This is 'Early Man' a textbook from 1965. Inside is a very famous but kind of misleading image,
the march of progress, which illustrates the gradual evolution of humans from a knuckle
walking ape into our modern bipedal form. In a way this makes perfect sense because our
closest relatives chimpanzees and gorillas are both knuckle walkers. It's so easy to get this
human-centric view of evolution where we're the star of the show and to believe that any
changes between us and them are the result of human evolution, human development. Especially
because our bipedalism is really quite unique. There are no other mammals in the forest today
walking around on two legs like me, probably none whatsoever. There are barely any other mammals
that are bipedal and the mammals that are bipedal do it extremely differently to us they don't
necessarily have this upright posture. There's kangaroos, a little shrew thing, pangolins,
they're all bipedal in a very different way to us. All primates can do it a little bit
but we're the only ones doing it day in day out. Despite this, the more we understand about our
evolution, the more fossils we uncover, the more this idea that we evolved from a knuckle walking
ape is being questioned. Maybe the last common ancestor of humans and chimpanzees
wasn't a knuckle walker but already partially bipedal just in the trees, maybe knuckle walking
is the evolutionary innovation. Looking at three fossils in particular let's see what we can learn
about the last common ancestor of humans and chimpanzees our extremely ancient ancestor.
Unraveling the behaviour of the last common ancestor of humans
and chimpanzees is uhhhh it's really tough to say the least.
As scientists we work with physical evidence, right, we work with the fossils
that we discover and there are very few fossils from the time period when we think humans and
chimpanzees last share a common ancestor. We can estimate using genetics that they last share a
common ancestor somewhere in the ballpark of about five to seven million years ago,
others have it closer to six to eight million, in the book i talk about six million years ago,
when the split is complete and you have a lineage leading to humans uh and a lineage that eventually
leads to chimpanzees and bonobos. Okay so if we want to know what was going on at that
crucial time period, at the dawn of of bipedal locomotion, we need fossils and the number of
early human fossils that we have from that time period um could fit in a grocery bag.
That's professor Jeremy DeSilva, author of 'First Steps: How Upright Walking Made Us Human' a great
book available now at all good retailers. As you just heard him say, unfortunately at the
moment we don't have many fossils from that key time period when this ancestor lived,
say five to eight million years ago, but we do have some and we do have fossils that sandwich
that time period which can obviously inform us about the behaviour of apes at that time.
Ardi, short for Ardipithecus ramidus, which means root ape in the Afar language,
was found in Northern Ethiopia in 1994 by professor Johannes
Haile Selassie and dates to around 4.4 million years ago. So this is about one to two million
years after the last common ancestor and one million years before the very famous Lucy,
who I made an absolutely banging video on by the way. As we travel further and further back in time
towards this last common ancestor, you would expect fossils to exhibit a mosaic of features
some more ape-like some more human, some more hominin and that's exactly what we see in Ardi.
What's interesting about Ardipithecus is that um the pelvis is shaped in a manner that would
support um a bipedal locomotion, it's short and squat and the positions of the muscles
and the hip joint would allow it to balance on a single leg when it was walking around on two legs.
Ummm and I've, I've studied the foot, I've looked at the foot very carefully and I think
their interpretation of the foot is more or less correct, that the outside of the foot,
it's really interesting, the outside of the foot looks a lot like yours and mine
and the inside of the foot looks like a chimpanzees,
and so the grasping ability of the inside of the foot is retained but the stiff
outside of the foot would allow it to push off the ground and propel itself into its next step.
This mosaic of features is exactly what we would expect from an animal
closer in time to the last common ancestor but if that ancestor was a knuckle walker then certain
bones are really thick, really big, with a lot of attachments for cartilage. This is
because this bone is required to almost constantly support their body weight when knuckle walking and
support their weight when hanging from a branch. Gorillas also have really large metacarpal bones
for the same reason. When we compare Ardi's metacarpal to these two bones, it's obviously not
as robust as a chimpanzees and that's not because Ardi was smaller than a chimp. They probably
weighed around 50 kilograms and stood at about 120 centimeters tall, which is really comparable to
a chimpanzee they're basically the same size. So, this does not appear to be the finger of an animal
that knuckle walked in any way, or of an animal that was recently descended from a knuckle walker.
Quick obligatory nuance here, there are of course people who disagree. When you're trying
to reconstruct the behaviour of an extinct animal from fragmentary remains of a different extinct
animal that lived a million years later, it would be crazy if there weren't disagreements. But
it is entirely reasonable to believe that Ardi was no descendant of a knuckle walker.
As we go deeper in time, if we evolved from something that was very chimpanzee like
then we should be finding features that are more and more and more and more like a chimpanzee, and
so with Ardipithecus ramidus, here we are going farther back in time to that common ancestor,
and they didn't find necessarily what they expected. It wasn't chimpanzee-like it all.
This is Udo found in 2019 by professor Madelaine Böhme,
and was given the species name Danuvius guggenmosi. Udo by the way isn't short
for anything but is named after a german rock star Udo Lindenberg who, extremely
conveniently for this video, is conducting a choir of chimpanzees. As you can probably
tell from their name Danuvius and the fact that they're named after a german rock star, Udo did
not live in Africa. 11.6 million years ago when they were alive, the world was quite different.
