OK, here's another impossible challenge of rock musicology.

When and how did the mixolydian mode come to be used in rock music?

To help answer, let's first ask ourselves: What was the first pop/rock/country/rockabilly/r&b/etc song to to use the "flat seventh" or "subtonic" chord? That is, if the song is in C, the use of B-flat major. The use of this chord *implies* the mixolydian mode. This chord does not exist in the major scale, but it does exist in the mixolydian. (You can also think of it as "IV of IV". Think of all those chord progressions that go: Bb F C. Bb is to F as F is to C... the weight of the whole thing collapses inevitably towards C with a force as strong as anything in western harmony.)

Anyway, these idea most definitely does not come from traditional Western musical theory. You will not hear this in classical music. Not very often, anyway. And I don't think you'll hear it very much in popular music before the rock era, either.