Re: why aren't there any bears in africa?

You're definitely right, once upon a time bears did exist in Africa.  So 
the real question would be why did they leave?

I can see two possible reasons: one, that they ran out of a food source 
that could sustain them for some reason; or two, that they went extinct 
(that particular population).

If they ran out of a food source, they would have left in search of food 
or simply died off.  This could be because the food source went extinct 
or they were outcompeted.  Bears are omnivores, and have a very heavy 
diet, they eat a lot.  If they were outcompeted for a major source of 
food, by say lions or something, then their numbers would have declined.  
Imagine if something could eat all or most of the salmon before bears got 
to it in Alaska.  Bears would go extinct there too, most likely, or have 
their numbers severly reduced.  Perhaps humans drove them off, or apes.  
Apes are quite the hunters when they want to be.

If they went extinct, you'd have to ask why.  Did something higher on the 
food chain come in and beat them up?  I doubt it, but maybe.  Perhaps 
another extinct predator migrated in and ate the bears.  I don't think 
there's anything in Africa now that could eat bears or anything, but 
there could have been.  So most likely they disappeared because of a food 
shortage.  

I don't know what caused that, could have been bears being outcompeted 
for food, some climate change that reduced the foods availability, or the 
food source itself becoming rare or extinct.  

So in short, I'm not sure.  Hopefully this puts you on the track for 
finding out the answers definitively.  Unfortunatly, most of the time in 
histortical ecology you never get clear cut answers, just probable 
reasons.  Good luck!

Moderator's Note:
The same could be said for pachyderms in North America - there used to be 
Mastadons all over the continent, now the only extant elephants are in Asia and 
Africa.  Probably for many of the same reasons given by Brian.