Discovering Urbanism has a nice post up about early 20th century urban planner Charles Mulford Robinson and his planning textbook, and it includes the following corrective to the notion that zoning originated as a way to separate polluting industry from places of residence and commerce:
There’s a common narrative about how zoning unfolded in America. First, planners needed to find ways to separate dangerous and unhealthy factories from the places where people lived. Once the legal basis for this tool was secured, it was eventually employed to separate businesses from residents. The final stage of zoning was to segregating different kinds of people from each other. That’s how we reached where we are today.
However, the Robinson textbook indicates that this progression was, if anything, reversed. In reality, residences at the time couldn’t be separated much from industry, because many of the working classes had to be within walking distance from their jobs. On the other hand, some of the very earliest uses of zoning were explicitly intended to separate “exclusive” neighborhoods from the lower classes, whether by requiring minimum densities or barring anything but detached single-family housing.