New Delhi, Nov. 20 -- A new evolutionary study is pushing the timeline of the "first kiss" far deeper into the past than expected. Researchers now say that the behaviour likely emerged more than 21 million years ago, long before modern humans appeared, and was almost certainly practised by the common ancestor of great apes.

According to Science.org, the team worked on a basic question: if kissing offers no clear survival advantage, why is it so common? They began by mapping where the behaviour appears in animals. That meant setting a narrow definition - non-aggressive, mouth-to-mouth contact, some movement, no food exchanged - as laid out in Evolution and Human Behaviour.

Once defined, examples started stacking up. Wolves. Prairie dogs....