July 30, 2025 • 28 mins
Is five a magic number? Why don't we have 3 fingers, or 7? Jorge gets the digits on this mystery with two evolution experts.
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Speaker 1 (00:01):
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
horhitch Ham, and today we're asking why do we have
five fingers and toes each hand or foot? Why not
three or four or six or seventeen? Who decided we
would have five digits in each hand? Now it turns
out there is a culprit which we'll get to at
(00:23):
the end. And to get there, we'll be talking to
two finger evolution experts who say there are three things
that explain the number of fingers in your hands, so
to recap count starting with your thumb will answer one question.
Talking to two experts learning three key facts, all under
fourty minutes, and the question is why do we have
(00:47):
five fingers? Enjoy? Hey everyone, Okay, here are some interesting facts.
Dogs and cats have five toes on the front legs,
but only four toes in each of their back legs.
Birds have three fingers on their wings and four toes
(01:08):
on their feet, Horses have one finger on each leg,
and cows and camels have two toes per leg. The
number of toes can even vary in the same type
of animal. Some salamanders have four front toes and five
back toes, and some have four toes all around. Clearly
there's a big variation in the number of fingers and
(01:30):
toes in the animal kingdom. But here's the weird thing.
There are no species of animals with six or more
fingers or toes on their limbs. Why is that it's five?
Somehow the maximum? What determines how many fingers each animal has?
And why do we have five fingers and toes? To
(01:51):
figure this out, I talk to two evolutionary biologists who
specialize in finger evolution. The first one is Jacob Scott,
a researcher at the University of Florida. Well, thank you
so much, Jacob for joining us.
Speaker 2 (02:07):
You are welcome, glad to be here.
Speaker 1 (02:09):
So my first question for you is how many fingers
am I holding up?
Speaker 2 (02:13):
Five?
Speaker 1 (02:15):
Good? Although you can't see my other hands, it was
kind of a trick question.
Speaker 2 (02:20):
That's true.
Speaker 1 (02:22):
Well, to get us started, I was wondering if you
could tell us where did fingers and toes come from?
Speaker 2 (02:28):
Yeah, so all vertebrates with limbs that live on land,
they ultimately evolved from something much more fish like or
a fish itself.
Speaker 1 (02:38):
Okay, so a quick refresher here about evolution. All vertebrates
with limbs meaning all animals with bones and arms and legs,
which include all mammals like us, dogs and horses, and
awesome birds, reptiles, and amphibians all came from a common
ancestor which evolved from fish and which had fingers on
(03:00):
its feet. But here's the shocker. That animal probably had
more than five fingers on each limb.
Speaker 2 (03:09):
They didn't have five fingers, they had somewhere between six
and eight. We've got fossil showing some of these like
sort of fish like tetrapods with six digits on their limbs,
with seven digits on their limbs, and with eight, so
we think it was more than five.
Speaker 1 (03:23):
Originally what we used to have more than five fingers. Yes,
that's right, our long ago ancestor and the ancestor of
all mammals, reptiles, birds, amphibians, et cetera, had potentially up
to eight fingers on each limb. Okay, let's back up
a little bit. I asked Jacob to lay out the
(03:44):
history of all land animals starting with this common ancestor.
Speaker 2 (03:50):
It was a very just an ancestor that looks, you know,
a lot more like a fish salamander type creature, and
they evolved in these tide pools or swamps just otherwise
shallow water where they kind of have the opportunity to
be embracing that fish side of it, but then occasionally
maybe would push up out of the water or move
between different tide pools. So the first land animals were
(04:13):
fishes that started adapting to living on dry land. They
started to develop lungs, their chests and hip bones started
to shift so they could push themselves out of the
water and kind of waddle around, and that's when fingers
or digits came into the picture. The conventional thinking right
now is that digits helped kind of in that transition
(04:36):
moving from water to land or transitioning between different shallow
water areas.
Speaker 1 (04:41):
So fingers kind of came from fins.
Speaker 2 (04:44):
Yeah, Ultimately, ultimately it came from fins.
Speaker 1 (04:47):
That's a short answer, right, Okay, here, it's worth taking
a little dive into fins. Almost all fish that are
alive today have what are called ray fins, like a
bunch of spikes coming out of the fish's body with
webbing between them, sort of like those traditional folding fans
that come from Asia. But those are not the fins.
(05:10):
Our arms and legs and Brut's wings come from about
four hundred million years ago. There were other species of
fish with what are called lobe fins, and these look
more like paddles or stubby little tails on the side
of the fish's body. They extended out with little armlike
bony segments, and they ended on a group of bones
(05:33):
which fanned out to form fins. And they just happened
to have four of those fins, and those are where
our arms and legs come from. Eventually, those low fin
fish started to wattle out of the water and they
used those paddle like fins to push themselves around. Then
there's a little bit of a gap in the fossil record,
(05:55):
but about three hundred and sixty million years ago you
start to see animals like a Cantostega gunari, which looked
like a cross between a salamander and the fish, or Idiostega,
which looked like a cross between a crocodile and the fish.
But here's the thing. Fossils of Acantostega and Ikiostega show
(06:18):
they had more than five fingers on their limbs. Idiostega
had seven toes in its back leg, and a Cantostega
had eight fingers in its front legs, So it's very
likely that our ancestors had a lot more than five fingers.
I mean, imagine if we still had eight fingers on
each of our hands. It might look weird, but think
(06:39):
about how much faster we could type, or what kind
of masterpieces we could play on the piano. Our numbering
system would be different. Instead of base ten, which is
based on how many fingers we have right now, we
would all be using Hexa decimal numbers, which begs the
question what happened. Well, according to our expert, what happened
(07:02):
was digit loss. That is, during evolution, you can lose fingers,
and actually it happens a lot.
