In 1974, the paleoanthropologist Donald C. Johanson led an expedition to Ethiopia to look for fossils of ancient human relatives.

In an expanse of arid badlands, he spotted an arm bone. Then, in the area surrounding it, Dr. Johanson and his colleagues found hundreds of other skeletal fragments.

The fossils turned out to have come from a single three-foot-tall female who lived 3.2 million years ago. The scientists named her species Australopithecus afarensis, and the skeleton was dubbed Lucy.

Four decades later, Lucy remains one of the most famous discoveries in paleontology. Finding a single bone of that age would have been reason to celebrate; finding so much from a skeleton revealed a tremendous amount about Lucy — and about human evolution in general.