must have been swapping saliva for hundreds of thousands of years following the two species split," explained the lead researcher.

Although this study identified when mouth contact developed it was could not resolve the mystery of why.

There are already a number of hypotheses – that it arose from cleaning behaviour in our primate ancestors or that it might offer an close way to assess the health and even the compatibility of a mate.

Dr Brindle hopes that the study will open a door to answering that question.

"It's crucial for humanity to recognize that this is a behavior we share with our non-human kin," she said.

"We should be investigating this behavior, not just dismissing it as frivolous because it has affectionate associations in people."