Speaker 2 (07:12):
It's actually really common to evolve digit loss, especially in mammals. Well,
I think the king of reducing your digits is the horse.
Horses have functionally just one digit on each foot. They
do have little bones from the palm, but they're fuse
to that one major digit. So if you look at
a horse leg, it sort of ends in just one digit,
(07:33):
which ends in a hoof. So they're basically standing on
just their middle finger.
Speaker 1 (07:38):
So whenever a horse rears up, it's giving you the
middle finger basically.
Speaker 2 (07:41):
Exactly, it's flipping you off. So, actually, horses are a
really cool example of this because we have an awesome
fossil record all through their digit loss. First they lost
their equivalent to their big toe. They lost it on
the hind limb first, and then you can see if
they would lose the pinky next, and then the pointer
finger and the ring finger were slowly just reduced in
(08:02):
length until they're basically nothing.
Speaker 1 (08:04):
Now, oh, we can see it in the fossil record. Yeah, yeah,
I had no idea. A horse's hoof is basically its
last remaining finger. Okay, as tacob to give me more
examples of animals losing fingers.
Speaker 2 (08:19):
I'll give you all the examples I can think of.
Most salamanders have lost at least one digit. Plenty of reptiles.
So modern birds they have three digits on their forelim
on their wing. The wing is the whole arm, and
then it ends in these three digits, and then four
digits on their hind limb. There's a big debate about
whether the identities of those digits on the wing are
(08:42):
some pointer middle or if they're pointer middle index.
Speaker 1 (08:46):
Wow, fascinating. So they're mixing it up three and four yeah.
Speaker 2 (08:50):
Yeah, So actually the digit loss tends to evolve more
or less independently between the forelam and the hind limb,
So there's plenty of species that only have digit loss
in one of the limbs and not the other. You'll
see different numbers in a lot of species, Like what
so dogs and cats they've both lost their big toe
on the hind limb. So if you go home and
(09:11):
look at your dog or your cat, you'll see that
they have, you know, full five digits on their front paws,
but only four on the back.
Speaker 1 (09:18):
Really, yeah, well, I've never thought to count dogs or
cats as fingers.
Speaker 2 (09:22):
It's something that I only like really noticed after I
started doing this research. I was like, oh, does my
dog also have you know, only four digits? And it
turns out yeah, he's a dog, so he will whoa.
And there's plenty of other examples, like guinea pigs have
four on their front legs and only three on the back.
Speaker 1 (09:41):
Are there any animals with two fingers or toes?
Speaker 2 (09:44):
Yes, So that's the group that includes things like cow
and bison and giraffes. So all three of those have
at least functionally two digits, And that's in comparison to
the horses, rhinos, and tape ears, which are odd toe animals,
so they'll always have one or three digits.
Speaker 1 (10:03):
It's like two big branches in evolution. Yep, exactly, all right,
So it seems like the common ancestor to all animals
with fingers started off with six to eight fingers in
their limbs. But now there are plenty of animals that
have much less than that we have five. Others have
evolved to have four, three, two, and even one finger
(10:28):
or toe per hand or foot. So my next question
was what causes this species to lose a finger or
a toe? All right, when we come back, we're gonna
put our finger on this mystery, which honestly sticks out
like a sore thumb. So stay with us. We'll be
right back. And we're back all right. We are counting
(10:59):
the number of fingers in our hand and asking why
do we have five fingers? Why not four or six?
In the last segment, we learned that the ancestor of
all four limb vertebrates, that is, the ancestor of mammals us, birds, reptiles,
and amphibians, lived about three hundred and sixty million years ago,
(11:22):
and fossils show it probably had six to eight fingers
or toes on each limb, but we have five fingers
and toes on each extremity. So what happened Well, according
to our expert scientists think one of the main reasons
is locomotion.
Speaker 2 (11:42):
For most animals, that's the primary use of their limbs
and their digits is getting around. Usually, the cause of
loss tends to be associated with the method of locomotion.
So a lot of animals that are really specialized for
running will lose digits. So think again, cats, horses, bison,
(12:02):
All of these animals that basically hunted, chase for food,
or try to get away from predators tend to have
reduced digits.
Speaker 1 (12:11):
Generally, why does losing your finger help you run?
Speaker 2 (12:15):
I can't give you a super confident answer on that,
but I can give you some associated evidence. One of
the thoughts on why horses were able to lose so
many digits was because they evolved that trait during an
ice age when the ground was more compact. If you're
trying to walk around on like marshy, swampy, muddy ground,
you kind of want more digits because you can disperse
(12:37):
your weight a little bit more evenly and not sink right, right,
But when the ground is harder, you can kind of
just slam your whole weight down on one point really
efficiently without the ground breaking around you.
Speaker 1 (12:48):
It's sort of like, I guess if you're walking on snow,
it's better to have a snowshoe.
Speaker 2 (12:52):
Yes, exactly, that's exactly right.
Speaker 1 (12:54):
But if you're running on a track like at the Olympics,
you don't want to be wearing snowshoes.
Speaker 2 (13:00):
That's a good way to think about it.
Speaker 1 (13:02):
You almost want sort of like a pogo stick on
your feet, right, Yeah, it's kind of what it is.
So one reason so many species of lost fingers or
toes is low commotion. Animals need to move and run fast,
and sometimes having a lot of toes can be a
bad thing. If you're trying to go as fast as
you can, think having a bunch of toes flapping around
(13:24):
when you hit the ground could actually slow you down.
