This video is brought to you by supporters on Patreon.
This is an x ray of a human hand.
It should look pretty familiar, some phalanges, 5 metacarpals, and exactly 8 carpal bones
or wrist bones.
But why 8?
Why not 5, or 10, or even just one bone?
After all, these few bones make the main joint with our radius to make our wrist, so why
not just fuse everything together?
My background is in kinesiology, so I know how the modern wrist moves, not how we got
the modern wrist.
On the other hand (hah, pun!) evolutionary biologists and anthropologists have a good
idea of how wrists evolved since they were so important to how our ancestors used tools,
and thus, how we survived as a species.
Going into this video, I thought I could read a handful of papers and get the gist of this
topic and make a quick video.
Ohh how wrong I was.
It might not seem like it at first glance, but our carpals are different from our ape
cousins.
Like this is the human pisiform bone, and this is the gorilla pisiform.
Little different, right?
As I continued researching, I found that each of these eight bones evolved independently
from one another and each has their own story.
So grab the peanut butter filled pretzels, this is gonna be a long video.
Skeletons started in the oceans around 500 million years ago.
At first, it was a couple bony fishes, then came tetrapods, or animals with four limbs.
We think that the first version of a tetrapod limb happened around 370 million years ago.
And because it was so fresh and so clean, these first limbs didn’t have a wrist or
a hoof or paw yet for that matter.
The carpal bones themselves, didn’t start evolving until tetrapods got digits, or what
we would call fingers and toes.
Those first digits were probably evolved radial bones like ray finned fish have today.
Tetrapods kept evolving in the ocean until they came on land which led to some big changes
in their body plan.
Like early aquatic tetrapods propelled themselves with their front limbs, while tetrapods on
land generate force with their hind limbs, which is why butt muscles and pelvises became
such a big deal.
As one of the papers I read put it, tetrapods had to evolve from “front wheel drive”
to “rear wheel drive”.
Which is just an adorable metaphor, ya know.
But that’s why we can recognize a lot of the same bones in horses, chinchillas, or
penguins is because they all came from this early tetrapod ancestor.
And all modern tetrapods have something between the digits and distal limb.
From there, the story of the wrist goes on some serious serious tangents and side quests,
with early tetrapods evolving six, seven, or eight fingers.
But for the most part after this body plan happened, the definition of wrist bones would
be solid — they were some kind of short bone interface between the long bones of the
forearm and fingers.
With a few modern exceptions: namely crocodilians and frogs and toads.
Their proximal row of carpal bones are long bones not short bones.
And at this point in evolution, we’re working with the traditional tetrapod skeletal plan
and we can start following homologs of specific anatomy.
These are anatomical features that start with a common ancestor but get reshaped by evolution
— just like we can follow the evolution of like fish across time, we can also follow
the evolution of specific bones.
Homologs can be developmental patterns and genetic similarities too.
But comparative anatomists base their study on anatomical homologs, which is what a lot
of the studies from my research used.
For instance, we can clearly see the radius and ulna bones as homologs in all kinds of
animals.
But sometimes we need to use multiple clues to show that one bone or muscle or whatever
is homologous to some other animal’s anatomy.
For example, us humans have muscles called interossei muscles between our fingers while
other primates have two muscles around that area.
So which muscle is the interossei muscle a homolog to?
We don’t have a solid answer.
Maybe it’s a homolog to one, but not the other, or maybe sometime in evolution, they
fused into one muscle for our hands.
And while I could spend hours going down rabbit holes of mammal wrist homologs, no literally,
this is a rabbit wrist, scientists are interested in the homologs of primates.
If you saw my video about human spine evolution, then you’ll remember that a huge factor
in anatomy is locomotion — how an animal moves.
And in general, we can split primates into four locomotion groups: quadrupeds which include
knuckle walkers like gorillas, tree swingers like gibbons, leapers like tarsiers, and bipeds
which are mostly humans, but other primates can temporarily walk on two legs too.
Since locomotion style is such a big deal in anatomy, we’d expect knuckle walkers
to have similar wrists to knuckle walkers, and tree climbers to have similar wrists to
tree climbers.
