Those of us who are serious users of UNIX or UNIX-like systems have no doubt looked at ric Lvnez's excellent UNIX Timeline at some point. If you haven't, I suggest that you do! The amount of information it offers is truly spectacular. But looking at it today, I came to a somewhat sad realization: the UNIX world has become quite homogenized.
This history of UNIX starts out in September of 1969. From then until after the release of UNIX TSS Fourth Edition in November of 1973, we see no forking or derivation. Between Fourth Edition and Fifth Edition, we see some forking starting to take place, in the form of PWB/UNIX and MERT. We witness more and more branching, up until 1981.
It is around that point that I think UNIX really starts to enter a 20-year period of significant growth and "individuality". Between 1980 and 1984, we see some pretty significant divergence. First, many of the major UNIX variants begin their lives. XENIX starts on August 25, 1980, while 4.0BSD is released in October of 1980. UNIX System III comes out in November of 1981. QUNIX (the precursor to QNX) hails from 1981, as well. HP-UX starts its life in 1982. Two of the most important UNIX variants begin at this time, too: SunOS 1.0 is released in February of 1982, followed by UNIX System V in January of 1983.
The period between 1984 and 1989 is truly a glorious time in the history of UNIX. SunOS blossomed during this period, with SunOS 2.0, 3.0 and 4.0 being released. We have the major 4.2BSD and 4.3BSD releases. Mach arises in 1985. The roots of AIX go back to 1986, which is also when IRIX began. With an impact still felt by Mac OS X users today, we have the release of NeXTSTEP 0.8 on October 12, 1988, and the release of NeXTSTEP 1.0 on September 18, 1989. Although not derived from UNIX itself, the development of Minix started during this time period (and we all know the impact it would later have on Linus and Linux).
Mind you, those are just the variants that ended up having the most significant impact on the UNIX computing world. As is clearly visible on the timeline, there were numerous other variants, with many focusing on a specific platform or domain. Regardless, what we notice is that this was an era of growth and innovation. There was a lot of diversity.
This trend continues into the 1990s. We have major events like the beginning of Linux in 1991, and the release of Solaris 2.0 in July of 1992. UnixWare came out in November of 1992. NeXTSTEP continued to evolve. On the BSD front, we see NetBSD, BSD/OS and FreeBSD arise. But now notice the trend on the timeline; we see far less sharing of code and ideas between the variants. This is especially evident between 1998 and 2001.
At this point, most of the activity is between Darwin, Mac OS X and Mac OS X server. We see some transfers of code and concept, such as XFS from IRIX to Linux. But otherwise, there's little interaction between the different variants.
Things are really starting to look bland between 2004 and today. In terms of actively-developed UNIX or UNIX-like operating systems, we're down to only a handful. The BSD world is perhaps the most diverse, where we have NetBSD, PC-BSD, DragonFly BSD, OpenBSD and FreeBSD. Other than that, the most active variants are Mac OS X, Linux, Solaris, HP-UX, Minix and AIX. IRIX has become mostly irrelevant, as have Tru64 UNIX, OpenServer and UnixWare.
We will have to wait and see what the future will bring. But it looks like it will likely be pretty isolated to only a few major UNIX or UNIX-like systems: FreeBSD, Mac OS X, Solaris, and Linux. Although activity will continue on HP-UX and AIX, no doubt, their influence may very well be minimal.
I have mixed feelings over how things have evolved. On one hand, we do have more powerful features concentrated in a smaller number of systems. And these systems are fairly prevalent, and well-constructed. But the diversity of the 1980s and early 1990s brought upon change and innovation at an exciting pace. A balance between the two extremes would likely be best, although it is suspect whether we will ever get there.