The nuclear family lived separate from the extended family. In separate cities. This is a new thing. You lived with your clan in history, not apart. The nuclear family is completely separated physically from their extended families, even across village lines.

Oh, I see. I agree with the other commenters, I think you may be focusing too much on the "nuclear" part of GP's comment. I interpreted that more as "immediate" family, and I think throughout history that people were most likely to rely on immediate family, then extended, then general kin.