UNIX on mainframes goes further back than one might think. I recall a attending a lecture from an Amdahl person working on UNIX, which they termed UTS on the Amdahl's in either the very late 1970's or very early 1980's. https://en.wikipedia.org/wiki/Amdahl_UTS
And, according to that Wiki on UTS, UTS was based on work done at Princeton in the mid 1970's - which seems quite reasonable and likely to me.
I actually use Ubuntu, but I am getting really annoyed with systemd. It makes troubleshooting systems that won't start properly a couple of orders of magnitude more difficult than it used to be. I have one system that I am basically ready to pull the data off of and throw in the trash can because it has startup errors that I have not been able to diagnose, and nothing shows up on forums or Google searches that quite matches. I tried figuring out the systemd conifg for it - but it eventually wasn't worth the time. Reminds me of the Windows registry.
UNIX on mainframes goes further back than one might think. I recall a attending a lecture from an Amdahl person working on UNIX, which they termed UTS on the Amdahl's in either the very late 1970's or very early 1980's. https://en.wikipedia.org/wiki/Amdahl_UTS
And, according to that Wiki on UTS, UTS was based on work done at Princeton in the mid 1970's - which seems quite reasonable and likely to me.