I mean, think about why us humans wear shoes to
walk and run. It essentially reduces our number of toes
to one. And this idea doesn't just apply to feet
and toes, It can apply to fingers and hands.
Speaker 2 (13:43):
There's some animals like spider monkeys that they swing from
vine to vine. They've lost their thumb because it's more
efficient to swing just kind of using your other forge
just as a hook, when the thumb would just get
in the way there. So that's the current thinking at least.
Speaker 1 (13:58):
Yeah, there are monkeys out there without opposable thumbs. If
you look up spider monkeys, their hands don't have thumbs.
So there are lots of examples in nature of animal
species that have loss fingers or toes. Our ancestors started
with six to eight. At some point that became five,
and since then some have gone down to four, three, two,
(14:22):
and even one finger or toe. This means the history
of fingers and toes in animals. It's not that some
animals evolved two digits and others four or five from scratch.
We all started with eight and progressively lost fingers and
toes over millions of years. But there is something odd
about this pattern, which is that you rarely see a
(14:45):
species evolved to gain a finger or a toe. Now,
sometimes an individual animal of a species might be born
with an extra finger or toe, but that's not quite
the same.
Speaker 2 (15:00):
Like in labs, we can knock out genes and it
will cause mice to develop extra digits. We know that
there are human congenital diseases where people will be born
with extra digits, or there's certain dog breeds that tend
to have extra digits, there's certain populations of cats, But
in terms of like a whole species, nothing has evolved
more digits stably, like a brand new digit with its
(15:23):
own identity being formed. It's very, very very uncommon, like
to the point where I can't think of a single
example offhand.
Speaker 1 (15:32):
Okay, what Jacob is saying is that while you might
get a person or an individual animal here and there
to grow an extra finger or toe, it usually doesn't stick. Overall.
It's extremely rare for a species to gain a finger
or toe, and yet, as you heard before, losing a
toe or losing multiple toes is extremely common in evolution.
(15:56):
What this means is that in nature there are no
species of anim animals with more than five fingers or toes.
We started with six or eight when fishes first of
all to walk on land, and then there spent a
gradual decline overall species, our branch of the evolutionary tree
of life got down to five fingers or toes, and
(16:17):
at least for us stop there. But other species kept
losing more digits, and no species alive today hasn't gained any.
So this answer is part of the question of why
we have five fingers. We don't have six fingers because
our ancestors lost that six finger a long time ago,
and it's super rare for any animal to gain a digit,
(16:41):
so we're left with five. But this brings up two
very important questions. Number one, why is finger evolution so biased?
Meaning why is it easier to lose a finger during
evolution but so hard to gain one? And number two,
we need to answer the second half of today's question,
(17:01):
which is why do we have five fingers and toes
not four or three? I mean, when's the last time
you use your pinky toe for anything? We'll answer both
those questions when we come back, so don't twiddle your thumbs.
Stay with us, we'll be right back. Welcome back.
Speaker 2 (17:32):
All right.
Speaker 1 (17:32):
We're asking why we have five fingers, and so far
we've learned that the ancestor of all animals with fingers
and toes had somewhere between six and eight fingers on
each limb, and we learned that basically it's all been
downhill from there. The history of the evolution of digits
is dominated by animal species losing fingers and toes and
(17:57):
almost never gaining them.
Speaker 2 (18:00):
It seems to be a.
Speaker 1 (18:01):
Pretty prevalent rule of thumb unintended. So another question is
why is that? Why is it easier to lose a
finger than to gain one? And also why haven't we
lost more fingers or toes? I mean, it's not like
our pinkies or our pinky toes or any toe for
that matter, are that essential for survival. Why do we
(18:24):
still have them? The point of finger At the answer,
I reached out to another finger evolution expert, doctor Friedson Gallis. Well,
thank you Tolis for joining us. Can please tell us
who you are and what you do.
Speaker 3 (18:39):
I'm an evolutionary biologist and I work at the Naturalist
Biodiversity Center in Liden, Than, Netherlands, and I am interested
in how body plans can change, what kind of mechanisms
make it difficult for evolutionary changes to occur, like why
we have two eyes two years and.
Speaker 1 (19:01):
Things like that. Great, I think that makes you the
perfect expert for our question here today we're trying to
find out why finger evolution is so one directional.
Speaker 3 (19:13):
Yeah, Well, what you see is the losses of digits
happen very often, but there are never gains of digits.
And I can explain why that is if you want
to know that.
Speaker 1 (19:26):
Okay, Doctor Gallas says. The secret to understanding why it's
easier in evolution to lose a finger than to gain
one is to understand how fingers grow in the body,
especially when we're just tiny embryos in the womb during
what's called development.
Speaker 3 (19:46):
So the number of digits is determined very early during development.
You have a very small embryo, and in this embryo,
to get development, you need to have interactions between different parts.
And because the embryo is so small at the stage,
these interactions there are in the very interactive stage, and
(20:09):
all parts are in communication with all other parts.
Speaker 1 (20:15):
Okay, what doctor Gals is saying is that during development,
when we're just a tiny clump of generic cells multiplying,
the way the cells know what to turn into is
by talking to each other. There's a complex network of
signals between all the cells that basically tell each other
you should be where the arm is, and you should
(20:37):
be where the brain is going to be, and you
should be where the lungs are. And this keeps going
until all of the body parts form and the shapeless
blob of cells that you were starts to look more
like an embryo and eventually like a baby animal. It's
all a very complex, coordinated process, and that's when fingers
(20:59):
start to get formed.
