From where, then, does this depraved nature of man come?

Heidelberg Catechism

Question 7. From where, then, does this depraved nature of man come?

Answer. From the fall and disobedience of our first parents, Adam and Eve, in Paradise,[1] whereby our nature became so corrupt[2] that we are all conceived and born in sin.[3]

[1] Gen 3;
[2] Rom 5:12, 18-19;
[3] Psalm 14:2-3, 51:5