One day during the Pliocene Epoch, a young adult ape died in the Awash Valley of East Africa. She was soon forgotten, and wouldn't be seen again for 3.2 million years. During that time, her species went extinct, new apes appeared across Africa and some evolved huge brains, helping them basically conquer the planet.

Then, 3.2 million years after that fateful day, two of these brainy apes finally stumbled across her skeleton in what's now Ethiopia. Realizing they'd found something historic, they began to carefully dig her out of the desert.

First, however, they gave their long-lost relative a name: "Lucy."

This discovery came in 1974, catapulting Lucy from forgotten fossil to worldwide celebrity. Scientists only found about 40 percent of her skeleton, but it was enough to tell a game-changing story about human evolution. And that story is not a quick read: Even today, 42 years after Lucy re-emerged from the Awash Valley, scientists are still making headlines with secrets they learn from her bones.

Here are a few interesting facts you may not know about Lucy, from groundbreaking revelations about her life to random trivia about her name(s):

1. She walked on two feet, but also hung out in trees.

Lucy the australopithecine at SMNH New research supports the idea that Lucy walked upright similar to modern humans, but also spent a lot of time in trees — as implied by this exhibit from the Smithsonian Museum of Natural History. (Photo: Tim Evanson/Flickr)

Lucy lived at a pivotal time for human-like apes known as hominins. Her species was transitional, with key traits of earlier apes as well as later humans. (It's worth noting, though, the "missing link" concept is a fallacy. It's based on an outdated belief that evolution is linear, and on a misinterpretation of inevitable gaps in the fossil record.)

Lucy walked on two feet, a major step in human evolution. We know this from several clues in her bones, such as the angle of her femur in relation to knee-joint surfaces — an adaptation that helps bipedal animals balance while walking. Her knee joints also show signs of carrying her full body weight, rather than sharing the burden with her front limbs, and various other indications have been found in her pelvis, ankles and vertebrae. Still, her skeleton couldn't have moved quite like ours does, and her big, chimp-like arms suggest she hadn't yet abandoned the trees.

This has fueled scientific debates since the '70s. Was Lucy fully bipedal, or did she still cling to the arboreal lifestyle of her ape ancestors? Her skull indicates she stood upright, and her muscular arms could just be a case of "primitive retention" — ancestral features that remain in a species even after they're no longer needed.

skull and skeleton of Lucy, Australopithecus afarensis Lucy's skeleton offers several signs of bipedalism, or walking on two feet. (Photo: Juan Aunion/Shutterstock)

It's possible Lucy's species had stopped climbing, but hadn't evolved smaller arms yet. And for years after her discovery, CT scans weren't advanced enough to see inside fossils. That kind of information could reveal a lot about Lucy's behavior, since usage affects how bones develop, but it wasn't an option until recently.

In November 2016, researchers published a study based on new, more sophisticated CT scans of Lucy's bones. It revealed heavily built upper limbs, supporting the image of a regular climber who pulled herself up with her arms. Plus, the fact that her foot was more adapted for bipedalism than for grasping suggests upper-body strength was especially vital to Lucy's way of life, resulting in robust arm bones.

This doesn't completely answer the question of how much time Lucy spent in trees, but it does shed valuable new light on this famous forebear. She may have nested in trees at night to avoid predators, the authors say, along with some foraging in daylight. Sleeping for eight hours a day would thus mean she spent at least a third of her time off the ground, explaining the need for her odd mix of adaptations.

"It may seem unique from our perspective that early hominins like Lucy combined walking on the ground on two legs with a significant amount of tree climbing," study co-author and University of Texas-Austin anthropologist John Kappelman says in a statement about the finding, "but Lucy didn't know she was unique."

2. She made us rethink the rise of big human brains.

brain size of Australopithecus afarensis The brains of Lucy's species, Australopithecus afarensis, were about one-third the size of a modern human brain. Pictured above is an endocast, a brain model based on the space in an animal's cranial vault. (Photo: Tim Evanson/Flickr)

Before Lucy, it was widely believed that hominins evolved big brains first, and then became bipedal later. Lucy, however, was clearly built for bipedal walking — an extremely rare adaptation for mammals — and yet her skull only had space for a brain about the size of a chimpanzee's. Her cranial capacity was less than 500 cubic centimeters, or roughly one-third as big as that of a modern human.

This mix of traits points to the payoff of walking upright, an adaptation that may have paved the way for later species like Homo erectus to evolve such big brains. It's still not entirely clear why Lucy and other hominins started walking like this, but it was probably at least partly a way to find new foods. And whatever the initial reason, bipedalism offered another perk for later species: It freed their hands for skills like gesturing, carrying stuff and — eventually — making tools.

Many hominins were expanding their diets during the Pliocene Epoch, including Lucy's species, Australopithecus afarensis. Studies of teeth and bones show a fading reliance on tree fruit, offset by a spike in "savanna-based foods" like grasses, sedges and possibly meat. Lucy herself may have been part of this trend: Fossilized turtle and crocodile eggs were found near where she died, leading some to speculate that her foraging skills included raiding reptile nests. Over time, as life on the ground grew more complicated for hominins, the importance of intelligence likely grew.

3. She was an adult, but only stood as tall as a modern 5-year-old.

child posing with Lucy skeleton A human child poses next to the skeleton of an adult Australopithecus afarensis. (Photo: James St. John/Flickr)

Lucy's brain may have been smaller than ours, but to be fair, so was her whole body. She was a fully grown young adult when she died, yet stood just 1.1 meters (3.6 feet) tall and weighed about 29 kilograms (64 pounds).

