Modern bicycles come in many varieties, but the overall design has been fairly stable for a long time. Once you get the proportions right to maximize comfort and efficiency, things don't need to change too much because our bodies aren't changing. But when the bicycle was first invented, there was a period of great innovation and change as various bike makers tried to get things just right. Below is a great minute-long video that shows some of those mutations, straight from the family tree of the bike that you (hopefully) ride often.
Here's the great 60 second animation:
Credit where credit is due:
Animation : Thallis Ville von Holck
Music : Flying Home by The Benny Goodman Sextet
If you want to see more ancestors of the modern bike, the poster above is for you. You can see a larger version and buy it here.