Final answer:
The first humans disobeyed God by eating forbidden fruit, an act that is central to Judeo-Christian teachings about the origins of sin and humanity's fall from grace.
Step-by-step explanation:
The first humans chose to disobey God by eating forbidden fruit from the Tree of Knowledge of Good and Evil, as depicted in the narrative found in the book of Genesis in the Bible.
This act of disobedience by Adam and Eve, which is known as 'The Fall', resulted in their expulsion from the Garden of Eden and the introduction of sin into the world. This narrative is a part of many Judeo-Christian teachings about the origins of humanity and the concept of original sin.