Everything has a backstory. No matter how hard we sometimes try to pretend that things just spring up out of thin air, there is always a context for stuff. We might not always know it, and sometimes we have to make peace with that.
Being self-contained units, memes are usually pretty good at identifying where they have come from (unless the mystery is part of the whole point). While the majority of people who enjoy memes are not going to be able to carbon date whatever they're looking at, they will probably get a sense of when and where they came about.
We all give off little clues about these things, whether we realize it or not, and the memes that we make are not different. It's just another part of that big old melting pot we call life, and it's pretty cool that we can see it in humor as well as more serious things.