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. Forty years later, learn the story behind the fossil that permanently changed scientists’ understanding of human origins.

Equally remarkable was how old she was. While the scientists couldn’t date the fossils directly, the age of the geological strata in the Hadar Basin indicated the Lucy skeleton was likely more than three million years old—far more ancient than other hominids. Once Lucy’s skeleton had been transported to the United States for further study, geologists used potassium-argon dating and other methods to fix the age of the basalt layer surrounding the discovery site at around 3.6 million years old. That number was later amended to 3.2 million years old using a more sophisticated radiometric dating process known as argon-argon dating.

Johanson’s first impulse was to label Lucy as part of the genus Australopithecus, a family of ape-like hominids that thrived between 2 and 4 million years ago. But when he took a closer look, he saw that many of her features were significantly more primitive than other Australopithecine specimens. Starting in 1976, Johanson and U.C. Berkeley anthropologist Tim White studied Lucy’s skeleton even further in an effort to place her in humankind’s family tree. The duo compared Lucy to a so-called “First Family” of some 13 other skeletal remains found at Hadar as well as to a collection of hominid footprints excavated by famed anthropologist Mary Leakey in Laetoli, Tanzania. Finally, in 1978, they released a groundbreaking paper classifying Lucy, the other Hadar fossils and the Laetoli footprints as all belonging to a brand new species of early hominid human ancestors. They called them “Australopithecus afarensis,” after the Afar Triangle in which Lucy was found.

Johanson and White’s bombshell promptly encountered backlash from the anthropological community. Many held that Australopithecus afarensis was not a discrete species, and still others—most notably the Kenyan paleoanthropologist Richard Leakey—balked at the assertion that fossils like Lucy were the direct ancestors of modern humans. The case for Australopithecus afarensis was strengthened in the early 1990s, however, when further research at Hadar yielded a wealth of other Lucy-like fossils including near-complete skulls of both male and female specimens. All told, Johanson and other researchers eventually collected fossils belonging to several hundred different Australopithecus afarensis individuals. These finds have helped bring Lucy and her kind into even sharper relief. Studies of afarensis shoulder bones have shown that Lucy most likely spent as much time climbing trees as she did walking upright on the ground, and other evidence has shown that she may have been a social animal that lived in small family units.

In the years since Lucy was lifted from her 3.2-million-year-old grave, anthropologists have gone on to find older and even more complete fossil remains of early human ancestors. In 2000, scientists in Ethiopia unearthed the remains of a 3.3 million year old Australopithecus afarensis baby dubbed “Selam.” The child was 100,000 years Lucy’s senior, but it’s now often known as “Lucy’s Baby.” Perhaps even more spectacular was “Ardi,” a 4.4 million year old Ardipithecus ramidus that displaced Lucy as the earliest known skeleton of a human ancestor. News of “Ardi” first came to light in 1994, but since then, scientists have found evidence of even older hominids dating back as far as 7 million years ago.

Despite these more recent breakthroughs, Lucy still stands as perhaps the most famous hominid fossil discovery ever made. Casts of her skeleton sit in museums around the world, and hundreds of thousands of people flocked to a traveling exhibition of the original Lucy fossils in the 2000s. In a 2009 interview with Time, Johanson speculated that Lucy’s enduring celebrity stemmed from the way she helped fill in the blanks in our understanding of early humans. “She showed us conclusively that upright walking and bipedalism preceded all of the other changes we’d normally consider being human…” he said. “She gave us a glimpse of what older ancestors would look like.”