Give some stream of consciousness and then the answer. The term monkey's uncle, most notably seen in the idiom "(well) I'll be a monkey's uncle", is used to express complete surprise, amazement or disbelief. It can also be used to acknowledge the impossibility of a situation, in the same way that "pigs might fly" is used. An example is if one says: "I may agree that if two plus two equals five, then I am a monkey's uncle". "I'll be a monkey's uncle" has been said to date from after 1925, the date of the widely publicized Scopes Trial in the United States, where the term first appears. The Oxford English Dictionary's earliest example is the phrase If that's a joke I'm a monkey's uncle, from an Ohio newspaper on 8 February 1925. It was originally a sarcastic remark made by creationists. The notion "that (people) were descended from apes was considered blasphemous ... by Darwin's contemporaries", and it was for this reason that the sarcastic phrase came into use.

Answer this question: where did the phrase i'll be a monkey's uncle come from?
The important part for answering this question is: "I'll be a monkey's uncle" has been said to date from after 1925, the date of the widely publicized Scopes Trial in the United States, where the term first appears.
Therefore, the answer is the widely publicized Scopes Trial in the United States.