But it’s not always that simple.
Some animals happen to evolve the same characteristic despite having different movement styles.
So in 2013, research published in BMC Evolutionary Biology described this big comparative morphology
study which brought in homo sapiens, apes, old world monkeys, new world monkeys, and
a few fossilized hominins all of which have different locomotion styles.
Some walk on two legs, some grab onto branches, some walk on their knuckles, and some, as
the paper put it, use “ricochetal brachiation” which is the science word for Swinging.
They could’ve literally said monkey bars and they didn’t.
And it turned out that our primate family members evolved a bunch of anatomy in parallel
with each other.
The study also pointed out that certain pieces of anatomy, including different carpal bones
evolve faster or slower than other pieces.
And they evolve independent of each other.
Yes, they all make up the same joint and kind of act as a module, but evolution of each
bone is different.
This is mosaicism — you can have anatomy change completely separately from other anatomy.
Some bones were heavily influenced by locomotion.
In particular the capitate, triquetrum, and scaphoid.
So you would expect those three bones to look different when you compare tree swingers to
knuckle walkers to full time bipeds like us because our locomotion styles are so different.
But others like the lunate and hamate, were influenced more by that animal’s clade than
how they moved.
So for those bones, your locomotion style was less important — it was your place in
the family tree that determined what these bones looked like.
Like gibbons.
These are the classic comparison monkeys for a tree swinging primate, so we’d expect
their hands to be built for it.
And their hamate bones are much longer and have a little hook called a hamulus, which
is an advantage for swinging through trees.
Compare that to spider monkeys — which are still tree swingers, but they don’t have
the cool hamate hooks.
Other studies have shown that these two species have pretty similar shoulders and elbows,
but their wrists are the big morphological difference.
So what about some of these other bones?
Well, it’s complicated.
Like orangutans and gibbons have really similar triquetrum and capitate bones even though
they don’t move around the same way — gibbons are tree swingers while orangutans kind of
scramble.
But... these styles of locomotion favor wrist mobility over wrist stability, so their similar
carpal bones match up from that perspective.
On the flipside, knuckle walkers favor a strong, stable wrist.
So they’re the opposite.
If we wanted to summarize things up until this point, we can explain some of the evolutionary
journey this way: knuckle walking wrists prioritize stability while tree dwelling wrists prioritize
mobility.
Which brings us to humans, almost.
Homo sapiens share a recent common ancestor with chimps and bonobos.
If we want to tell the story of our wrists, we need to ask how this ancestor moved around.
Were they knuckle walkers, tree climbers — what was their deal?
According to the most recent review that I could find, the best hypothesis that experts
have is that we likely evolved from knuckle walkers, but there’s conflicting evidence.
Hominoids, or apes, are special.
Of all the primate clades, there’s a ton of carpal bone variation among apes.
Especially the hamate, triquetrum, lunate, and capitate.
This suggests that the apes are under increased selective pressure compared to other species,
at least when it comes to wrists.
For example, our homo sapien triquetrum looks more like gorillas and chimps than neanderthals’,
which is weird, since our locomotion style and tools usage is closer to neanderthals
than gorillas.
According to the BMC paper, this suggests that we evolved similar triquetrum bones at
the same time as chimps and gorillas.
Maybe we evolved these beefy triquetrum because tool use put extra stress on that side of
our wrists like how gorillas put weight on their knuckles, but we haven’t really been
able to test that.
And then there’s the scaphoid, which is the research topic that ultimately nosedived
my descent into madness for this script.
And it’s because of this thing, the os carpi centrale — a free floating bone near the
intersection of the trapezoid, scaphoid, and capitate bones.
Humans and gorillas are usually born with this bone fused to their scaphoid — sometimes
it fused to the trapezoid or capitate, but almost always the scaphoid.
The most popular thought is that when os centrale is fused to the scaphoid, it makes the wrist
more rigid for knuckle walking.
And for modern humans, it might contribute to wrist stability.
The pisiform bone is a little different.
This little dude hangs on the ulnar side of the wrist.
Apes have longer pisiforms which gives them some extra torque when swinging or knuckle
walking.