When Lucy's brain size is considered in proportion to the rest of her body, it doesn't seem as tiny. In fact, her brain is actually larger than what's normal for a modern, nonhuman ape of her body size. This doesn't necessarily mean her intelligence could rival ours, but it is a reminder that she wasn't just an upright chimpanzee.

4. She may have died by falling out of a tree.

Lucy falling out of a tree This illustration shows a leading — but not universally accepted — theory about Lucy's death. It's from a 2016 study that concludes she suffered fatal injuries from a fall, 'probably out of a tree.' (Image: John Kappelman/UT-Austin)

For all we've learned about Lucy's life over four decades, her death has remained mysterious. Her skeleton doesn't show signs of gnawing by carnivores or scavengers (aside from a single tooth mark on one of her bones), so scientists doubt she was killed by a predator. Otherwise, though, they've been stumped.

Then, in August 2016, a team of U.S. and Ethiopian researchers announced a break in Lucy's cold case. Their study concluded her death "can be attributed to injuries resulting from a fall, probably out of a tall tree." They used high-resolution CT scans to make 35,000 virtual "slices" of her skeleton, one of which showed something odd. Lucy's right humerus had a type of fracture uncommon in fossils: a series of sharp, clean breaks with bone fragments and slivers still in place. Along with other, less severe fractures at the left shoulder and elsewhere, this is consistent with a long fall in which the victim tries to break the impact by extending an arm before landing.

Aside from shedding light on Lucy's final moments, this cause of death would also support the idea that Lucy's species still dwelled in trees, points out John Kappelman, who also worked on the other 2016 study about Lucy's arms.

"It is ironic that the fossil at the center of a debate about the role of arborealism in human evolution likely died from injuries suffered from a fall out of a tree," Kappelman says in a statement. Not all experts agree with this conclusion, arguing the bone damage could have occurred after she died, although the study has been widely lauded. And beyond the potential scientific insights, learning how Lucy died can also help modern humans relate to her on a more personal level.

"When the extent of Lucy's multiple injuries first came into focus, her image popped into my mind's eye, and I felt a jump of empathy across time and space," Kappelman says. "Lucy was no longer simply a box of bones, but in death became a real individual: a small, broken body lying helpless at the bottom of a tree."

5. Her English name comes from a Beatles song.

When paleoanthropologist Donald Johanson and graduate student Tom Gray found Lucy on Nov. 24, 1974, they gave her the prosaic name "AL 288-1." Despite everything this australopithecine has taught us, she might not be a household name if that clunky title had stuck. Fortunately, a party broke out that night at the expedition team's camp, and it offered inspiration for a better alternative.

As the scientists celebrated, someone was playing the Beatles' 1967 song "Lucy in the Sky with Diamonds" over and over in the background. "At some point during that night, no one remembers when or by whom, the skeleton was given the name 'Lucy,'" according to the Human Origins Institute at Arizona State University. The name stuck, and 40 years later, it can be hard to think of her as anything else.

6. Her Ethiopian name, Dinkinesh, means 'you are marvelous.'

Lucy the australopithecine, Australopithecus afarensis A reconstruction of Lucy greets visitors at the Field Museum in Chicago. (Photo: Tim Boyle/Getty Images)

The name "Lucy" has humanized this creature for many people, pushing us to imagine a relatable individual, not just a faceless extinct animal. But while it resonates widely, it doesn't have the same cultural salience for everyone.

And so, although the world mainly knows her as Lucy, that isn't her only modern moniker. In the area where she actually lived, now part of Ethiopia, she's known as Dinkinesh in the Amharic language. Lucy is a nice name, but there's unique reverence encoded in Dinkinesh, which translates to "you are marvelous."

7. We're all still walking in her footsteps.

Laetoli footprints These footprints were made 3.6 million years ago, most likely by Australopithecus afarensis. They were found in Laetoli, Tanzania, two years after the discovery of Lucy in Ethiopia. (Photo: Tim Evanson/Flickr)

Lucy belonged to one of many species in the extinct Australopithecus genus. She hails from heady times in human evolution, long before we were the last hominins left standing. It's widely believed that one australopithecine species launched the entire Homo genus — which includes eggheads like Homo habilis, Homo erectus, neanderthals and us — but we still aren't sure which is our direct ancestor.

We may never know, and some experts doubt we're descended from A. afarensis, citing other species as likelier candidates. Still, Lucy remains a popular possibility. Her species has a lot in common with Homo, and since our genus arose roughly 2.8 million years ago (about the same time A. afarensis died out), the timing works.

Even if we aren't Lucy's direct descendants, however, she's still a titan of hominin history. As perhaps the most famous australopithecine of all time, she has come to symbolize not just her species or her genus, but the very idea of small, upright apes setting the stage for humanity. We now have a rich fossil record of Australopithecus, including other species and more evidence of Lucy's kind, like the Laetoli footprints pictured above. These all help us clarify what life was like for our pre-human ancestors, providing valuable context for our own species' recent success.

After all, Homo sapiens only evolved about 200,000 years ago. We've accomplished a lot in that brief time, but we've stayed so busy it's easy to forget how briefly we've been around. Fossils suggest Lucy's species lived between 3.9 million and 2.9 million years ago, for example, which would mean this humble hominin existed for about 1 million years — or five times longer than we've made it so far.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.