RIO DE JANEIRO — This city’s history as the main gateway for African slaves brought to the Americas has come into sharper focus in recent years as a construction boom in the city unearthed mass graves and other traces of that era’s slave trade.

On Sunday, the United Nations Educational, Scientific and Cultural Organization took an important step toward preserving that grim legacy by adding the Valongo Wharf, where hundreds of thousands of slaves disembarked, to its World Heritage List.

“From a historic point of view, this is a testimony to one of the most brutal episodes in the history of humankind,” Unesco says in its justification for the designation.

Historians believe that as many as 50 percent of the estimated 10.7 million slaves brought to the Americas arrived in Brazil. Valongo was the entry point for an estimated 900,000. Brazil imported more slaves than any other nation and in 1888 became the last in the Americas to officially abolish slavery.