[TUHS] Other POSIX Candidates?

[segaloco via TUHS]

I'm paraphrasing here but I've read in a few places something to the effect that UNIX was "selected" as the basis on which to build a portable operating system standard, which of course we all know as POSIX.  However, I got thinking on the implications of that phrasing, and have to ask, was there actually a "selection" made picking UNIX over some other candidate, or was it pretty much established from the outset of pursuing a standard that UNIX was going to get standardized?

Another way to put it would be as a chicken and egg, which came first, desire for a portable base system definition that UNIX happened to fit nicely, or the ongoing need for UNIX standardization finding sponsorship by the working groups, IEEE, etc.?  Did any other OS contend for this coveted honor?

- Matt G.