Why do some black people say that "all white knowledge came from the black man"? Is there any evidence of this?

No, none at all.

no but it could be said that most of the wealth that the white man has came from the toil of the black slaves

IF all white knowledge comes from the black man, then why is the black man just a tiny portion of the general populace?

One needs to understand that blacks are desperate for respect since, on the whole, most people do not respect black people and indeed look upon them as Satan's children. It is a pity that they are so inept but more so that they are destructive, dangerous and a drag on all societies - this includes African societies of course. They know they are the children of Satan and have contributed nothing (but JAZZ - and that IS as incredible contribution). So to make themselves feel better they make up wild, unsubstantiated BS and try to believe these wild absurdities

Yes, Africa being the cradle of mankind. The oldest human fossils were discovered in Africa.