"I don't believe anybody's ever claimed cats turn into birds or lizards turn into zebras."
No, nothing that crazy, they just claim that prokaryotes turned into humans. You're right to point out how delusional any poster is if they joke about how lizards turned into zebras. Clearly, it was dinosaurs that turned into zebras. There, now we've straightened out that problem.
And it only took 3.5 billion years, too. Nobody pretends that a prokaryote will grow two arms and two legs over night and graduate from the Harvard Law School. Your incredulousness stems from a poverty of vision on your part, not from any problem with evolutionary theory.