Back in 2005/2006, I remember catching parts of an old serial on TV. It was from the '30s/'40s, and it involved some mad scientist/lost civilization operating out of an African jungle, using science to turn ordinary tribal villagers into these hulking, simple-minded brutes. I recall that the brutes were all played by the same black actor, and at least one of the chapters featured a group of old black men who had been transformed into old white men.
Found it. It’s The Lost City.