Atlantic: South America was once a land of giants. Its plains and forests were home to elephant-sized ground sloths, mace-tailed armadillos the size of a car, and huge creatures that resembled humpless camels. Hunting them were a large subspecies of jaguar, sabre-toothed cats, and the gigantic short-faced bear, several times heavier than a grizzly. All of these giantsthe aptly named megafaunaare now gone. Why? In the 1930s, Junius Bird, an American archaeologist in the mold of Indiana Jones, found a clue...