ok well first, we are free to not take the first 11 chapters of Genesis literally.
Our literal history starts with Abraham. even then, in the New Testament, in the Gospels, the writers are not concerned with history, they are concerned with the theology of what it all means.
Genesis was written while the Jews were in exile and taken into Babylon.
sin does not come from original sin. we as finite, imperfect creatures are free to live good lives and move towards God, or to move away from God and be corrupted in evil. Adam and Eve already had the propensity to sin, they were created imperfect. They chose disobedience.
from the catechism
Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight."122 Sin sets itself against God’s love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods,"123 knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God."124 In this proud self- exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.125
The root of sin is in the heart of man, in his free will, according to the teaching of the Lord: "For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man."128 But in the heart also resides charity, the source of the good and pure works, which sin wounds.
1873 The root of all sins lies in man’s heart. The kinds and the gravity of sins are determined principally by their objects.