Speaker 3 (21:03):
So this number of digits is determined, then the number
of eyes and the number of little structures are determined. Then,
because there are all these many interactions with all the
other parts, if you make a change, you get more changes.
So maybe what is easy to imagine is the eye
development is very early and there are many interactions with
(21:27):
their brain. Now, if you get an extra eye or
you have one less eye, that has incredible effect on
the brain development.
Speaker 1 (21:36):
What doctor Gallas means is that it's such a complex
coordination process when all your cells are trying to figure
out how to grow into you, that if you mess
with one thing, like say an eye, it's going to
mess up the process and all the other parts are
not going to grow the way they're supposed to grow.
Speaker 3 (21:56):
You get all kinds of monstrosities you have that that
are not viable. And then the same happens with digit
because the development of digits is connected with also all
kinds of other parts of the body. So if you
change the number of digits, you get other changes as well.
Speaker 1 (22:17):
Okay, so this explains why any mutation can be potentially
disastrous for any growing embryo. Suddenly growing, say an extra finger,
could mess up the complex signaling between the different parts
that are growing, and it could cause your lungs, or
your spinal column or your head to grow the wrong way.
(22:40):
But this limitation applies to both gaining a finger and
losing a finger. We still haven't explained why in the
evolution of fingers it's way more common to lose fingers
than to gain them. Well, according to doctor Gallis, the
reason for that is that there's a way to fool
your developing embryo.
Speaker 3 (23:01):
There's a way evolutionary way around it. What you get
is that you have the earliest development, and this needs
to happen because these interactions you need them. So what
you have is that you get your digits, then development
stoops after this stage, and then it gets destroyed.
Speaker 1 (23:22):
Okay, So the trick is that let's say you're a
horse that could run faster if you had fewer toes,
or a monkey that could swing easier with fewer fingers.
You don't want to not grow your fingers because that
would mess up the signaling in your embryo. Instead, you
grow all five fingers when you're a young embryo, and
once the signaling for that extra finger stops or becomes
(23:46):
less important, you stop growing that finger and let it
shrivel off. In other words, you sort of fake out
your embryo. You fool it into thinking it's going to
have all five fingers and toes, but then you drop
the ones you don't want, which makes for some interesting
surprises when the embryo is growing.
Speaker 3 (24:07):
Yeah, and for example, horses during development, they still have five.
Speaker 1 (24:11):
Digits really like a horse embryo has five toes exactly.
Speaker 3 (24:17):
And then what happens and then the development stops and
it's broken down.
Speaker 1 (24:22):
Huh and only one finger remains. Yes, And this is
why it's easier during evolution to lose a finger than
to gain one. It's easier to fool the embryo into
thinking it's getting a finger and then later stop growing it.
Then it is to fool it into thinking it's not
getting an extra finger, and then at the last minute
(24:44):
grow a whole finger out of the blue. Okay, so
this explains why we don't have six fingers. It's hard
to gain a finger as a species once you lose it.
Now the question is why don't we have four fingers?
If we're under the thumb of this bi towards loosing fingers,
how come we haven't lost anymore. I mean, it's not
(25:05):
like the pinky toe is that useful, or any of
our toes for that matter. And here, as I promise,
we have a very specific culprit, one that I've been
hinting at throughout the whole episode, and it has to
do with the order in which fingers grow when we're embryos.
They don't all grow at the same time. They start
(25:26):
one after the other, and it turns out that the
order in which they start to grow determines which one
is most likely to get lost. Here's Jacob Scott again.
Speaker 2 (25:39):
So it tends to be the first digit that gets
lost in evolution usually is the last digit to form
during development. So in mammals, digit one, so equivalent to
the thumb or the big toe, that's the last one
to form, and it's usually the first one to be lost.
Speaker 1 (26:00):
This makes sense. Remember I said that to lose a finger,
you kind of have to fool the embryo so you
don't mess up the coordination between all the different parts. Well,
it's easier to do this if you're the last finger
to form, because being the last one you sort of
fly under the radar. But the time you come around,
(26:20):
things are more settled. And in our case, the last
digit to form is the thumb.
Speaker 2 (26:29):
I would say in humans, the digit were most likely
to evolve the loss of would be our thumb, and
we really need our songs.
Speaker 1 (26:38):
I see, I see. It's like, evolutionarily, the thumb would
be the first to go, but because it's so useful,
it's like, uh, you know, it's not going anywhere, right, Oh,
that's fascinating. Even though the pinky is maybe the least
useful finger. It's like, way in the back of the line,
it's like the thumb is running cover for the pinky.
Speaker 2 (26:58):
Yeah, so we're we're stuck with the pinkies.
Speaker 1 (27:03):
And there you have it. The reason we have five
fingers and not four is the thumb. Evolutionarily, the thumb
would be the next to go, but it's so useful
we can't get rid of it. And because we can't
get rid of it, it ends up protecting all the
other fingers. In other words, we can't lose the pinky
(27:25):
without first losing the thumb, which for us is too
useful to lose. And if you go back through the episode,
you'll notice I was hinting at this from the start.
You can cut the number of times I used the
word thumb with your fingers. Okay, I think we answered
today's question. Why do we have five fingers? Well, we
(27:46):
don't have six or more because once you lose a
finger during evolution, it's really hard to get one back.
And we don't have four or fewer because our thumbs
are too good to lose. So that's our show. I
hope you give it a thumbs up. See you next
time you've been listening to Science Stuff. Production of iHeartRadio
(28:11):
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and please tell your friends. We'll be back next Wednesday
(28:34):
with another episode.
Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm
horhitch Ham, and today we're asking why do we have
five fingers and toes each hand or foot? Why not
three or four or six or seventeen? Who decided we
would have five digits in each hand? Now it turns
out there is a culprit which we'll get to at
(00:23):
the end. And to get there, we'll be talking to
two finger evolution experts who say there are three things
that explain the number of fingers in your hands, so
to recap count starting with your thumb will answer one question.
Talking to two experts learning three key facts, all under
fourty minutes, and the question is why do we have
(00:47):
five fingers? Enjoy? Hey everyone, Okay, here are some interesting facts.
Dogs and cats have five toes on the front legs,
but only four toes in each of their back legs.
Birds have three fingers on their wings and four toes
(01:08):
on their feet, Horses have one finger on each leg,
and cows and camels have two toes per leg. The
number of toes can even vary in the same type
of animal. Some salamanders have four front toes and five
back toes, and some have four toes all around. Clearly
there's a big variation in the number of fingers and
(01:30):
toes in the animal kingdom. But here's the weird thing.
There are no species of animals with six or more
fingers or toes on their limbs. Why is that it's five?
Somehow the maximum? What determines how many fingers each animal has?
And why do we have five fingers and toes? To
(01:51):
figure this out, I talk to two evolutionary biologists who
specialize in finger evolution. The first one is Jacob Scott,
a researcher at the University of Florida. Well, thank you
so much, Jacob for joining us.
Speaker 2 (02:07):
You are welcome, glad to be here.
Speaker 1 (02:09):
So my first question for you is how many fingers
am I holding up?
Speaker 2 (02:13):
Five?
Speaker 1 (02:15):
Good? Although you can't see my other hands, it was
kind of a trick question.
Speaker 2 (02:20):
That's true.
Speaker 1 (02:22):
Well, to get us started, I was wondering if you
could tell us where did fingers and toes come from?
Speaker 2 (02:28):
Yeah, so all vertebrates with limbs that live on land,
they ultimately evolved from something much more fish like or
a fish itself.
Speaker 1 (02:38):
Okay, so a quick refresher here about evolution. All vertebrates
with limbs meaning all animals with bones and arms and legs,
which include all mammals like us, dogs and horses, and
awesome birds, reptiles, and amphibians all came from a common
ancestor which evolved from fish and which had fingers on
(03:00):
its feet. But here's the shocker. That animal probably had
more than five fingers on each limb.
Speaker 2 (03:09):
They didn't have five fingers, they had somewhere between six
and eight. We've got fossil showing some of these like
sort of fish like tetrapods with six digits on their limbs,
with seven digits on their limbs, and with eight, so
we think it was more than five.
Speaker 1 (03:23):
Originally what we used to have more than five fingers. Yes,
that's right, our long ago ancestor and the ancestor of
all mammals, reptiles, birds, amphibians, et cetera, had potentially up
to eight fingers on each limb. Okay, let's back up
a little bit. I asked Jacob to lay out the
(03:44):
history of all land animals starting with this common ancestor.
Speaker 2 (03:50):
It was a very just an ancestor that looks, you know,
a lot more like a fish salamander type creature, and
they evolved in these tide pools or swamps just otherwise
shallow water where they kind of have the opportunity to
be embracing that fish side of it, but then occasionally
maybe would push up out of the water or move
between different tide pools. So the first land animals were
(04:13):
fishes that started adapting to living on dry land. They
started to develop lungs, their chests and hip bones started
to shift so they could push themselves out of the
water and kind of waddle around, and that's when fingers
or digits came into the picture. The conventional thinking right
now is that digits helped kind of in that transition
(04:36):
moving from water to land or transitioning between different shallow
water areas.
Speaker 1 (04:41):
So fingers kind of came from fins.
Speaker 2 (04:44):
Yeah, Ultimately, ultimately it came from fins.
Speaker 1 (04:47):
That's a short answer, right, Okay, here, it's worth taking
a little dive into fins. Almost all fish that are
alive today have what are called ray fins, like a
bunch of spikes coming out of the fish's body with
webbing between them, sort of like those traditional folding fans
that come from Asia. But those are not the fins.
(05:10):
Our arms and legs and Brut's wings come from about
four hundred million years ago. There were other species of
fish with what are called lobe fins, and these look
more like paddles or stubby little tails on the side
of the fish's body. They extended out with little armlike
bony segments, and they ended on a group of bones
(05:33):
which fanned out to form fins. And they just happened
to have four of those fins, and those are where
our arms and legs come from. Eventually, those low fin
fish started to wattle out of the water and they
used those paddle like fins to push themselves around. Then
there's a little bit of a gap in the fossil record,
(05:55):
but about three hundred and sixty million years ago you
start to see animals like a Cantostega gunari, which looked
like a cross between a salamander and the fish, or Idiostega,
which looked like a cross between a crocodile and the fish.
But here's the thing. Fossils of Acantostega and Ikiostega show
(06:18):
they had more than five fingers on their limbs. Idiostega
had seven toes in its back leg, and a Cantostega
had eight fingers in its front legs, So it's very
likely that our ancestors had a lot more than five fingers.
I mean, imagine if we still had eight fingers on
each of our hands. It might look weird, but think
(06:39):
about how much faster we could type, or what kind
of masterpieces we could play on the piano. Our numbering
system would be different. Instead of base ten, which is
based on how many fingers we have right now, we
would all be using Hexa decimal numbers, which begs the
question what happened. Well, according to our expert, what happened
(07:02):
was digit loss. That is, during evolution, you can lose fingers,
and actually it happens a lot.
