Before South Park even existed in nascent forms, animator Trey Parker was making student films at the University of Colorado. And in 1992, Parker co-directed with Chris Graves American History, an irreverent overview of critical events in the history of the United States.
According to production lore, Stone recruited his Japanese roommate Junichi Nishimura to recap the entirety of American history, which the directors in turn set to stop-motion paper cut-outs. You can see early South Park flourishes in this 20-year-old cartoon, namely the deer-in-headlights eyes.
And for more early work from the creators of South Park, here's Parker's 1989 live-action student film The Giant Beavers of Southern Sri Lanka (a kaiju parody) and Parker and Matt Stone's play turned Troma movie, Cannibal! The Musical.
[Via Cartoon Brew]