Humans aren’t the only ones who kiss—monkeys do it, polar bears do it, and now research suggests that the practice may go back more than 21 million years. Scientists have reconstructed the evolutionary history of kissing and found that the common ancestor of humans and other great apes likely engaged in this intimate behaviour.

The study, published in the journal Evolution and Human Behaviour, even suggests that Neanderthals may have kissed—and that humans and Neanderthals could have shared smooches with one another.

Kissing has long been something of an evolutionary mystery. While it has no obvious survival or reproductive purpose, it is widespread not only in human societies but also across the animal kingdom. By identifying similar behaviours in other species, researchers were able to

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