Humans picked up kissing from Neanderthals around 50,000 years ago. Yeah, those Neanderthals were the original romantics! Scientists think that when they kissed early humans, we started doing it too.
And now, kissing is a big part of how we show love, affection, or even just say hi.
It's not just humans, though - some monkeys and apes like chimpanzees and orangutans kiss too. This makes scientists think that kissing is a pretty old behavior, rooted in our evolution.
Today, kissing means different things in different cultures, but it's mostly a way to connect with others.