The moment a theme is stated, it wants to do something. What?
Like all lifeforms, it wants to replicate, mutate, transform…become something. That something is musical structure.
How we get from a single note to a theme (motif), to a full-blown musical structure (song, free improvisation, symphony) is seemingly a mystery. And yet it’s not.
Due to a naturally occurring acoustical phenomenon known as the overtone series, one note is not actually one note—there are in fact many notes vibrating above the single note (the fundamental) we think we’re hearing exclusively.
But consciously or not, we have an innate awareness of those other notes, the overtones that ring out from the fundamental. And that awareness, at some point in human history, led us to pluck those notes out of the air, string them together into themes.
And then what did we do? We repeated those motifs, and they become something larger. First simple melodies. Then, as we repeated, we varied: shifted a pitch here, altered the rhythm there, played the motivic idea from another starting point in the scale.
In no time (though nobody knows how many centuries or millennia “no time” took to unfold) we had the beginnings of musical structure.
Music exists in time, evolves in time. As soon as you repeat something over time, and then vary it, you are effectively creating an incipient structure—whether you intend to or not.
At some point in musical history, humans began to mean it—to order notes intentionally. But that intention always leads back to one note which has within it the potential to become all notes—themes, melodies, songs, and larger structures.