"Which is two questions, I know, but you can see where this was going. The assumption seems to be that since so many fossils of so many different types of hominids have been found there, that that is where the evolution must have occurred. But is that a good assumption?"
I think it's more the fact that we find a long line of things that sort of lead up to humans in Africa - and we don't find that anywhere else. Fossilization is a rare event, but not to find any humans anywhere else in anything other than finished form gives us reason to think that humanity started where we find the apparent lineage. It's not a logical conclusion without some other assumptions, of course. Also we don't find humans anywhere until long after they have appeared in Africa. It's not an absolutely required conclusion, but it seems like the best conclusion given the data we have.
Whenever one makes a measurement other than counting of small numbers there is an error bar around the measurement. Sometimes the error bars are very big. Also, estimation techniques improve the more time we have to consider them - well, ideally. There's the case of estimating the circumference of the Earth. The estimate of Eratosthenes back in BC was actually far better than later measures until quite recently (about 1600's I think).
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