I see the history of prog as this following allegory: Brian Wilson and The Beatles were the field where the seeds of prog have been planted. Their innovations are too important to be ignored.
Procol Harum and the Moody Blues were the seeds of prog. They brought key elements (instruments/sounds) of the genre into play while not using these in a Prog goal/environnement.
King Crimson's first album was the sprout. It was the first album where all the elements of Prog were assembled in a coherent package with a definitive sound.
ELP, Yes, Tull and the 70's bands were the plant/trees. They are the foundation of what is prog.
Then finally came the fruits. These are the prog bands who do not come from the "classic" era. Some years produced good fruits, while some others had rotten harvests or no harvest at all.
And I hear you ask "Where's Pink floyd?". Well, in the 60's, PF was the farmer who decided one day to do an Acid drop and then proceeded to water the seeds of prog. The years passed, the drops continued, and one day, the farmer awoke to see the trees of prog. He thought that it would be great to be one with the trees, and thus proceeded to do an astral voyage and became one with the trees.
The end.
Yes, I know, none of it makes sense and there's probably around fifty historical mistakes in there. I just thought about it for fun.
My point is that I think KC's first album is prog's real starting point. See, that sentence alone would have made a pretty boring post.
