I hear that differently. But I refer to the very beginning of the piece, not to a bass line. Maybe I misunderstood where the part is.
The first chord is a major chord, not a minor one. The descending line starts from the octave, then minor seven, minor sixth, natural fifth and fourth and then before jumping back to the octave very shortly at least in the background a major third. A second is not played, so theoretically a minor or a major one can be part of the scale.
So it is either the fifth mode of melodic minor (major second) or the fifth mode of harmonic minor (minor second).
After that it is indeed natural minor.