Ours is short and stubby while gorillas and chimps have growth plates in theirs.
So /our/ pisiform is smaller than all of our evolutionary cousins, but our calcaneus is
wider and thicker than other apes.
Now, why am I mentioning a bone in the foot all of a sudden?
The pisiform and calcaneus are paralogs of each other — two different structures that
evolved in parallel.
Basically, the calcaneus is the foot version of the pisiform and the pisiform is the hand
version of the calcaneus.
A paper published in EvoDevo in 2019 described how they’re the only carpal and tarsal that
have two ossification centers and a growth plate.
If you’ve never heard of evo devo, or evolutionary developmental biology before, it’s basically
when scientists learn things about evolution by studying development.
So if we look at how these bones develop in utero, the pisiform is the hand version of
the heel bone.
That tells us that as we evolved bipedalism, the pisiform kind of shrank and lost a growth
plate over time while the calcaneus got bigger and kept its ossification center and growth
plate.
Now, around the time our ancestors got out of the trees and started walking on two legs
is when we start seeing a record of this big new heel bone, but the not the pisiform yet.
That gives us the idea that the calcaneus evolved in relation to walking upright while
the pisiform was an adaptation to tool use.
And tool use wouldn’t just shape the pisiform — it was such a big deal that it would reshape
the rest of the hand.
I had one lingering question while researching all of this.
Is tool use /that/ powerful of a driver of evolution?
Like does it impact survival or reproduction so much that evolution will select wrists
that favor complex tool usage?
There are two hypotheses that go together for this that say
Humans can move their hands and grip tools in totally unique ways, and if that’s true,
we’ll have evolved unique carpal anatomy as well.
Basically, it’s this loop of form influencing function influencing form and so on.
To that end, humans do have pieces of unique anatomy that lets us do unique things with
our hands:
Hand proportion and robust thumbs — we have short fingers relative to our thumbs, especially
when you compare them to chimps and gorillas and those thumbs are surrounded by muscles.
Including a unique mechanical advantage for the opponens pollicis muscle, the muscle that
lets do opposition of the thumb.
We have some differently shaped carpals bones compared to other apes.
Like we have larger trapezium bones to accommodate all the compressive force from squeezing stuff.
Plus, the carpometacarpal joints are oriented in a way that allows humans to cup their hands,
making it possible to do the baseball grip our digits have big bumps or tuberosities
on the far end of our relatively flat fingers.
Finally, we have such strange wrist mobility — we can do all kinds of weird combined,
double axis movements.
And that shows up in the things that /we/ can do with our hands that other primates
can’t.
Like chimps straight up can’t do certain precision grips, this style of grip, while
baboons and capuchins can precision grip but not very forcefully.
This is why capuchins pound open nuts with rocks.
It’s adorable.
So far, macaques are the closest at having a strong and precise grip, but scientists
are figuring out how that became a thing.
Now, humans have all kinds of unique gripping styles.
Key grip lets us hold a flake and use it to cut things.
The baseball grip lets us shape tools precisely, and the hammer grip lets us use tools to smash
or hammer things.
Then there’s dart thrower’s motion which is unique to humans, so according to this
hypothesis, our wrists evolved in a way that allowed that motion.
How that became the wrist of modern day humans is still a mystery, but a report from 2010
tried.
They gave it the amazing title “The advantage of throwing the first stone”.
[Bible references!
Hey ohh!]
But there are still tons of unanswered questions in this story.
We know what /we/ can do, but we want to figure out what our ancestors could do with their
hands, because we still don’t know.
My sources go back decades and so many of them end with “we’d still love to know
how our last common ancestor with chimps used their hands!”.
And speaking of a smooth transition to the Patreon plug, this script took over forty
hours to research and write.
I always post an annotated version of my script a few days early for Patrons.
If you want early access to scripts and videos, and want to support your local neighborhood
anatomy teacher in the process, head over to Patreon.
You can also check out some of my other videos here and if you haven’t please subscribe
and hit the bell icon so you get notified when I post a new video.
I appreciate all of you who have watched this far.
Have fun, be good.
Thanks for watching.