Speaker 2 (07:12):
It's actually really common to evolve digit loss, especially in mammals. Well,
I think the king of reducing your digits is the horse.
Horses have functionally just one digit on each foot. They
do have little bones from the palm, but they're fuse
to that one major digit. So if you look at
a horse leg, it sort of ends in just one digit,
(07:33):
which ends in a hoof. So they're basically standing on
just their middle finger.
Speaker 1 (07:38):
So whenever a horse rears up, it's giving you the
middle finger basically.
Speaker 2 (07:41):
Exactly, it's flipping you off. So, actually, horses are a
really cool example of this because we have an awesome
fossil record all through their digit loss. First they lost
their equivalent to their big toe. They lost it on
the hind limb first, and then you can see if
they would lose the pinky next, and then the pointer
finger and the ring finger were slowly just reduced in
(08:02):
length until they're basically nothing.
Speaker 1 (08:04):
Now, oh, we can see it in the fossil record. Yeah, yeah,
I had no idea. A horse's hoof is basically its
last remaining finger. Okay, as tacob to give me more
examples of animals losing fingers.
Speaker 2 (08:19):
I'll give you all the examples I can think of.
Most salamanders have lost at least one digit. Plenty of reptiles.
So modern birds they have three digits on their forelim
on their wing. The wing is the whole arm, and
then it ends in these three digits, and then four
digits on their hind limb. There's a big debate about
whether the identities of those digits on the wing are
(08:42):
some pointer middle or if they're pointer middle index.
Speaker 1 (08:46):
Wow, fascinating. So they're mixing it up three and four yeah.
Speaker 2 (08:50):
Yeah, So actually the digit loss tends to evolve more
or less independently between the forelam and the hind limb,
So there's plenty of species that only have digit loss
in one of the limbs and not the other. You'll
see different numbers in a lot of species, Like what
so dogs and cats they've both lost their big toe
on the hind limb. So if you go home and
(09:11):
look at your dog or your cat, you'll see that
they have, you know, full five digits on their front paws,
but only four on the back.
Speaker 1 (09:18):
Really, yeah, well, I've never thought to count dogs or
cats as fingers.
Speaker 2 (09:22):
It's something that I only like really noticed after I
started doing this research. I was like, oh, does my
dog also have you know, only four digits? And it
turns out yeah, he's a dog, so he will whoa.
And there's plenty of other examples, like guinea pigs have
four on their front legs and only three on the back.
Speaker 1 (09:41):
Are there any animals with two fingers or toes?
Speaker 2 (09:44):
Yes, So that's the group that includes things like cow
and bison and giraffes. So all three of those have
at least functionally two digits, And that's in comparison to
the horses, rhinos, and tape ears, which are odd toe animals,
so they'll always have one or three digits.
Speaker 1 (10:03):
It's like two big branches in evolution. Yep, exactly, all right,
So it seems like the common ancestor to all animals
with fingers started off with six to eight fingers in
their limbs. But now there are plenty of animals that
have much less than that we have five. Others have
evolved to have four, three, two, and even one finger
(10:28):
or toe per hand or foot. So my next question
was what causes this species to lose a finger or
a toe? All right, when we come back, we're gonna
put our finger on this mystery, which honestly sticks out
like a sore thumb. So stay with us. We'll be
right back. And we're back all right. We are counting
(10:59):
the number of fingers in our hand and asking why
do we have five fingers? Why not four or six?
In the last segment, we learned that the ancestor of
all four limb vertebrates, that is, the ancestor of mammals us, birds, reptiles,
and amphibians, lived about three hundred and sixty million years ago,
(11:22):
and fossils show it probably had six to eight fingers
or toes on each limb, but we have five fingers
and toes on each extremity. So what happened Well, according
to our expert scientists think one of the main reasons
is locomotion.
Speaker 2 (11:42):
For most animals, that's the primary use of their limbs
and their digits is getting around. Usually, the cause of
loss tends to be associated with the method of locomotion.
So a lot of animals that are really specialized for
running will lose digits. So think again, cats, horses, bison,
(12:02):
All of these animals that basically hunted, chase for food,
or try to get away from predators tend to have
reduced digits.
Speaker 1 (12:11):
Generally, why does losing your finger help you run?
Speaker 2 (12:15):
I can't give you a super confident answer on that,
but I can give you some associated evidence. One of
the thoughts on why horses were able to lose so
many digits was because they evolved that trait during an
ice age when the ground was more compact. If you're
trying to walk around on like marshy, swampy, muddy ground,
you kind of want more digits because you can disperse
(12:37):
your weight a little bit more evenly and not sink right, right,
But when the ground is harder, you can kind of
just slam your whole weight down on one point really
efficiently without the ground breaking around you.
Speaker 1 (12:48):
It's sort of like, I guess if you're walking on snow,
it's better to have a snowshoe.
Speaker 2 (12:52):
Yes, exactly, that's exactly right.
Speaker 1 (12:54):
But if you're running on a track like at the Olympics,
you don't want to be wearing snowshoes.
Speaker 2 (13:00):
That's a good way to think about it.
Speaker 1 (13:02):
You almost want sort of like a pogo stick on
your feet, right, Yeah, it's kind of what it is.
So one reason so many species of lost fingers or
toes is low commotion. Animals need to move and run fast,
and sometimes having a lot of toes can be a
bad thing. If you're trying to go as fast as
you can, think having a bunch of toes flapping around
(13:24):
when you hit the ground could actually slow you down.
I mean, think about why us humans wear shoes to
walk and run. It essentially reduces our number of toes
to one. And this idea doesn't just apply to feet
and toes, It can apply to fingers and hands.
