Question: How and when did idolatry start?
Answer: Since there are many ways to define idolatry, I could come up with a variety of answers. I could say that idolatry began the first time someone knowingly acted against the will of G-d. By doing so one is saying that G-d does not rule, and if G-d doesn’t rule, then something else must rule, often a person’s own self. Placing someone other than G-d as ruler is a form of idolatry, and under that definition idolatry began in Gan Eden (Garden of Eden).
If battling G-d is idolatry, then perhaps idolatry begins at Bavel (the Tower of Babel).
If worshiping intermediaries is idolatry, perhaps when people prayed to the Sun, Moon, stars, and nature to act on their behalf was the beginning of idolatry. From such beginnings, it doesn’t take long before people forget that intermediaries are intermediaries, and begin to worship them as gods in their own right.
Regards, Eliahu Levenson