MONTEVIDEO, URUGUAY—Paleontologists have unearthed more than 1,000 fossils from a bone bed at Arroyo del Vizcaíno in southern Uruguay. Many of the bones, which date to 29,000 to 30,000 years old, are remains of three extinct species of giant ground sloth. Some of these sloth bones bear marks that could have been made by tool-wielding human hunters. But the site of Monte Verde, in Chile, is the earliest known human settlement in South America, and it is just 14,000 years old. “So, it’s strange and unexpected,” said Richard Fariña of Uruguay’s Universidad de la República.