Why when astonished would someone say, “Well, I’ll be a monkey’s uncle”?

During the famous Scopes trial in 1925, a Tennessee schoolteacher, John T. Scopes, was accused of breaking that state’s law by teaching Darwin’s theory of evolution rather than the Biblical origins of mankind. The trial was a sensation and astonished many who had never heard that humans might be related to the apes, and from this came the expression, “Well, I’ll be a monkeys uncle.”