I am not sure you would ever find the wellspring from whence all "this" came from... though I think I still have some old 386BSD disks someplace.
It's true that Linux derived from Unix, however both have evolved well beyond expectations and neither represent their origins all that much. Well, in some ways they do; filesystems, controls, hell, even some of the daemons are the same. (If it ain't broke don't fix it!)
Linux is, more or less, a unix-like kernel surrounded by differing packages which altogether make up what is called Linux. Distro to Distro the kernel should be the same -- which is good.
FreeBSD, OpenBSD, NetBSD, etc et al. Now those are closer to Unix ; in fact in all but name -- as the name is legally protected -- they *are* unix.
The other choices are Solaris Unix, which is not bad and is used heavily in IT, so if you were looking to move into a new career path....
However, Solaris for x86 architecture sucks wind. It's a slow, bloated beast reminiscent of that product from Redmond. Solaris on the USparc chipset is much better, and what it was truly designed for.
Besides, if you learn unix, you can carry that knowledge forward. There are differences, but under the covers most of the commands are similar in the extreme.
AIX, and IBM Unix product is another. Unless you have a serious piece of hardware around I wouldn't even think about it. Besides... smitty pisses me off.
There are of course other ones out there; HPUX, Compaq Tru64 Unix, IRIX -- shudder shudder, cough cough.
As well as many dead or dying variants too numerous to mention.
The one thing they all have in common is the foundation for which they were built.
If you want to throw something on one of your old systems to play around and teach yourself -- good for you, BTW -- I suggest FreeBSD, for the following reasons:
It's free
It's well supported through the mailing list and mailing list archives. Smart people generally answer your questions free of charge -- sometimes it's even the folks who write the darn thing.
It's closer to "true" unix than anything else that's free. (A debatable point if you happen to be in the SYS V camp -- which I am not).
It's more stable as a running platform for services and serving.
The daemon is so much cooler than the ridiculous looking penguin.
http://www.freebd.org
Cheers.