There are no middle stages. To say there is a middle stage implies that what we currently are at is the end state, which isn't true. We ourselves are also a middle stage for some future species. Species merely change over time based on the natural selection pressures of their environment, and random mutation. Make sense? Also we do have fossils of "middle stage" apes. http://humanorigins.si.edu/resources/intro-human-evolution [spoiler]follow the menu on the right side of the link screen. The how do we know tab is especially good.[/spoiler] This is a good resource for your questions. It'll take some reading, but if you're genuinely interested the answers to your questions are there.