The Morenada dance tells the story of the African slaves who worked in the mines. These characters' costumes weigh something like 100 lbs, and they still manage to dance (or at least twirl and sway) in unison for hours.
In the past, these carnaval dances were the exclusive realm of the lower classes, but over time, they've taken on a kind of prestige among the rich and middle classes as well. Rather than being a religious festival, this parade was a competition between La Paz's universities and their various schools. It was also an excuse to have a huge party.