Very few people actually sit down and make up jokes, yet everyone is always telling jokes. Where do they come from?
They come from earlier forms of jokes that get embellished every time a new person passes them on. Gradually they change into new jokes. Sometimes you can still see the family resemblance to the earlier joke in the later development. That's why we find ourselves saying "You know, when I heard that joke it wasn't about President Clinton, it was about President Ford. It went like this..." Or some such thing.
This is a great example of evolution in action. Every time a joke is passed on, it's like a new generation being born. Each generation has the chance of changing a little bit from the previous one. A Gerald Ford joke won't get passed on very much these days; but if it changes into a Clinton joke, it may. It's the survival of the funniest.
Now, ask yourself this question. Where did chickens come from? Was there a day somewhere in the past when no chickens existed, and then the next day there were chickens?
No. Chickens, like all animals, came into being by a process of gradual change. They emerged from a previous species, let's call it the proto-chicken, that changed a little bit with each generation. Like better jokes being passed on, birds with more useful traits tended to survive and reproduce. Eventually a new species altogether emerged. Just in time to cross the road.