Donald C. Johanson and a coworker on November 29, 1974, discovered small bones on the slope of a desert channel at Hadar located in Ethiopia. There were the bones of a unique hominid that did not resemble anything discovered earlier. It was named "Lucy" (after the Beatles' song "Lucy in the Sky with Diamonds"). The small skeleton was an incredible discovery and a significant link in the search for our ancestors.

The term "hominid" has a very flexible definition. It generally means an erect-walking primate that is an ancestor of present humans. A hominid can be an ancestor of "true" or modern humans, or a relative, such as a modern primate. The few fossils that had been found before 1925 were from different geographical regions. They were also different from one another, and no one knew exactly what they were, how they were related, or their age.

Early efforts to discover the ancestors of humans centered in and around Europe. Hominid fossils had been found in the Neanderthal Valley of Germany, Beijing, and Java to name a few. Then, in 1924, Raymond Dart discovered a skull found in South Africa that did not resemble a human skull or that of a baboon or a chimpanzee. The skull was nicknamed the Taung baby, since it was found at Taung and was estimated to be the skull of a six-year-old hominid. Additional discoveries of fossils by the 1950s convinced most scientists that two types of hominids had existed in South Africa: Australopithecus africanus, a slender type, and Australopithecus robustus, a more primitive, robust type.

In 1959, Louis S. B. and Mary Leakey discovered the skull of a large Australopithecus robustus at Olduvai Gorge in Tanzania. They named their discovery Zinjanthropus boisei ("Zinj" also known as Nutcracker man or East Africa man), because they believed the hominid was sufficiently different from the australopithecines that it represented a different species; it was later reclassified as one of the robust australopithecines. It was the first australopithecine found outside South Africa and the first to be reliably dated, at 1.8 million years old. With the publicity surrounding the Leakeys' find, particularly through the National Geographic Society, paleoanthropology became fashionable to the general public and more funding was made available for further studies.

In 1972, Richard E. Leakey, the son of Louis and Mary, discovered a hominid skull in Kenya. He asserted that the skull was definitely that of a human and that it was approximately 2.9 million years old. This skull was the oldest known fossil of a human. If the more advanced genus Homo (to which humans belong) existed at the same time as the more primitive australopithecines, then theories that Hominids evolved from australopithecines were incorrect. Later, more accurate dating placed the age of the skull at about 1.9 million years.

In November, 1974, during an international expedition to Ethiopia, two of the oldest and finest hominid jaw fossils were discovered. A few days later, a third jaw was found. Richard and Mary Leakey visited the site and confirmed Johanson's suspicion that the jaws could be Homo with excessively primitive features. The jaws were dated at approximately three million years old, which made them the oldest known Homo fossils.

On November 30, 1974, a few days after the Leakeys had left the Hadar excavation site in Ethiopia, Johanson found the half-complete skeleton of Lucy. For three weeks, everyone at the site collected several hundred pieces of bone, which made up approximately 50 percent of the skeleton. Lucy was a tiny-brained individual, approximately 3.5 feet (a little more than 1 meter) tall. The sex of the skeleton was confirmed by the pelvic bones, which must be larger in females in order to permit the birth of large-skulled babies. Lucy walked erect, which confirmed theories that hominids walked erect three million years ago.

More hominid fossils were found in 1975 and 1976. At site 333, the fragments of at least thirteen individuals of various ages and sexes were found scattered on a slope. These fossils were Homo and very different from Lucy. The 1976 season also yielded stone tools, which strengthened the theory that the site 333 fossils were Homo, since there is no evidence that australopithecines made or used tools. Johanson and Tim White carefully compared the Hadar fossils and the fossils found at Laetoli, Tanzania, where Mary Leakey and White were working. These comparisons indicated that the Hadar and Laetoli hominids were similar and represented a developmental stage in between apes and humans. This determination was a departure from Johanson's early belief that the fossils were Homo. Johanson and White decided that the Hadar and Laetoli hominids were an early, distinct australopithecine. They named these hominids Australopithecus afarensis.

The discovery of Lucy was a significant development in the search for clues to understanding hominid evolution. Lucy was unique and as she was a very old, primitive, and small hominid that did not fit into the known hominid types. Although only 40 percent of the skeleton was recovered, bones from both sides of the body were present, allowing paleoanthropologists to reconstruct approximately 70 percent of her skeleton by using mirror imaging. With mirror imaging, existing bones are used to determine what the missing counterpart on the other side of the body looked like. Because of the evidence of upright walking in a hominid estimated to be millions of years old and because of the small brain size, the question of why hominids began walking upright had to be reexamined.

One previous theory was that manual dexterity, increased tool use, and brain development had forced some humans to stand erect in order to carry more with their hands. Lucy's hands were similar to those of modern humans, but no evidence has been found to suggest that australopithecines made or used tools. Various other theories explaining erect walking were suggested or considered.