Before Mac OS X, and even before Rhapsody, there was A/UX.

Introduced in 1988, it was Apple's first foray into producing a UNIX operating system. Based on System V (with some features from BSD), it ran on a very select few Macintosh models: only those with a floating-point unit and paged MMU could run A/UX. Interestingly this meant that an SE/30 could run it, but a Quadra 840AV (which is the fastest M68K Mac produced) could not.

Having A/UX meant that Apple could bid on large US Government procurement contracts; it is fully POSIX-compliant, and basically runs a version of System 7 as a process on top of System V UNIX.

Eventually the plan was to join forces with IBM and their AIX UNIX, as well as simultaneously release Pink (part of the Copland project). This plan fell apart, eventually the entire deal fell apart, and Apple went on to implement features from Copland into Mac OS 8 and later. A/UX 3.1.1 was the final release, in 1995, by which point the entire platform was essentially abandoned.

A/UX can be successfully emulated by the QEMU project - see setup instructions here. I am not sure if my installation went as planned as I asked it to install the X11 components, but it didn't appear to actually do so and I didn't care enough to fix it.

Here is the Macintosh booting...but it's actually booting a copy of System 7, which it uses as a sort of crude "bootstrapper" for A/UX.

Here now we are booting A/UX, which is apparently meant for a Workgroup Server machine (I am emulating a Quadra 800).

Here is an unfamiliar sight if you are used to Mac OS (of the time anyway)...a login screen! Yes, there is actual security and actual user accounts at play here, as you might expect out of a UNIX operating system.

   
The login about dialog, which when clicked flashes a bunch of signatures of members of what I presume are the development team.