Speaker 2 (13:43):
There's some animals like spider monkeys that they swing from
vine to vine. They've lost their thumb because it's more
efficient to swing just kind of using your other forge
just as a hook, when the thumb would just get
in the way there. So that's the current thinking at least.
Speaker 1 (13:58):
Yeah, there are monkeys out there without opposable thumbs. If
you look up spider monkeys, their hands don't have thumbs.
So there are lots of examples in nature of animal
species that have loss fingers or toes. Our ancestors started
with six to eight. At some point that became five,
and since then some have gone down to four, three, two,
(14:22):
and even one finger or toe. This means the history
of fingers and toes in animals. It's not that some
animals evolved two digits and others four or five from scratch.
We all started with eight and progressively lost fingers and
toes over millions of years. But there is something odd
about this pattern, which is that you rarely see a
(14:45):
species evolved to gain a finger or a toe. Now,
sometimes an individual animal of a species might be born
with an extra finger or toe, but that's not quite
the same.
Speaker 2 (15:00):
Like in labs, we can knock out genes and it
will cause mice to develop extra digits. We know that
there are human congenital diseases where people will be born
with extra digits, or there's certain dog breeds that tend
to have extra digits, there's certain populations of cats, But
in terms of like a whole species, nothing has evolved
more digits stably, like a brand new digit with its
(15:23):
own identity being formed. It's very, very very uncommon, like
to the point where I can't think of a single
example offhand.
Speaker 1 (15:32):
Okay, what Jacob is saying is that while you might
get a person or an individual animal here and there
to grow an extra finger or toe, it usually doesn't stick. Overall.
It's extremely rare for a species to gain a finger
or toe, and yet, as you heard before, losing a
toe or losing multiple toes is extremely common in evolution.
(15:56):
What this means is that in nature there are no
species of anim animals with more than five fingers or toes.
We started with six or eight when fishes first of
all to walk on land, and then there spent a
gradual decline overall species, our branch of the evolutionary tree
of life got down to five fingers or toes, and
(16:17):
at least for us stop there. But other species kept
losing more digits, and no species alive today hasn't gained any.
So this answer is part of the question of why
we have five fingers. We don't have six fingers because
our ancestors lost that six finger a long time ago,
and it's super rare for any animal to gain a digit,
(16:41):
so we're left with five. But this brings up two
very important questions. Number one, why is finger evolution so biased?
Meaning why is it easier to lose a finger during
evolution but so hard to gain one? And number two,
we need to answer the second half of today's question,
(17:01):
which is why do we have five fingers and toes
not four or three? I mean, when's the last time
you use your pinky toe for anything? We'll answer both
those questions when we come back, so don't twiddle your thumbs.
Stay with us, we'll be right back. Welcome back.
Speaker 2 (17:32):
All right.
Speaker 1 (17:32):
We're asking why we have five fingers, and so far
we've learned that the ancestor of all animals with fingers
and toes had somewhere between six and eight fingers on
each limb, and we learned that basically it's all been
downhill from there. The history of the evolution of digits
is dominated by animal species losing fingers and toes and
(17:57):
almost never gaining them.
Speaker 2 (18:00):
It seems to be a.
Speaker 1 (18:01):
Pretty prevalent rule of thumb unintended. So another question is
why is that? Why is it easier to lose a
finger than to gain one? And also why haven't we
lost more fingers or toes? I mean, it's not like
our pinkies or our pinky toes or any toe for
that matter, are that essential for survival. Why do we
(18:24):
still have them? The point of finger At the answer,
I reached out to another finger evolution expert, doctor Friedson Gallis. Well,
thank you Tolis for joining us. Can please tell us
who you are and what you do.
Speaker 3 (18:39):
I'm an evolutionary biologist and I work at the Naturalist
Biodiversity Center in Liden, Than, Netherlands, and I am interested
in how body plans can change, what kind of mechanisms
make it difficult for evolutionary changes to occur, like why
we have two eyes two years and.
Speaker 1 (19:01):
Things like that. Great, I think that makes you the
perfect expert for our question here today we're trying to
find out why finger evolution is so one directional.
Speaker 3 (19:13):
Yeah, Well, what you see is the losses of digits
happen very often, but there are never gains of digits.
And I can explain why that is if you want
to know that.
Speaker 1 (19:26):
Okay, Doctor Gallas says. The secret to understanding why it's
easier in evolution to lose a finger than to gain
one is to understand how fingers grow in the body,
especially when we're just tiny embryos in the womb during
what's called development.
Speaker 3 (19:46):
So the number of digits is determined very early during development.
You have a very small embryo, and in this embryo,
to get development, you need to have interactions between different parts.
And because the embryo is so small at the stage,
these interactions there are in the very interactive stage, and
(20:09):
all parts are in communication with all other parts.
Speaker 1 (20:15):
Okay, what doctor Gals is saying is that during development,
when we're just a tiny clump of generic cells multiplying,
the way the cells know what to turn into is
by talking to each other. There's a complex network of
signals between all the cells that basically tell each other
you should be where the arm is, and you should
(20:37):
be where the brain is going to be, and you
should be where the lungs are. And this keeps going
until all of the body parts form and the shapeless
blob of cells that you were starts to look more
like an embryo and eventually like a baby animal. It's
all a very complex, coordinated process, and that's when fingers
(20:59):
start to get formed.
Speaker 3 (21:03):
So this number of digits is determined, then the number
of eyes and the number of little structures are determined. Then,
because there are all these many interactions with all the
other parts, if you make a change, you get more changes.
