Where did kissing originate from?

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I have been wondering where kissing originated from. My grandson is 17 months old and he is learning to kiss. He kisses Oma which is me when we say our good byes. And when his mommy is holding him and his daddy kissing him good bye he will reach for his mommy and daddy to kiss. I have often wondered how kissing came to be. It is one of my most fun things to do. Does anybody know where it originated from?