My understanding is that the Amidah was recited only by the Shat'z during a period before the printing of Siddurim and, perhaps, when Hebrew proficiency was weak.
I'm assuming that because the Kedusha has Congregational responses, that this was introduced when Siddurim were more readily available and people had access to the text. But, I may be incorrect, and, perhaps, the Shat"z recited the Kedusha by himself?
When was it introduced - approximate date / era?