I suspect they were stumpers to everyone. Was there a real point? As in, "Where is the fossil record leading to chimps and bonobos?"
Probably with the fossil record leading to bats (but in the latter case I'm purely guessing). If it happened in an eroding, forested upland, you won't find much fossil trace.
So forest creature evolution is undocumentable? Or does this unfortunate accident of nature only happen to chimps?