Aaron was the first head of the priesthood that bears his name. He was “called of God” (Hebrews 5:4) to this position of God’s power. When the Lord first gave the law of carnal commandments as preparatory gospel, he gave a preparatory power in priesthood to administer the lesser law. (Hebrews 7:12; Inspired Version, Exodus 34:1-2.) This lesser priesthood was conferred upon Aaron and his sons after him. (Exodus 28, 29, and 30; Leviticus 1:11, 3:2, 13:2; Numbers 18).
The Church of Jesus Christ of Latter-day Saints is operated by the Melchizedek Priesthood. Everything it does is by that agency: “All other authorities or offices in the church are appendages to this priesthood.” (D&C 107:5).