While hunting for fossils in Ethiopia's Afar Triangle on November 24, 1974, paleoanthropologist Donald Johanson and graduate student Tom Gray stumbled upon the partial remains of a previously unknown species of ape-like hominid. Nicknamed “Lucy," the mysterious skeleton was eventually classified as a 3.2 million-year-old “Australopithecus afarensis"—one of humankind's earliest ancestors. The headline-grabbing find filled in crucial gaps in the human family tree, but it also shook up ideas about early human evolution and upright walking.