The discovery of a 1.8 million-year-old skull has offered evidence that humanity's early ancestors emerged from Africa as a single adventurous species, not several species as believed, drastically simplifying the story of human evolution, an international research team said Thursday.

The skull—the most complete of its kind ever discovered—is "a really extraordinary find," said paleoanthropologist Marcia Ponce de Leon at the University of Zurich's Anthropological Institute and Museum, who helped analyze it. "It...