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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.
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