God humbled Himself to be born of a human being, and so humbled Himself that when the Angel Gabriel announced God's will to the Virgin, she still had the option of saying, "No." That is the free will that makes us sons not slaves, as St. Paul's letter in the lectionary today makes clear.
So while Mary is a very great lady, the pattern of a Christian, the Mother of God, conceived without sin, it is the will of God that puts her in that high place. But without Mary's free cooperation in saying, "Fiat mihi secundum verbum tuum" - "Let it be done to me according to thy word" - the Incarnation would not have occurred. But that also is God's will.
Makes you dizzy, but praise God for His great mercy!
We should all be grateful for Mary's faith and obedience to the word and giving birth and raising our Savior. But there is nothing to suggest that another person could not have been chosen by God to have done it had Mary not.
Well I guess Mary did have a PHD...She spoke Latin as well...