[Stefan] What did the world look like 6 to 10 million
years ago? What was the environment that these apes were living in,
and and where were they living because I think that's not what people might be expecting.
[Jeremy] This time period is known as the Miocene, the late miocene, six to ten
uh million years ago or so. It's, you know, the earth itself is not too different than it is
today in terms of the continental positions and things like that. There are subtle differences uh
in in the size and shape of say the mediterranean. The mediterranean actually um is what we call
at the time that the the Tethys Sea or the Paratethys sea, and around the mediterranean
uh it was quite forested. Throughout Southern Europe um and into Africa and that's where we find
fossil apes. In Spain, and France and Greece and Italy and Germany and Turkey, Hungary uh and
so it was a very different world in which all of these different um ape experiments were going on.
Udo probably would have felt really at home where I am today in the woods, wet, warm, lots of trees,
but what is it about their anatomy that can give us clues to the origins of our bipedalism?
The first is that the set to the ankle joint, right at the bottom, as I put my my pen there,
it's perfectly perpendicular. So it forms a right angle. If this was an ape,
it would be angled it'd be angled that way, and it's because their feet twist inwards and
that helps them grab onto their the branches they're climbing. So they have what's called
an inverted foot or twisted in foot and that helps them clim. Whereas humans have a foot where
the bottom of our foot contacts the ground, we have an arch too, but uh the ball the foot
and the heel are more or less in line with the ground and then our shin bone comes straight up,
and that's what you see here with that angle. And the other thing is that the knee joint,
the knee joint in apes is very curved, it's very convex and it's because there's all
this mobility that happens at the knee. Humans have very flat knee joints and so does this,
now i'm not saying that that this is a homonin, it's not in our lineage it's way too old for that,
this is telling us about function not about relatedness. And so those two anatomies there,
these little clues um that this thing was behaving in a slightly different way
from a modern ape, and actually aligns it a little bit more with what we see in these early hominins.
You can probably imagine Udo behaving in a similar way to a gibbon perhaps, an animal certainly
predisposed to life in the trees but when it absolutely had to get on the ground,
it didn't crawl on its knuckles, it scurried about on its two little legs. So we have Ardi, roughly
1.5 million years after the last common ancestor, which seemingly was not a descendant of a knuckle
walker. Udo before the last common ancestor with adaptations for some sort of arboreal bipedalism.
What about that shopping bag-sized pile of remains from the time of the last common ancestor?
So if we say okay humans and chimps share this common ancestor six or
seven million years ago and we're imagining this common ancestor, well we have fossils,
and so here's a here's a fossil from seven million years ago and this is Sahalanthapus tchadensis.
Sahalanthopus was found in the Central African country of Chad in 2001 and published in 2002
and again this is this is the time period when the common ancestor existed. Well without genetics we
can never really know if this is the common ancestor, right? Um but but it's gonna be as
close to it as anything we've ever found. Okay, so what do we know about it? Is it a chimp? No
it's not a chimp at all. It's face is more gorilla like. It's got a big brow ridge like a gorilla,
its skull uh is uh is chimpanzee like in the size of the brain, in some aspects of the shape of the
skull, the back of the skull is gorilla-like, but the hole at the bottom where the neck comes out
is positioned like it is in you and I, and so it's more human-like and so that would
suggest that this thing was quite upright. And recently the team that found this skull
has produced a pre-publication of two arm bones, two ulna, and a femur that they found at the site.
And the femur, according to the team, has hints of of bipedal adaptations in it um and the ulna
don't show the same adaptations you would expect for a knuckle walker. So again this
this body pattern that's emerging of something that is is upright in the trees maybe, and then
coming down to the ground and moving on two legs, exactly when the common ancestor is, is existing.
One of the most significant implications of this idea is that knuckle walking must have evolved
twice. First when our lineage split with gorillas, then again when we split with chimpanzees, and in
fact there is some evidence to support that idea. Chimps and gorillas don't knuckle walk in the same
way, they have a slightly different wrist posture. So that could perhaps be a sign that it evolved
independently. In many ways these three specimens have flipped the march of progress on its head.
Instead of this downward progression, our ancestors could have already been
pretty bipedal and it's chimpanzees and gorillas that have been on this downward progression.
And so now when we're trying to understand why did bipedalism evolve, we don't necessarily have
to think about how did a knuckle walker turn into a biped. How did it go through these intermediate
stages where it's kind of this clumsy, awkward, bent hip, bent knee, wobbling thing that can't,
that can't move with great efficiency. And so the idea that that would be an adaptive way to move
for an animal that already is slow, um seems kind of bizarre, and so this makes much more
sense to me that the common ancestor would be already predisposed to this kind of locomotion
and then as environmental changes are occurring, and you're getting the spreading of grasslands
and the patchiness of those forests, now getting from point a to point b means moving on two legs,
and you were already doing that just in the trees. And so the question now
for a paleoanthropology, is not why did bipedalism evolve it's why did knuckle walking evolve?
Even though we can learn so much from studying our cousins,
these three fossils are a brilliant reminder that we can't simply view them as time machines.
They've been on their own evolutionary journey for the past 10 million years or so, just like us,
and it's entirely possible that our ancestors were never knuckle walkers. Now i'm going to walk to
the camera to finish this video because I don't know what else to do and also we could find more
fossils that could totally contradict this finding and we shouldn't be surprised if that happens.