So maybe what is easy to imagine is the eye
development is very early and there are many interactions with
(21:27):
their brain. Now, if you get an extra eye or
you have one less eye, that has incredible effect on
the brain development.
Speaker 1 (21:36):
What doctor Gallas means is that it's such a complex
coordination process when all your cells are trying to figure
out how to grow into you, that if you mess
with one thing, like say an eye, it's going to
mess up the process and all the other parts are
not going to grow the way they're supposed to grow.
Speaker 3 (21:56):
You get all kinds of monstrosities you have that that
are not viable. And then the same happens with digit
because the development of digits is connected with also all
kinds of other parts of the body. So if you
change the number of digits, you get other changes as well.
Speaker 1 (22:17):
Okay, so this explains why any mutation can be potentially
disastrous for any growing embryo. Suddenly growing, say an extra finger,
could mess up the complex signaling between the different parts
that are growing, and it could cause your lungs, or
your spinal column or your head to grow the wrong way.
(22:40):
But this limitation applies to both gaining a finger and
losing a finger. We still haven't explained why in the
evolution of fingers it's way more common to lose fingers
than to gain them. Well, according to doctor Gallis, the
reason for that is that there's a way to fool
your developing embryo.
Speaker 3 (23:01):
There's a way evolutionary way around it. What you get
is that you have the earliest development, and this needs
to happen because these interactions you need them. So what
you have is that you get your digits, then development
stoops after this stage, and then it gets destroyed.
Speaker 1 (23:22):
Okay, So the trick is that let's say you're a
horse that could run faster if you had fewer toes,
or a monkey that could swing easier with fewer fingers.
You don't want to not grow your fingers because that
would mess up the signaling in your embryo. Instead, you
grow all five fingers when you're a young embryo, and
once the signaling for that extra finger stops or becomes
(23:46):
less important, you stop growing that finger and let it
shrivel off. In other words, you sort of fake out
your embryo. You fool it into thinking it's going to
have all five fingers and toes, but then you drop
the ones you don't want, which makes for some interesting
surprises when the embryo is growing.
Speaker 3 (24:07):
Yeah, and for example, horses during development, they still have five.
Speaker 1 (24:11):
Digits really like a horse embryo has five toes exactly.
Speaker 3 (24:17):
And then what happens and then the development stops and
it's broken down.
Speaker 1 (24:22):
Huh and only one finger remains. Yes, And this is
why it's easier during evolution to lose a finger than
to gain one. It's easier to fool the embryo into
thinking it's getting a finger and then later stop growing it.
Then it is to fool it into thinking it's not
getting an extra finger, and then at the last minute
(24:44):
grow a whole finger out of the blue. Okay, so
this explains why we don't have six fingers. It's hard
to gain a finger as a species once you lose it.
Now the question is why don't we have four fingers?
If we're under the thumb of this bi towards loosing fingers,
how come we haven't lost anymore. I mean, it's not
(25:05):
like the pinky toe is that useful, or any of
our toes for that matter. And here, as I promise,
we have a very specific culprit, one that I've been
hinting at throughout the whole episode, and it has to
do with the order in which fingers grow when we're embryos.
They don't all grow at the same time. They start
(25:26):
one after the other, and it turns out that the
order in which they start to grow determines which one
is most likely to get lost. Here's Jacob Scott again.
Speaker 2 (25:39):
So it tends to be the first digit that gets
lost in evolution usually is the last digit to form
during development. So in mammals, digit one, so equivalent to
the thumb or the big toe, that's the last one
to form, and it's usually the first one to be lost.
Speaker 1 (26:00):
This makes sense. Remember I said that to lose a finger,
you kind of have to fool the embryo so you
don't mess up the coordination between all the different parts. Well,
it's easier to do this if you're the last finger
to form, because being the last one you sort of
fly under the radar. But the time you come around,
(26:20):
things are more settled. And in our case, the last
digit to form is the thumb.
Speaker 2 (26:29):
I would say in humans, the digit were most likely
to evolve the loss of would be our thumb, and
we really need our songs.
Speaker 1 (26:38):
I see, I see. It's like, evolutionarily, the thumb would
be the first to go, but because it's so useful,
it's like, uh, you know, it's not going anywhere, right, Oh,
that's fascinating. Even though the pinky is maybe the least
useful finger. It's like, way in the back of the line,
it's like the thumb is running cover for the pinky.
Speaker 2 (26:58):
Yeah, so we're we're stuck with the pinkies.
Speaker 1 (27:03):
And there you have it. The reason we have five
fingers and not four is the thumb. Evolutionarily, the thumb
would be the next to go, but it's so useful
we can't get rid of it. And because we can't
get rid of it, it ends up protecting all the
other fingers. In other words, we can't lose the pinky
(27:25):
without first losing the thumb, which for us is too
useful to lose. And if you go back through the episode,
you'll notice I was hinting at this from the start.
You can cut the number of times I used the
word thumb with your fingers. Okay, I think we answered
today's question. Why do we have five fingers? Well, we
(27:46):
don't have six or more because once you lose a
finger during evolution, it's really hard to get one back.
And we don't have four or fewer because our thumbs
are too good to lose. So that's our show. I
hope you give it a thumbs up. See you next
time you've been listening to Science Stuff. Production of iHeartRadio
(28:11):
written and produced by me or hitch Hamp credited by
Rose Seguda, executive producer Jerry Rowland, and audio engineer and
mixer Jasey Peckram, and you can follow me on social media.
Just search for PhD Comics and the name of your
favorite platform. Be sure to subscribe to Sign Stuff on
the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts,
and please tell your friends. We'll be back next Wednesday
(28:34):
with another episode.
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