In a follow-up article, Porter cites the source of the melody to Coltrane's "Big Nick": Francis Poulenc's "Impromptu #3." See Porter's article for recorded examples.
In a comment below the second article, a reader points out that the source of the melody to the first half of "Giant Steps" was pretty definitely Harold Shapero's String Quartet (1941). Check the recording below! Shapero moves his theme through quite a few transpositions, including some chromatic third relations, though he doesn't follow "Coltrane changes."
The second half of "Giant Steps,” both chords and melody, is definitely borrowed from Nicholas Slonimsky. This is widely known (see this previous post for an image). It was a monumental achievement for Coltrane to combine the Shapero melody, Coltrane changes, and the Slonimsky example into a cohesive whole, not to mention working up the technique and applying the improvisational creativity necessary to perform it.
While checking other sources for this post, I ran across an assertion that the intro to "So What," written by Gil Evans, is borrowed from Debussy's "Voiles." I hear Debussy, but I don't particularly hear that piece in the "So What" intro. I also read in several places that "So What" derives in some way from Ahmad Jamal's recording of Gould's "Pavanne" - I'd have to be convinced. However, it's a fact that the horn riff in James Brown's "Cold Sweat" was derived from "So What" - as stated by Alfred "Pee Wee" Ellis, who wrote the horn parts.
Update 3/28/21 - Regarding "So What," see the comments below. It seems pretty clear that Miles used the template of Ahmad Jamal's "New Rhumba" (not the Jamal "Pavanne" recording cited in Wikipedia), distilling it into the modal, ultra-cool piece that had so much to do with setting the mood of Miles' "Kind of Blue" album.
