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As I've understood it, "classic" Mac OS isn't an ancestor of the modern macOS. OS X had emulation support at some point (dropped years ago, though), but the internals descend from NeXTSTEP (a honestly somewhat odd Mach / BSD hybrid kernel). Edit: ah, nevermind, it's just a confusing graph:
And so we arrive at OS X, the child of the classic Macintosh OS and NeXTSTEP. The best way to think about OS X is that it took the consumer focus and interface paradigms of the Macintosh and layered them on top of NeXTSTEP’s technology. In other words, the Unix side of the family was the defining feature of OS X.