As made famous by Stevie Wonder
Original key is Eb minor while we play in D minor, one step down. (Original is in Ebm)
The song is mainly D minor / Dorian, the main difference between minor and Dorian being to generally play B as a passing note rather than Bb. Dorian scales are the same as minor but with a raised 6th (B). Think of D Dorian as A minor but starting and ending on D, and A becoming the fifth of the scale. Unless B or Bb need to be played here, generally bass can vamp between D and A without much difference.
Links:
Full Song Transposed (Slightly different arrangement but correct key)
Example Bassline Verse to Bridge
This song stays centered on D, primarily using the D Dorian scale for verses. This scale has no sharps or flats, essentially sharing a key signature with C Major.
D – E – F – G – A – B – C – D
The chorus deviates more into more complex chords, taking some more chromatic turns outside of what is established in the verses. Overall the chorus functionally centers on A7 as a functional dominant with additional harmonies, before pulling down to F7. This lands the progression without finality, before going back to Dm7 verses. These elements add to the funk style of the song.