Fun fact! NeXT was a commercial att unix fork. The transition from the unix base to the BSD base did infact happen at apple after the acquisition. The value of next was in its application library, which would eventually become the mac foundation libraries like coreaudio and cocoa etc. The earliest releases of Rhapsody are very illuminative about the architecture of XNU/OSX. I don't doubt that linux was considered. There's a specific time when the actual move of rhapsody to a freebsd base occurred and it was at apple sometime in 97 or 98 iirc.
