Paul and his adviser, whom I forget, wrote it in the early 1980s at Stanford. MIT liked it and decided to manage its development. It was after Xerox Parc and Apollos distributed graphics system, but before Sun, Apple and MicroSoft. Following Unix's minimalist toolkit naming convention "X" was the command stream and "W" the display api. Our Stanford computer joined in at version #5 on a VAX. It was commercially supperted around version 10 by DEC and Sun. And "froze" at version 11, going into 2nd and 3rd digit numbering after that.
There was always the intent to make it objected-oriented, hence the tootlkit kludge called Motif. The early 80s was in flux over OO languages Xerox MESA, way-to-slow Smalltalk, ObjectPascal, etc. C++ and ObjectiveC wouldnt be around for a few more years.