When Jobs left Apple he started his own computer company called "Next". It started out with a similar business model as Apple (hardware and software) but used BSD (I think) Unix with a proprietary shell. The system was midway between a typical PC and a minicomputer (for the time). It did okay with univeristy research folks but failed commercially. So they stopped hardware development and concentrated on the OS. I have never used the OS, but it was reported at the time to be a very clean, very powerful system. They began developing a variant to run on microprocessors in addition to Motorola.

Fast Forward. Jobs goes back to Apple.

Apple decdies that the Mac OS will never successfully migrate to a mutiprocessing, networked environment in its native code. Jobs shuts-down Next and brings most of those folks into Apple with instructions to write a Mac look-alike shell from all the Next code. It blows the doors off of the native Mac team's best efforts so the native Mac development team gets shut down and Mac System X gets released, essentially a Mac-like shell placed on Unix BSD.

Recall that the Next software was ported to other computer systems, including Intel. So Jobs never shuts that group with a group down and instead, keeps them secretely chugging along working on porting the Mac System X (BSD Unix) to Intel hardware (and probably other computer systems, too. Wanna bet?).

First Motorola (couldn't get the processor speed fast enough, soon enough in large enough numbers) and later, IBM (couldn't get the heat and power requirements to where Apple wanted to be with small laptops and smaller thinner devices planned for) have trouble meeting Apple's expectaions of microprocessor design. Apple gets frustrated, being thwarted in their plans by suppliers outside of their ability to direct so they search for an innovative solution.

And here we are.