For decades after the discovery of “Lucy” (in the 1970s), it was thought that humankind first walked the Earth about 3 million years ago. But fossil finds in the mid-1990s pushed the evolutionary start-point for humans to more than 4 million years ago. Further astonishing finds in the early 2000s led researchers to conclude that human ancestors first walked the Earth nearly 6 million years ago.

In November 1974 American Donald C. Johanson (1943-) made one of paleoanthropology’s most widely publicized finds when he discovered a partial skeleton at Hadar, Ethiopia. More than 3 million years old, the female skeleton was the most complete hominid fossil ever found, but the skull was not recovered. The creature stood three and a half feet tall and, although apelike, had definitely walked upright. When Johanson officially announced his find in 1979, “Lucy” (named for the Beatles song “Lucy in the Sky with Diamonds,” which was popular in the camp at the time the fossil was found) became known as the mother of all humankind. (Her sex was confirmed by the pelvic bones.) Since she was an erect-walker, the finding gave certainty to theories that hominids walked erect at 3 million years B.C.

After the discovery of Lucy, older hominid fossils were unearthed by researchers in the field. In 1994 anthropologist Meave Leakey (1942—) and her associates found fossils of a 4.1-million-year-old bipedal species near Lake Turkana, Kenya. These were designated Australopithecus anamensis. (Technically, the first fossils of the Australopithecus anamensis were discovered in 1965 by a Harvard expedition in Kenya, but they were not identified as such until later finds were made.) Also in 1994, University of California at Berkeley paleoanthropologist Tim White and an international team published their 1993 discovery of the fossils of a chimplike animal in Ethiopia; they classified it as Ardipithecus ramidus. These fossils were dated to nearly 4.4 million years ago. Fossils discovered between 1997 and 2001 in the Middle Awash region of Ethiopia were determined to be nearly 6 million years old. In 2001 Yohannes Haile-Selassie (Cleveland Museum) and coauthors Tim White and Gen Suwa (University of Tokyo Museum) reported the monumental findings in the journal Nature: The hominid named Ardipithecus kadabba was thought to “represent the first species on the human branch of the family tree just after the evolutionary split [from chimpanzees].”