Why Do We Have Five Fingers?

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

You are welcome, glad to be here.

So my first question for you is how many fingers am I holding up?

Five?

Good? Although you can't see my other hands, it was kind of a trick question.

That's true.

Well, to get us started, I was wondering if you could tell us where did fingers and toes come from?

Yeah, so all vertebrates with limbs that live on land, they ultimately evolved from something much more fish like or a fish itself.

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 its feet. But here's the shocker. That animal probably had more than five fingers on each limb.

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.

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 history of all land animals starting with this common ancestor.

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

So fingers kind of came from fins.

Yeah, Ultimately, ultimately it came from fins.

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. Our arms and legs and Brut's wings come from about

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, which ends in a hoof. So they're basically standing on just their middle finger.

So whenever a horse rears up, it's giving you the middle finger basically.

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 length until they're basically nothing.

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.

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 some pointer middle or if they're pointer middle index.

Wow, fascinating. So they're mixing it up three and four yeah.

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 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.

Really, yeah, well, I've never thought to count dogs or cats as fingers.

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.

Are there any animals with two fingers or toes?

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.

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 or toe per hand or foot. So my next question was what causes this species to lose a finger or

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, All of these animals that basically hunted, chase for food, or try to get away from predators tend to have reduced digits.

Generally, why does losing your finger help you run?

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

It's sort of like, I guess if you're walking on snow, it's better to have a snowshoe.

Yes, exactly, that's exactly right.

But if you're running on a track like at the Olympics, you don't want to be wearing snowshoes.

That's a good way to think about it.

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 when you hit the ground could actually slow you down. I mean, think about why us humans wear shoes to

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.

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, and even one finger or toe. This means the history

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

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. What this means is that in nature there are no species of anim animals with more than five fingers or toes.

All right.

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 almost never gaining them.

It seems to be a.

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 still have them? The point of finger At the answer, I reached out to another finger evolution expert, doctor Friedson Gallis. Well,

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.

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.

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.

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.

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 all parts are in communication with all other parts.

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 be where the brain is going to be, and you

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 their brain. Now, if you get an extra eye or you have one less eye, that has incredible effect on the brain development.

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.

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.

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. 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

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.

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 less important, you stop growing that finger and let it shrivel off. In other words, you sort of fake out

Yeah, and for example, horses during development, they still have five.

Digits really like a horse embryo has five toes exactly.

And then what happens and then the development stops and it's broken down.

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 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.

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.

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, things are more settled. And in our case, the last digit to form is the thumb.

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.

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.

Yeah, so we're we're stuck with the pinkies.

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 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