RIO DE JANEIRO — Sailing from the Angolan coast across the Atlantic, the slave ships docked here in the 19th century at the huge stone wharf, delivering their human cargo to the “fattening houses” on Valongo Street. Foreign chroniclers described the depravity in the teeming slave market, including so-called boutiques selling emaciated and diseased African children.

The newly arrived slaves who died before they even started toiling in Brazil’s mines were hauled to a mass grave nearby, their corpses left to decay amid piles of garbage. As imperial plantations flourished, diggers at the Cemitério dos Pretos Novos — Cemetery of New Blacks — crushed the bones of the dead, making way for thousands of new cadavers.

Now, with construction crews tearing apart areas of Rio de Janeiro in the building spree ahead of this year’s World Cup and the 2016 Summer Olympics, stunning archaeological discoveries around the work sites are providing new insight into the city’s brutal distinction as a nerve center for the Atlantic slave trade.

But as developers press ahead in the surroundings of the unearthed slave port — with futuristic projects like the Museum of Tomorrow, costing about $100 million and designed in the shape of a fish by the Spanish architect Santiago Calatrava — the frenzied overhaul is setting off a debate over whether Rio is neglecting its past in the all-consuming rush to build its future.