Books of Chronicles
Prior to the appointing of Aaron and his sons to function as priests it was customary amongst the congregation of Israel that, just as all professions attract certain character types who excel therein, the duty of priesthood was best served by the firstborn of each household. As it is the firstborn who is, by nature, inclined and qualified to be an "emissary of the congregation" in the eyes of God. According to the "Targum Yosef" to Books of Chronicles, individuals from the Tribe of Reuben (the firstborn of Jacob) where leading as the High priest, up until the establishment of Aaron to the same (Targum Joseph to Books of Chronicles 1 5:1-2).