Seamus, go and listen to any Dark Star from each year starting from 1968 until 1974. You will see how much the song evolved from year to year. It is not so much how it was written as how it WASN'T written.
The structure was so loose that it allowed for a modal jam in A mixolydian. Modal playing is something that became popular through jazz music. It allowed players to improvise freely without thinking so much about hitting strict chord changes, just staying in key and doing whatever felt right.
There's a lot to be said about any given version of Dark Star and the song as a composition that evolved through time and space (as the Dead as a band also did) but Dark Star is best when it speaks for itself.
a whole new gameball