Walking on two legs is one of the main things that sets humans apart from other mammals. And we really want to know more about why we do this, and how, and when we made the switch from four legs to two. We thought we had it figured out. Our ancestors used to live in trees, but then Africa dried out and forests got further apart, and they needed a good way to get from tree A to tree B. So they put one foot in front of the other, headed out into the open grassland, and the rest is history. For many years, this was the most popular explanation of how we got here. But over time, doubts started to creep in. And then, in the 90s, one fossil species threw a major wrench in that idea. So let’s talk about Ardipithecus.
The Ultimate Evolutionary Upgrade:
Anthropologists talk about bipedalism a lot because it’s kind of a weird thing for a species of mammal to do. I mean, there aren’t many of us, and most of the bipeds hop instead of striding like we do, one foot at a time. And bipedalism is arguably the thing that made our species as we know it possible. After all, walking on two legs meant that our hands were free to become more dexterous and to carry and use stuff like tools. To riff on an old saying: when you’ve finally got a hammer, you begin to see the nails.
So bipedalism gave us everything from banana splits to Mario Kart, which is why researchers want to know as much as possible about its origins. And as for how this happened, we can get a good grasp on what parts of our bodies had to change in order to get our hands off the ground, thanks to comparisons between our bodies and other apes.
For example, the bones and muscles of our pelvis have all shifted to stabilize our torsos and let us balance on one leg at a time. Even the size and angle of our femurs changed, bringing our legs directly under our center of gravity and lining the knees up directly above the ankle joints, which helps us support our weight more efficiently. Our knee joints also got wider to support that weight, and we started to be able to lock our knees, which lets us hold our leg at full extension without using any muscles, which saves us energy. There were even changes to our spines.
If you feel your lower back, you can see how it curves inwards. The spines of four-legged animals don’t do that. This curve helps our backs work like a spring, which basically absorbs shock as we’re walking along. And while our hands got more dexterous over time, our feet and toes lost their grasping power. We said farewell to opposable big toes and gained an arch in the foot along the way, which helps transmit the weight of the body from your heel to the ball of the foot as you take a step. All of these changes gave us a stable foot that can hold our weight as we walk, run, skip, dance, or do whatever else we wanna do on two legs.
Why The Old Story Never Quite Fitted:
But when we think about why all these things happened, we’re really asking about what changed not just in our ancestors’ bodies, but in their lives. Evolution favors the organisms that are most well-adapted to their niche and their environment. So to see humans split off so dramatically from our quadrupedal ape relatives like gorillas and chimpanzees, something must have happened to that niche and environment. So what was it in our evolutionary trajectory that made the first upright hominins more well-adapted than their quadrupedal peers? Like we said at the beginning, one idea that scientists were keen on from the get-go was that it was some sort of gigantic environmental shift.
Since so many of our ape relatives live life as forest-dwelling quadrupeds, again, think about gorillas and chimps, scientists assumed that was kind of our ancestral condition. And because bipedalism only really makes sense to do if you’re walking on the ground, whatever made our ancestors switch over must have been connected to when our ancestors stopped calling the forests home.
Darwin’s Clue and the Australopithecus Find:
Even before we had much in the way of fossil evidence, this idea was popular. Both Darwin and his scientific rival, Lamarck, speculated about it, for example. But this idea really grew legs, pun very much intended, with the discovery of an ancient human relative known as Australopithecus africanus in the 1920s. Today, we know this creature lived in Southern Africa about 3.3 to 2.1 million years ago. And based on the shape of its skull as well as its teeth, its discoverer, Raymond Dart, believed it to be an ancient human, rather than an ape. And one that walked upright.
Dart’s major clue was a hole in the skull called a foramen magnum. This is where the spinal cord connects to the brain. And the placement of this foramen tells us the angle at which the spinal cord sticks out of the skull. If you’re gonna stand upright, that hole has to be at the bottom of the skull so that you can look forward, instead of on the back of the skull like it is for quadrupeds. Cause they’re looking like this. And further discoveries since then have backed this up. Today, we have found Australopithecus fossils with curved spines, broad pelvises, angled femurs, and big toes that were lined up next to all the others, not stuck out like chimp toes. All of that is what you’d expect out of a biped.
Eating Your Way to the Grassland:
And we’ve even found preserved footprints that show these hominins walked on two legs only. As for whether these hominins lived in the forest or in open plains, one major clue came in their teeth. Australopithecus had thick enamel on its molars, which you need if you’re eating an abrasive diet of leaves and plant matter, like you would find in an open environment. Combined with the fact that southern Africa is at least 45% savannah today, Dart’s find seemed to support the savannah hypothesis. Then, in the late 50’s and early 60’s, a few big publications cemented the idea that our ancestral landscapes were similar to modern savannahs. Especially open grassland savannahs.
Why Walk When You Can Knuckle-Walk?
But over the years, this idea about the savannah being key to the evolution of bipedalism had its detractors as well. For starters, it’s kinda… vague? Like, it gives us a sequence of events, but it’s not actually a great explanation for why bipedalism had to be the way to get around on the ground. After all, chimps and gorillas spend plenty of time walking around on the forest floor; they just do it on all fours to do it. And over the years, scientists have kind of looked around and remembered that while, yes, Africa does have some large savannahs, it’s not all desolate expanses with long hikes between trees. Some parts are pretty woodsy, even the parts that we’d call grassland. And that was likely the case even in the past as well. So our prevailing scenario still has some gaps to fill. And then a fossil named Ardi hit the scene.
