History

The emergence of the bourgeois and the proletariat is pretty straightforward. The proletarii were a class designated in the Roman census. Basically they were legally free (not slaves) but owned very little wealth. They were by no means the majority of people in Rome.

Fast forward 1700 years. There's precious metals and unfinished goods flooding europe and this emergent class of small merchants that want to make stuff to sell now that people have the gold to buy it. They're pretty ingenuitive so they create new machines and processes to produce, but they need a ton of labor-power. Serfs don't really make the cut and slaves are too expensive so they turn to the free labor, the proletariat. Hired out just for the cost of their labor and totally severable. A few centuries later, capitalism and technology have overtaken basically every aspect of our society, displaced all the serfs, freed most of the slaves, and those merchants are basically hiring everyone as a worker and there are very few people left of the other old classes.