How did every human descendant become a fallen creature?
Every human descendant became a fallen creature by inheritance from Adam, not by personally eating forbidden fruit. Because all are sons of Adam, God decreed that everyone coming from Adam would be fallen, born spiritually dead, spiritually destitute, and spiritually an enemy of God.
Les explains that we are not guilty sinners because we ate a forbidden fruit or because we sinned in the similitude of Adam's sin. That was what Adam did; we did not. Instead, we inherited it: we are sons of Adam and are immediately born into this world spiritually dead, spiritually destitute, and spiritually an enemy of God. Les says it is hard to explain, but it is God's business, and He makes no mistakes. We take by faith that God created Adam innocent, sinless, and perfect, but Adam fell. God then decreed that everyone who came from Adam would be a fallen creature. Yet God did everything to bring that fallen creature back to his original state if they would just believe. It has always been by faith—Adam and Eve were restored into fellowship by faith, and so it was all the way through the Old Testament.