Imagine yourself in Tanzania, in eastern Africa, more than three and a half million years ago.

A strange, unfamiliar creature walks past you -- on two legs -- across a bed of wet

volcanic ash, leaving footprints behind.

It's small, with an ape-like face and long arms.

But the tracks it leaves are unmistakably human-like, a lot like the prints you'd make

in the wet sand on a beach.

Now, fast forward to 1978.

Thanks to a lucky confluence of events, those footprints have fossilized, at a site called

Laetoli.

And when they're discovered by scientists, they revolutionize the way we think about

the history of our species.

The researchers who found these tracks, led by anthropologist Mary Leakey, dated them

to 3.6 million years ago, which was important, because it contradicted a longstanding idea

about human evolution.

The traditional view of our evolutionary history was that we got smart before we stood up.

In other words, the first thing that made us human was thought to be evolving a large

brain, with the rest of our human-like features, like walking on two legs, coming later.

But judging by the presence of their fossils in that same sediment layer, those tracks

were probably made by a species of human ancestor called Australopithecus afarensis.

And it had a really small brain, about the size of a chimpanzee's.

So, these footprints were evidence that, maybe what first distinguished our lineage wasn't

being brainy.

Maybe it was walking upright.

But if Australopithecus was already striding across the landscape 3.6 million years ago,

who started them on that path?

What species pioneered this style of locomotion?

Who was the first to walk?

To get to the origins of how we came to walk on two feet, you first have to be able to

recognize the evolutionary hallmarks of bipedalism in your own body.

And they can be found literally from head to toe!

Think about your skeletal anatomy: Your head is balanced on top of your spine, with the

spinal cord connecting to the brainstem at the base of your skull, rather than at the

back.

Your spine has a series of curves in it that position your torso above your hips.

Your pelvis is shaped kind of like a bowl, with your hip bones curving around your sides.

Your thigh bones, meanwhile, angle inward from your hips, putting your knees closer

to the midline of your body.

And your feet are very different from those of your living ape cousins.

You've got a big heel bone called a calcaneus, your feet have arches in them, and you've

got short toes with a big toe, or hallux, that's in line with the rest of them.

You're definitely not going to be grasping any tree branches with it.

All of these features are adaptations to bipedalism, especially those in your pelvis and your feet.

They allow you to get around efficiently, using less energy than a chimp does walking

on all fours.

But there isn't a single species of human ancestor that suddenly appears with all of

these features.

And fossils are often fragmentary.

So anthropologists are still studying what this transition looked like and which features

likely appeared first.

Adding to the puzzle, it can be hard to tell whether a species still climbed trees but

could walk on two feet if it needed to -- or if it was fully committed to life on the ground.

Now that we know what we're looking for, we can ask again: who was the first human ancestor

to walk upright?

Well, if we walk back in time from Australopithecus afarensis at Laetoli, the next possible contender

for that title is Australopithecus anamensis

It lived in Eastern Africa between 4.2 and 3.9 million years ago.

And there are two leg bones from this species that tell paleoanthropologists that it was

a biped.

One of these is a partial femur, or thigh bone.

It looks a lot like the femur of Australopithecus afarensis, but it's bigger.

And because we have a lot more of the skeleton of Australopithecus afarensis, we can tell

that it walked bipedally, so we're pretty sure that means that the femur of anamensis

also came from a biped.

Even better evidence for bipedalism in this species comes from the shin bone, the tibia.

In 1994, paleoanthropologists excavating a site in northern Kenya found both the upper

and lower ends of a tibia, the parts that form the knee and ankle joints.

They then compared these bones to those of humans and chimpanzees.

Because, in humans, the tibia comes straight up from the ankle, better for standing upright.

But in chimps, the tibia slants diagonally outward, which suits their preferred mode

of locomotion, called knucklewalking.

And the bones from Kenya turned out to be straight, like a human's.

So experts think Australopithecus anamensis was probably a biped, too.

Now, let's take yet another step back in time, to 4.4 mya in Ethiopia.

Enter another ancient human ancestor, Ardipithecus ramidus.

This species is best known from a partial female skeleton that got a lot of buzz in

2009; she was nicknamed “Ardi.”

Ardi was small, just under 1.2 meters tall, and, based on the fossils of animals found

around her, she lived in a wooded environment.

And her skeleton told a surprising story.

Ardi had an opposable big toe, like modern apes have, but the rest of the preserved bones

of her foot suggest that her foot didn't work like a grasping hand, as it does in living

apes.

Instead, her foot seems to have acted like a rigid lever, propelling her forward the

way that feet of humans and Old World monkeys do.

Along with her strange feet, Ardi's skeleton also included a pelvis – and this part of

her anatomy had another unique combination of features.

The top of Ardi's pelvis looked a bit like that of Australopithecus afarensis – short

and broad, with the beginnings of a bowl shape.

But, the bottom of Ardi's pelvis -- the bones you're probably sitting on right now -- resembled

the pelvis of a climbing ape, like a chimp, with an angled muscle-attachment point for

powerful hamstrings!

So what did this mosaic of traits mean for Ardi?

Was she a climber or a biped?

Her discoverers think that she was both – an adept climber in the trees, but also an effective

biped on the ground.

