I know I said that I was going to write about the evolution of the Hagaddah, but actually what I was really interested in was the origins of the Seder itself. But before I get into the origins of the Seder, I will include some information about how our current Hagaddah came to be.
The basic form was set down in Mishnaic times, that is by about 200 CE, and much was added during or after Talmudic times which is where all those Rabbis got into the book. The aspect of the Ma Nishtanah, the Four Questions, is rather old but it has changed over time. The order of questions is different as are some of the questions themselves.
Originally there was a question saying "On all other nights we eat meat roasted, stewed or boiled, but on this night, roast only." (See Pesachim 116a). The dipping question was moved up and the sitting/reclining question filled in.
Also, at the end, the songs of Chasal Siddur Pesach, Dayeinu, and Chad Gadya were added around the turn of the last millenium. Rav Amram Gaon, who headed the Babylonian Yeshiva of Sura between 856-876 CE ended the Hagaddah after the fourth cup, Rav Saadia Gaon (882 CE - 942 CE) did not have Dayeinu or Chad Gadya in his version but Rashi (1040-1105) did have Dayeinu. In fact, Chad Gadyah didn't appear in the Hagaddah until 1590 in Prague.
It is really strange thus knowing that Chad Gadyah came so late into the text that later and contemporary Rabbis try so hard to come up with a "deep" explanation of it. The truth of the matter is that that song came from a late medieval German folk song ("Der Herr der schickt den Jokel aus") which, in turn, was based on an old French nursery song.
Now, given the variable and mutable nature of the Hagaddah for its entire history, it does seem strange that Orthodoxy today is so against any modern changes to it - even though they continue regardless.
Anyway, the origins of the Seder itself are actually that of the Greek symposium. Since the whole Seder was set up after the fall of the Temple and sacrifices were no longer brought, it sensibly took the form of an already formed system. Among the practices described by the Greek sources were: a ritual wine libation and washing of the hands; the eating of various hors d'oeuvres before the main meal, including lettuce and various fruit and nut salads resembling charoset, sometimes in the form of sandwiches (reminiscent of Hillel's famous custom); the singing of hymns to an assortment of gods, whose praises might make up the central topic of discussion; and the posing of a set of questions to set off the conversation. They would also commonly lay on their left sides as that was the method of dining at the time. Carpas is clearly a Greek word as is afikoman. (Although the Greek idea of an "epikomion" was rather different from the stale piece of matzah Jews would be familiar with.)
And here yet again we see famous Rabbis trying to give special retroactive meanings to these clearly borrowed practices. Like that charoset is to symbolize the mortar, etc. Not that there is anything necessarily wrong with it, but in doing so they hide the actual origins. Indeed, some perhaps for that very purpose.