If we discount disobedience as a sin in Abrahamic texts then eating from the Tree of Knowledge of good and evil would be the original sin.

We can go a little further than that.

What ability does a naive being unlock by attaining knowledge of good and evil? That would be judgment. The ability to separate and label what is good and what is bad. In that sense, our first sin came at the expense of living in dualism thus making us mortals.

Following this train of thought it would make sense why all Abrahamic religions tell you that only God can judge and almost all Eastern religions guide you to nondual living / out of the lower chakras of survival ( analytical mind )