The Australopithecus has been around for a while now—and so has our knowledge of that human ancestor. The species Australopithecus africanus ("the southern ape of Africa") was first classified based on a skull found in 1924, which seemed to have characteristics of both humans and apes but which clearly belonged to a creature that walked upright, based on the position of the spinal cord.
For the next 50 years or so, new human ancestors were discovered every now and then, including different Australopithecus species—but it was in the 1970s that a "surge of discoveries" brought a new level of understanding to human origins. One of those big discoveries was the famous skeleton known as Lucy, who was found on this day, Nov. 24, in 1974:
In 1972 Maurice Taieb, 40, of France's National Center for Scientific Research, and Donald Carl Johanson, 34, of Case Western Reserve University and the Cleveland Museum of Natural History, found stone tools dating back 2.6 million years in the Afar region of Ethiopia. Two years later their team made an even more dramatic discovery. Not far from their first find, they [later] uncovered the fossilized remnants of a 20-year-old female Australopithecus lying in a layer of sediment 3 million years old. Unlike most other fossils of early man —a tooth here, a bone fragment there, occasionally a portion of a skull—this one comprised a good part of the skeleton.
Named after the Beatles' song Lucy in the Sky with Diamonds, Lucy was a small creature, not much more than a meter tall, with a brain capacity about a third that of modern man. Lucy 's skeleton gave scientists their best clues yet to the proportions of Australopithecus, and revealed her to be surprisingly short-legged. But the find left no doubts that she walked erect. The shape of her pelvis showed clearly that she was bipedal.
A few years would pass, however, before the full importance of Lucy would become clear. It was early 1979 when TIME declared her a "front-page celebrity" after Johanson announced the Lucy was a specimen of Australopithecus afarensis, a whole different species from those previously known Australopithecus examples. Significantly, she dated to a period before hominids split into the brand that led to us and the one that led to extinction. "The implications, says Johanson, are profound," TIME noted. "First, the old notion that man became bipedal as his brain grew is certainly false: Lucy was small-brained, but could stand erect. Second, because Lucy is basically so primitive, man may have split from his ape ancestors much later than 15 million years ago, as is commonly supposed."
Others in the field, however, like Richard Leakey (son of the famous anthropologists Louis and Mary Leakey and a believer that the common ancestor lived earlier than Lucy did), disputed some of Johanson's analysis that afarensis was a single, separate species.
Despite quibbles over classification, there's no question that Lucy was and is important to our understanding of human evolution. In 2012, scientist Derek Rossi nominated the spot where she was found for TIME's list of the most influential places in history: "The single most significant place in human history is where hominids first evolved and emerged," he said. "More specifically, the Afar region of Ethiopia has been the site where many of the most significant early hominid fossils have been unearthed, including the Australopithecus afarensis fossil find by Donald Johanson, dubbed Lucy."
Read a 1977 cover story about Richard Leakey's work, here in the TIME Vault: Puzzling Out Man's Ascent