On November 24, 1974, paleoanthropologist Donald Johanson was looking for something very specific in the gravel-strewn landscape of northeastern Ethiopia. A year earlier, Johanson and a colleague had made a thrilling discovery in the very same region—a perfectly preserved knee joint from an ancient human that walked those hills 3.4 million years ago. That was nearly a million years older than the oldest known human fossil at the time.

But a single knee joint wasn’t enough to prove that Johanson had found the oldest fossil of a hominin, the scientific classification for modern humans (Homo sapiens) and all of our extinct human ancestors. Johanson needed more evidence—a skull, ideally, and some teeth. So he and a graduate student were back in the Afar region of Ethiopia scanning the barren hills for bones.

“I spotted this little piece of elbow only about an inch-and-a-half or two inches long,” says Johanson, now a professor at Arizona State University and the founding director of its Institute of Human Origins. “And I thought maybe it's a primate like a baboon; there are lots of baboon fossils there. But as I picked it up and looked at it more closely, I said to my student, Tom Gray, ‘This is from a human—a human ancestor.’”

Stepping back, Johanson realized there was more: 47 bones in all, including a thigh bone, a pelvis, and incredibly, fragments of a skull and jawbone. That was plenty to construct a detailed anatomical model of a human that lived more than 3 million years ago.

“This was damn significant," Johanson. "It was a hominin skeleton from a geological layer that was estimated at that time to be older than 3 million years. So this was a moment of incredible exhilaration. I mean, here it was right at my feet.”

With Johanson's discovery of “Lucy”—as the fossilized skeleton became known—scientists were forced to rewrite the human evolutionary timeline. This was an ancestor with a mix of humanlike and apelike traits. How many other ancient human species inhabited Africa, and how did they all fit into the genealogy of modern humans?

A half century later, Lucy remains one of the most iconic finds, offering clues to solving the mystery of human origins.

The site in Ethiopia where Johanson and Gray found Lucy in 1974 is called Hadar, and it turned out to be an incredibly rich location for recovering ancient bones. Millions of years ago, Hadar was home to a large lake. Today those long-buried lake sediments are eroding to the surface and revealing ancient remains, both animal and human.

“Animals either fell into the lake or were dragged into the lake, and slowly decayed and over time covered by sand and clay,” says Johanson. “We don’t know how Lucy got into the lake. Maybe she was down by the water getting a drink and was taken by a crocodile.”

In 1975, a year after discovering Lucy’s skeleton at the Hadar site, Johanson’s team made an even more astonishing find: the ancient remains of a group of 17 individuals, probably related, which also dated back 3.2 million years. Among scientists, the group is known as the “First Family.”

Prior to Lucy, the oldest known human fossil—a 2.5-million-year-old skull recovered from a cave in South Africa—was from an ape-like hominin species called Australopithecus africanus. But Johanson’s hominins dated nearly a million years earlier and came from East Africa, a completely different region.

In 1978, after carefully comparing Lucy and the First Family with the recovered remains of other ancient humans, Johanson’s team claimed with confidence that Lucy and the other Ethiopian fossils represented a previously unknown species of human. The species was named Australopithecus afarensis after the Afar region of Ethiopia.

According to Darwin, one of the main characteristics that separate hominins (humans) from hominids (great apes like gorillas, chimpanzees and orangutans) is bipedalism—walking upright on two feet. To prove that Lucy was indeed an ancient human, Johanson had to show that she and her fellow Australopithecus afarensis species walked on two legs.

The evidence for bipedalism started with that first knee joint that Johanson found in 1973.

“One of the first people I showed it to was an orthopedic surgeon,” says Johanson. “And I said, ‘What do you think of this? But be careful, it’s 3 million years old.’ He said, except for the small size, it’s identical to what we use in a knee replacement.”

The same was true for Lucy. Even though Lucy would have only stood 3.5 feet tall, everything about her knees, hips and pelvis pointed to upright walking.

“Lucy had a pelvis that wasn’t tall and narrow like a quadruped, but short and squat like a human,” says Johanson, “like the one we're all sitting on.”

What sealed the deal for Johanson was a separate discovery in Tanzania of a stunning set of human footprints captured in fossilized ash. The footprints, found in 1978, dated to 3.7 million years and were accompanied by fossilized teeth and jaw fragments that matched Lucy’s species.

“The footprints that were found are identical to the footprints you and I leave in the beach sand,” says Johanson. “There’s really no longer any controversy about Lucy upright walking.”

At the time of Johanson’s discovery in 1974, Lucy was by far the oldest human fossil on record. In the past five decades, though, even older hominin bones have been found in Africa belonging to other species that either predate or overlap with Lucy.

Australopithecus anamensis, which also inhabited the Afar region, dates to 4.4 million years ago, and some scientists believe they’ve found bone fragments that push the origins of humanity back as far as 7 million years.

Although she’s no longer the oldest, Johanson believes that Lucy still occupies a special place in the hominin family tree—as the last common ancestor of two major branches of humans, both modern and extinct.

Lucy and her species Australopithecus afarensis went extinct around 3 million years ago. But Johanson and other paleoanthropologists have found traces of Lucy's anatomical characteristics in later hominins, including the genus Homo, to which modern humans belong. The issue was a time gap between the last of Lucy’s species (~3 million years) and the first evidence of Homo (~2 million years).

The “missing link” between Lucy and the earliest Homo species may have been discovered in 2013. Another team working in Hadar found a mandible (jawbone) that didn’t fit any known human species.

“The front of the mandible has this backward sloping profile heavily reinforced with a lot of bone like afarensis [Lucy’s species],” says Johanson, “But the posterior parts of the mandible where it attaches to the lower part of the cranium look like Homo.”

The jawbone dates to 2.8 million years ago, just 200,000 years after the disappearance of Lucy’s species. That makes it a strong candidate for being a link between Lucy and modern humans.

Johanson and other paleoanthropologists have used similar anatomical comparisons to identify Lucy’s species as the ancestor of a completely different branch of extinct hominins known as “robust australopiths” for their powerful jaws and grinding teeth.

“The family tree I presented hypothesized that afarensis was a common ancestor to both the Homo lineage and the continuing Australopithecus lineages,” says Johanson. “This was (pardon the expression) one of the great bones of contention, because there were very few fossils known between 2 and 3 million. Over the past 50 years, that hypothesis has been tested over and over again. Fortunately, we now have evidence for our genus Homo at 2.8 million, just 200,000 years after afarensis disappears.”

Johanson still visits Lucy from time to time. All of her original fossils are stored safely in the National Museum of Ethiopia.

“I believe very strongly that all these fossils that are recovered in African countries belong in their country of origin,” says Johanson.

Johanson also notes that it wasn’t his idea to name her Lucy. As a newly minted Ph.D., he would have preferred something more serious and sober, like the fossil’s technical name: A.L.288-1, which stands for “Afar locality 288.” But the night Lucy was discovered, the team threw a little party to celebrate.

“We had a great celebration that night,” says Johanson. “We were listening to Sgt. Pepper's Lonely Hearts Club Band by The Beatles and the song “Lucy in the Sky with Diamonds” came on. One of the expedition members, Pam Alderman, said, ‘Why don’t we call the skeleton Lucy?’ I thought it deserved some scientific name. But it was too late. The name stuck and has become sort of a touchstone.”