The Fossil That Broke the Model:
Ardi, or, more properly, Ardipithecus, is a genus of hominins dating to between 5.8 and 4.4 million years old, so a good bit before Australopithecus hit the scene. The oldest species we’ve found was Ardipithecus kadabba, which lived about 5.8-5.2 million years ago. Unfortunately, we don’t have very many fossils of this guy, so we can’t really say how it moved or grooved in its environment. The real species of interest in the two-legged race is Ardipithecus ramidus. Ramidus’s skeleton was found in Ethiopia in 1994 and dates to around 4.4 million years ago. The Ethiopia part is important, so remember that. There are now at least 110 fossils of Ardipithecus ramidus, including a partial skeleton from one individual, which is rare. And the picture they all paint is… weird.
The Hybrid Hominin:
Because Ardi looks bipedal… but not in the way we’d expect. There are some parts of the skeleton that look basically like early forms of what we have today. The pelvis has a rotation that may indicate muscles were shifting for stability, like much later hominins. Its knees were in line above the feet, instead of being all bow-legged and chimp-y. And it had its foramen magnum right at the bottom of the skull, indicating that it stood upright, like us.
Plus, while Ardi’s hands were shaped like chimp hands, with long grasping fingers, their wrists were also much more flexible and less stable than those of modern apes. This meant that Ardi wouldn’t have been able to use its hands for knuckle-walking like chimps and gorillas do. So far, Ardi is looking like it walked the bipedal walk a lot like we do.
The Big Toe That Stuck Out:
But then we get to the feet. In particular, the big toes. They were abducted, meaning they stuck out, like a chimp’s big toes or our own thumbs. If you’ve ever jammed your pinky toe on a coffee table, imagine what that would feel like if it were your big toe instead, and it would wrench far away from the rest of your foot. That’s a yikes.
The Forest That Never Dried:
Plus, it would make shoe shopping an absolute nightmare. And remember how we said where Ardi lived was important? It turns out that in Ethiopia, 4.4 million years ago, the environment actually hadn’t really dried out yet, and the researchers suggested that where Ardi lived was a dense forest. That means you wouldn’t need to walk on the ground very far, or maybe at all, to get from tree to tree, which is what we thought drove Australopithecus’s lineage to become full-time terrestrial bipeds in the first place. Basically, if you add this all up, Ardi couldn’t have walked exactly like us, or like chimps, or like anything else that’s around today.
A New Theory Takes Root:
This meant that researchers had to take a new approach to Ardi. They started with the basics: what movements would this hominin have been good at making? The hip says that they would be stable on one leg, but the foot is flexible and capable of grasping. The hands are strong enough to grip things, but not as useful for walking around on all fours.
Their final assessment was this: Ardi likely walked around on two legs in the trees, using those strong, grippy hands to hold onto branches above its head for extra support and stability. If it did need to go to the ground, it could have walked around bipedally, though probably not for as long or as well as humans can. All this suggests that at least some ancient human relatives were shifting to some form of bipedalism without the trees disappearing.
Evolution’s Garden Path:
Based on studying Ardi, scientists are starting to understand that the path to bipedalism isn’t as straightforward as we thought. Less of a highway, and more of a garden path. Maybe even one with several branches, since in 2012, scientists reported fossils from an unnamed species that may have come after Ardi, and been contemporary with Australopithecus, but which still had that grasping big toe. So there may have been a time when both walking and clambering humans lived on Earth.
The Lasting Questions About Ardi:
Of course, there are still some questions about Ardi. Like, while we know it’s part of our hominin family, we don’t really know if Ardi is our direct ancestor or more of an evolutionary cousin. And, of course, there’s debate around exactly how heavily forested Ardi’s home was. But overall, between Ardi and other forms of evidence, while the savannah hypothesis isn’t dead and is, in fact, still useful in a lot of ways, it’s become clear the story is a lot more complex than those early scientists supposed. Ardi’s story is fascinating because it highlights how, while we know a lot about human evolution at this point, there are still many mysteries to uncover. Decades of research were turned on its head, thanks in part to this one species and its funky toes.
Conclusion:
The story of how and why humans began walking on two legs is far from settled. For decades, we thought the answer lay in the drying savannahs of Africa, but fossils like Ardipithecus have completely rewritten that narrative. Ardi showed us that bipedalism might have started in the trees, not on the open plains, and that our journey to upright walking was gradual, messy, and full of unexpected twists. The truth is, we may never find a single cause for why we stand tall, but that mystery is part of what makes our evolution so fascinating. Every fossil, every bone, every odd big toe adds another clue to the story of what it means to be human.
FAQs:
1. Who was Ardi?
Ardi, short for Ardipithecus ramidus, was a 4.4-million-year-old early human relative found in Ethiopia.
2. Why is Ardi important?
Because her fossils challenged the old savannah theory and suggested bipedalism began in forested environments.
3. Did Ardi walk like humans do today?
Not exactly—Ardi could walk upright but still had grasping feet for climbing trees.
4. What was the Savannah hypothesis?
It was the idea that humans started walking on two legs after forests disappeared and open grasslands spread.
5. Was Ardi our direct ancestor?
Scientists aren’t sure—she could be a direct ancestor or a close evolutionary cousin.
6. Why don’t scientists agree on how bipedalism evolved?
Because the fossil record is incomplete and different species evolved walking abilities in unique ways.