It'll probably take finding a more complete, less fragmentary pelvis to tell for sure.

Now, when you look at our ancestors before Ardi, the picture starts to get murkier.

But there are three other species that some paleoanthropologists think might have been

bipeds.

And, given their ages, and that bipedalism is traditionally considered one of the defining

traits of our lineage, these three creatures are also contenders for the title of the first

known hominin.

A hominin is anything that's more closely related to us than it is to chimpanzees.

So, this includes us and all of our extinct relatives that existed after the last common

ancestor of chimps and humans.

Fewer fossils of these three potential first hominins have been found, so paleoanthropologists

know less about their anatomy and are less certain about whether they were bipeds.

The first of these is an older species of Ardipithecus called Ardipithecus kadabba.

It was found in the Middle Awash region of Ethiopia and dates to between 5.8 and 5.2

million years ago.

Evidence that it might have been a biped comes from the shape of the joint of one of the

bones of its big toe… and that's it.

The second species is called Orrorin tugenensis.

Discovered in the Tugen Hills in central Kenya, it lived around 6 million years ago.

There are 13 fossils of this species, and two of them are partial femora, or thigh bones.

And the latest research on these bones has found that their shape was part-way between

that of later australopithecines and those of earlier apes.

So, some experts think Orrorin was a biped, but that it didn't walk quite like we do

– it might have had its own kind of swagger.

Finally, the oldest contender for the earliest biped is Sahelanthropus tchadensis.

Its partial skull was excavated in Chad and dated to around 7 million years ago.

And its discoverers think that it was bipedal based on the position of the hole at the base

of the skull, where the spinal cord connects to the brain.

Because, remember, in bipeds, this hole is underneath the skull, rather than on the back.

But there's still some debate about whether this is good proof of bipedalism or not, and

no other fossils of this species have been found.

So who was the first biped?

Most paleoanthropologists would agree that Australopithecus anamensis is currently the

the safest bet.

But others would be willing to go with Ardi or Orrorin.

It all comes down to how neat the transition from tree climber to upright walker was – because

it didn't happen all at once.

But this raises another really important question: Why did bipedalism happen at all?

For a long time, the go-to explanation for why we became bipeds was the known as the

“savannah hypothesis.”

It proposed that changes in Africa's climate caused forests to shrink and grasslands to

grow, and moving between the remaining patches of trees was easier on two legs.

This idea spawned a number of spin-offs theories: like, that walking upright exposed less of

our bodies to the sun than walking on all fours did, making it easier for us to regulate

our body temperature;

And that freeing up our hands to carry things was key to our development, which is what

Charles Darwin thought.

In fact, chimpanzees do sometimes walk on two legs when they need to carry something,

like highly-prized foods.

Also, walking on two legs is more energetically efficient than walking on four, when you're

a primate on the ground.

But wait – remember when I said that Ardi was found in a wooded environment?

That throws a bit of a wrench into the savannah hypothesis.

The geochemistry of the Orrorin site also suggests that it lived in a forested area.

So, maybe early hominins didn't have to leave the trees to become bipedal.

Maybe life in the trees was actually pushing them in that direction.

In contrast to the savannah hypothesis, some researchers have suggested that our upright

body plan comes from there being an advantage to standing on branches while feeding.

This idea is based largely on the behavior of some living apes that stand on branches

to feed.

Also since finding a lot more fossil apes – we see that many of their skeletons indicate

they held their torsos upright.

So, we don't know for sure why bipedalism evolved.

Answering these kinds of questions about the deep past is always difficult.

In this case, it's made even harder by that fact that we're totally unique in this type

of locomotion among mammals – we have a living sample size of one.

What we can say is that the earliest potential hominins, one of whom might've been our

direct ancestor, had some bipedal features.

And it could probably move around on the ground and in the trees in a lot of different ways

– a lot like apes today.

To truly solve this puzzle, all we can do to keep looking for more fossils and more

footprints.

Even Laetoli still has more to tell us.

Researchers digging in 2015 found the tracks of two more individuals, moving in the same

direction and at the same speed as the ones Mary Leakey's team found in 1978.

So with more study, and more time, we may yet understand how that fascinating, human-like

creature managed to walk past you on an African bed of ash, more than 3 million years ago.

Now, I want to thank CuriosityStream for supporting PBS Digital Studios.

With CuriosityStream you can stream documentary films, and programs about science, nature,

and history, including CuriosityStream originals!

For example, there's Rapidly Evolving Human, an exploration into how changes in our genetic

code have made us who we are, and how our genome continues to change!

CuriosityStream offers unlimited streaming, and for you Eons viewers, you can check it

out at curiositystream.com/eons and use the promo code EONS.

If you're following us on social media you already know this, but we have something new

for you!!

Check out our brand-new Eons sticker, available at DFTBA.com.

Just click the link in the description.

And if you're a fan of bonus content, join the Eonites over at patreon.com/eons!

We recently posted the full clip from my Ashfall internship over there - a few seconds of which

made it into the Yellowstone supervolcano episode.

We want to thank all of our patrons, and we want to especially thank our Eontologists:

David Reed Rasmussen, Steve, Jon Ivy, and Eric Lawrence.

And as always, let us know what you want to learn about!

And don't forget to go to YouTube.com/eons and subscribe!