Jesus was the Son of God and the Son of man. He was 100% God and 100% human. He got his DNA from Mary, which is how He became one of us. Jesus was not another human. He became one of us through Mary, who is also from the line of David.
The reason behind the Immaculate Conception is because Catholics understood Mary to be the New Eve. In Genesis, the first Eve was called "woman" because she was the mother of the living.
Genesis 2:22-23 Then the rib which the LORD God had taken from man He made into a woman, and He brought her to the man. And Adam said: "This is now bone of my bones and flesh of my flesh; She shall be called Woman, because she was taken out of Man."
God made a promise in the Old Testament about the coming Savior. According to the Bible (the bold is mine):
Genesis 3:15 And I will put enmity between you and the woman, and between your seed and her seed; He shall bruise your head, and you shall bruise His heel.
The above Scripture was not referring to Adam's wife, Eve. It was referring to Mary, and scripture says "HER seed"....meaning that the DNA of Jesus came from Mary.
In the Old Testament, Jesus called His mother, "Woman". This is the same name that was given to the first Eve. By calling her "woman", Jesus is saying that His mother is the New Eve. Just as the first Eve was born sinless, the second Eve (Mary) was also conceived without sin.
John 2:3-4 And when they ran out of wine, the mother of Jesus said to Him, “They have no wine.” Jesus said to her, “Woman, what does your concern have to do with Me? My hour has not yet come.”
John 19:26 When Jesus therefore saw His mother, and the disciple whom He loved standing by, He said to His mother, “Woman, behold your son!”
Just as there was a first Adam, there was a second Adam (Jesus). Just as there was a first Eve, there was also a second Eve (Mary). In the Old Testament, the first Eve came out from the rib of man, and man fell into sin. In the New Testament, it was man (the second Adam) who came out from the womb of Mary, and man did not fall into sin, but was redeemed. Mary was the New Eve whom God spoke about in Genesis 3:15. She was conceived without Original Sin. Of course, this doesn't mean that she didn't need a Savior. Like everyone else, she needed a Savior. God was was her Savior.
Luke 1:46 And Mary said: “My soul glorifies the Lord and my spirit rejoices in God my Savior, or he has been mindful of the humble state of his servant. From now on all generations will call me blessed, for the Mighty One has done great things for me— holy is his name.
Mary was the new Eve and became the new mother of the living.