I don't want to derail the conversation, but this era of retro-computing is a passion of mine, so I'm obligated to point out that (AFAIK) XNU was created from s

I don't want to derail the conversation, but this era of retro-computing is a passion of mine, so I'm obligated to point out that (AFAIK) XNU was created from scratch, based on the Mach 4-derived kernel from OSF/1, and the non-encumbered 4.4BSD "lite" with patches from its forks. (You can see the kernel in the "osfmk" directory in XNU). It makes sense when you consider that 4.3BSD could never have been used in a product without tainting it, such that every copy of OS X would have needed a unix license (like NeXTSTEP did). This was also a factor in replacing Adobe Display Postscript with a similar, but lighter and unencumbered internally developed solution. (I might be wrong as I'm not the world's biggest NeXT/Darwin fan, but I see it as incredibly unlikely that they'd have kept any code and risked license contamination, especially when Mach 4/OSFMK were super incompatible with their proprietary "Mach 2.5", and there was no preexisting PowerPC support in NeXT)