Nope... we don't KNOW... Read what Stringy posted again, because I think you missed it. There is always the possibility we are completely WRONG in our findings. As I pointed out, we have not explored every fossil layer in every area of the planet, and we can't... it's physically impossible. We don't know that some sparse "clan" of humans weren't living in a cave somewhere and we've just not yet discovered that particular fossil layer because of its obscurity.
This is the crux of my argument in a nutshell, people like YOU who think we KNOW as a matter of FACT, when we don't really know because we don't ever know everything! I have no problem with you saying "we think" or "we are inclined to believe" but when you start proclaiming something as a FACT, it requires you PROVE the fact beyond any shadow of a doubt, and you simply haven't done that with this, and you can't.