Briefly, the argument goes (as you would recognise, given your cultural background) that, prior to the city state, humans had spent hundreds of thousands of years in relatively egalitarian groups. Greed, etc. were there, but were controlled by group norms & rules.
The city state bred mass slavery - class hierarchy in the sense of permanent, racist othering (born a slave, always a slave)
Below is about the origins of class hierarchy as an extension of gender hierarchy.