I'm addressing two points. First that differentiation existed in the past. Though with the dependence on both ag and human muscle power, the size of the nonfatm, nonlaboyr workforce was likely far smaller. But also that those roles had a strong persistence trend within families? -- if your father worked in a field (and it was generally only men who had careers), then you were likely to follow a similar path. How much of that was socially determined and how much based on psychological and other heritable traits I don't know. Gregory Clark's book, The Son Also Rises looks at family status trends in Europe, which is related but not the same though has similarities.
The classifications I posted above show that how specialized you see work depends much on how the classifications are defined. My sense is that periods of rapid change in employment structure lead to increases in classifications. E.g., the 1920 set. Which actually covered 1900 - 1920. See Vaclav Smil's recent articles on the innovation of the 1880s.
I didn't want to convey the impression
that social differentiation isn't a binary, either/or
process. Differentiation has been going on for millenia with some setbacks along the way.
The classifications I posted above show that how specialized you see work depends much on how the classifications are defined. My sense is that periods of rapid change in employment structure lead to increases in classifications. E.g., the 1920 set. Which actually covered 1900 - 1920. See Vaclav Smil's recent articles on the innovation of the 1880s.