WHEN WERE THE LEVITES ASSIGNED TO MINISTER?

Number 3:6-7 Bring the tribe of Levi near, and present them before Aaron the priest, that they may minister unto him.  And they shall keep his charge, and the charge of the whole congregation before the tabernacle of the congregation, to do the service of the tabernacle. Deuteronomy 10: 7-8 From thence they journeyed unto Gudgodah; and from Gudgodah to Jotbath, a land of rivers of waters.  At that time the Lord separated the tribe of Levi, to bear the ark of the covenant of the Lord, to stand before the Lord to minister unto him, and to bless in his name, unto this day.

I'm not seeing what the contradiction is supposed to be, unless the critics are supposing that Aaron and God are one and the same.  In the events in Numbers, they're assigned to minister unto Aaron.  In the Deuteronomy passage, they're assigned to bear the ark and minister unto God.  Not a contradiction.