Deep inside a cave 30 miles from Johannesburg, South Africa, a tight crevasse guards the passageway to what was, until recently, the grave of at least 15 human-like individuals. Their bones and teeth—more than 1,500 fragments in all—lay in a heap in the bottom of a pitch-black chamber for ages, until two skinny spelunkers with flashlights squeezed into the earth deep enough to find them. Now those bones are in the hands of scientists who say they belong to a new species of prehumans, with a mix of features typically associated with modern man or fossils belonging to Australopithecina, a...