Lucy at 50: How the world’s most famous fossil was discovered

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Lucy, a fossilized skeleton unearthed 50 years ago this month, transformed scientists' understanding of human evolution.

The discovery by American paleontologist Don Johanson and graduate student Tom Gray on November 24, 1974, in Ethiopia opened a new chapter in the human story, offering proof that ancient hominins were able to walk upright on two feet 3.2 million years ago — a trait once thought to have evolved more recently in tandem with big brains and tool use.

Assigned to a new species, Australopithecus afarensis , but best known by her simple nickname, Lucy had a mixture of ape and humanlike traits that suggested she occupied a pivotal branch in the human family tree. She has fueled five decades of scientific research and debate as well as ignited a broader public fascination with human origins.

While there are now fossil hominins twice as old as Lucy, she remains a paleoanthropological rock star. Made up of 47 bones from the same individual, she was the oldest known and the most complete skeleton of an early human ancestor when she was found.

CNN spoke to Johanson, 81, founding director of the Institute of Human Origins at Arizona State University, to hear the story of Lucy's discovery and why her significance endures.

This conversation has been lightly edited and condensed for clarity.

CNN: Take us back to 50 years ago.

Don Johanson: It was a Sunday morning, and temperatures were well over 100 degrees (Fahrenheit). I was in the Afar region of Ethiopia, which is one of the lowest places on the planet. There are ancient geological strata there, layers that date back millions and millions of years.

I was walking on sediment 3.2 million years in age searching for the fossilized remains of various kinds of animals, but particularly the remains of our ancestors. And I happened to look over my right shoulder. If I had looked over my left shoulder, I would have missed it.

What I saw was a little fragment of bone, a little part of the elbow and a part of a forearm, and I could tell immediately that it was from a human ancestor. It wasn't from an antelope. It wasn't from a baboon. It wasn't from any other kind of animal.

My student and I kneeled down to have a closer look. We looked up the slope, and there were fragments of the skull and fragments of a pelvis and fragments of an arm bone and the leg bone. And I realized at that moment that here was the childhood dream. ... I'd always wanted to go to Africa to find something and by golly this was something. But we didn't know how much it would become an icon in the study of human origins.

CNN: How long did it take to excavate the skeleton?

Johanson: (The bones) were very fragile. They had been mineralized, changed into stone, and we did a very careful crawl to pick up the obvious pieces, and then we took off the top layer, square by square, and then put those in the burlap bags, and then water washed them in the stream through very fine screening. The whole process took about 2 1⁄2 weeks.

It was wonderful to see her come together on the lab table in the field. The femur there was only about a foot long, or 28 centimeters, long.

What is this? I thought. Is this a child? Well, let's look at the jaw. The wisdom teeth had erupted so she was an adult. But my god, if this was an adult, it had to have been only about 3 1⁄2 feet tall, a meter tall.

CNN: How did the fossil get the name Lucy?

Johanson: Because of the delicate nature of the bones and the short stature, we felt she was probably a female. (Subsequent fossil discoveries revealed that males were much bigger than females.)

That night in camp we were playing the Beatles' (album) "Sgt. Pepper's Lonely Hearts Club Band," and "Lucy in the Sky With Diamonds" was playing when one of the camp members said, "Why don't you call her Lucy?"

It was just serendipity that it happened, but it stuck. It was, in many ways, an attractive name. People could identify with it. It made those bones a person. It drew you in and made you want to know more. And she's been the touchstone (of human origins) for the average person.

CNN: What kind of world would Lucy have lived in?

Johanson: It certainly was not what you see when you are riding around the Serengeti in a four-wheel drive vehicle today. It wasn't open plains. It was forested but not dense tropical rainforest — sometimes with open bush, and so you have available lots of different kinds of fruits and nuts and so on for food, probably robbing bird's nests, crocodile nests, turtle nests.

It really looks like upright walking is something that appeared in considerably more forested environments than we (initially) thought. The original view was that, once the forest disappeared, we stepped out onto the savanna, and we stood up to look over the tall grass. However, there (are) carnivores faster than your house cat (on the savanna), and you're not going to last long. I think bipedalism, from a logical point of view, was something that developed in the forest.

CNN: What was Lucy's significance at the time that she was discovered?

Johanson: She was the oldest, most complete hominin known at that time. This was terra incognita in the early '70s. Very few people had been to this region of Ethiopia, and people began launching their own expeditions and finding even more exciting things in some ways.

But I think that Lucy was the spark. She ignited a new stage in human origins research. What she did, most importantly, was she broke the 3 million-year time barrier, and the site of Hadar , which is a local name, is very fossil-rich. And it turned out to produce an enormous number of fossils of her species and gave us a really important benchmark by which all other discoveries that were made in the Afar could be judged.

CNN: In the field of human evolution, what has surprised you most over the past 50 years?

Johanson: One of them is that we have Neanderthal genes in us. Many, many years ago, before we knew that, we thought of ourselves as a very different species from Neanderthals. We could not exchange genes.

Work in paleogenetics, of course, has revealed that we carry 1% to 4% Neanderthal DNA. I have 2.1% — more Neanderthal than some people. And for the recent stretch of human origins, where we have Neanderthals living with Homo sapiens, a surprising discovery was made from (bones found in) Siberia. They found DNA, but it wasn't human, and it wasn't Neanderthal. It was some other species that we know very little about. They call them Denisovans .

How have views of Lucy changed over the last 50 years?

In the early days, there was a question of, really, how do we know that she's over 3 million years (old)? And argon dating (a method to date rock) was really developing very significant breakthroughs in the early '70s. So that (question) went away very quickly.

There were critics who said Lucy probably walked with a bent hip, bent knee gait. One of the most important things that was found subsequently were the footprints that Mary Leakey's team found in northern Tanzania in 1978. Obviously, these people weren't wearing shoes, leaving imprints in ash that was wet was like leaving a footprint in a beach sand. Here was the evidence that they were walking almost similar to us.

CNN: Was Lucy a direct ancestor to humans?

Lucy's species did not give rise directly to modern humans, but her pivotal place on the human family tree led to all later hominin species, most of which went extinct. The Homo lineage persisted and ultimately gave rise to us, Homo sapiens.