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 90’s, one fossil species  threw a major wrench in that idea. So let’s talk about Ardipithecus. [♪ INTRO] Anthropologists talk about bipedalism a lot, because it’s kind of a weird thing  for a species of mammal to do.

I mean, there’s not 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 you hold your 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. But when we think about why  all these things happened, we’re really asking about what changed not just in our ancestor’s  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 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. 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 1920’s. 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 that 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. 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. 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. A little hominin with a big, and problematic, toe… But before we get there, all research  needs funding, including ours. So here’s a quick ad break!

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Incogni works to keep your  private information private. And, ultimately, they help  you stay on the internet so you can keep finding  cool stuff like this video! 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. 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 on 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.

But then we get to the feet. In particular, the big toes. They were abducted, meaning they stuck out, like chimp 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 was your big toe instead, and it would wrench far away  from the rest of your foot. That’s a yikes. 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. 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. 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 where both walking and clambering humans lived on Earth.

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 was turned on its head, thanks in part to this one  species and its funky toes. [♪ OUTRO]