Icaticus

Wiki has an excellent article with a detailed history on it. In short "Punk" begins with early Iggy Pop (The Stooges - listen to Now I Wanna Be Your Dog) and Patty Smith but its essentially a very difficult answer to give. It was more of an evolution that took place as a backlash from over-complication and codification.

If you consider it to begin at Sex Pistols then your thinking of Hardcore Punk. Thats a little easier to answer, Bad Brains and D.O.A. are its Godfathers. Thats partly opinion partly fact. You see Bad Brains started during punks move into the second wave punk and influenced massively the transition to hardcore by its then unique thrashing fast paced sound. DOA also predated the term Hardcore Punk and the split of hardcore punk. The easiest way to tell where a person stands on the issue is what they think of the Ramones. Although the Ramones are the most prolific three-cord punk band, they dont really sound like what a Hardcore Punk fan would call Punk. Since the split began in Southern California, the split is more noticeable as you get further west in the US. So as you can see... the answer is far more difficult than would at first be apparent.

Post-Punk qualification is more of a sound. Its was after the 1978 Punk vs Hardcore split and pre 1981 second British invasion. If its from... say post '89 it would more likely be part of the Punk Revival.

Unfortunately, although Punk as an art form is very diverse, the culture creates this central and narrow view of what "Punk" is thus limiting its experimentation and growth. Since Punk is such an "inside" community those that diversify themselves outside it loose the punk support base. Its important for anyone looking to learn more about punk to realize that the purpose of creating the art form was to address the over-complication issue and make modern music more accessible to the masses, thus a sort of electrified and modernized folk.

So really... where did punk begin? With the concept of folk music turned modern.