WEBVTT - Why Do We Have Five Fingers? 0:00:01.560 --> 0:00:05.040 Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm 0:00:05.040 --> 0:00:07.880 horhitch Ham, and today we're asking why do we have 0:00:08.240 --> 0:00:12.960 five fingers and toes each hand or foot? Why not 0:00:13.080 --> 0:00:17.680 three or four or six or seventeen? Who decided we 0:00:17.720 --> 0:00:21.120 would have five digits in each hand? Now it turns 0:00:21.160 --> 0:00:23.680 out there is a culprit which we'll get to at 0:00:23.680 --> 0:00:26.040 the end. And to get there, we'll be talking to 0:00:26.200 --> 0:00:30.280 two finger evolution experts who say there are three things 0:00:30.320 --> 0:00:33.800 that explain the number of fingers in your hands, so 0:00:34.000 --> 0:00:37.760 to recap count starting with your thumb will answer one question. 0:00:38.240 --> 0:00:42.720 Talking to two experts learning three key facts, all under 0:00:42.880 --> 0:00:46.680 fourty minutes, and the question is why do we have 0:00:47.320 --> 0:00:57.080 five fingers? Enjoy? Hey everyone, Okay, here are some interesting facts. 0:00:57.400 --> 0:01:00.960 Dogs and cats have five toes on the front legs, 0:01:01.240 --> 0:01:04.360 but only four toes in each of their back legs. 0:01:05.160 --> 0:01:08.680 Birds have three fingers on their wings and four toes 0:01:08.920 --> 0:01:12.399 on their feet, Horses have one finger on each leg, 0:01:13.000 --> 0:01:16.800 and cows and camels have two toes per leg. The 0:01:16.880 --> 0:01:19.280 number of toes can even vary in the same type 0:01:19.319 --> 0:01:23.440 of animal. Some salamanders have four front toes and five 0:01:23.600 --> 0:01:28.240 back toes, and some have four toes all around. Clearly 0:01:28.360 --> 0:01:30.920 there's a big variation in the number of fingers and 0:01:30.959 --> 0:01:34.000 toes in the animal kingdom. But here's the weird thing. 0:01:34.400 --> 0:01:37.640 There are no species of animals with six or more 0:01:37.720 --> 0:01:42.640 fingers or toes on their limbs. Why is that it's five? 0:01:42.760 --> 0:01:47.760 Somehow the maximum? What determines how many fingers each animal has? 0:01:48.200 --> 0:01:51.640 And why do we have five fingers and toes? To 0:01:51.720 --> 0:01:55.360 figure this out, I talk to two evolutionary biologists who 0:01:55.360 --> 0:01:59.880 specialize in finger evolution. The first one is Jacob Scott, 0:02:00.440 --> 0:02:06.160 a researcher at the University of Florida. Well, thank you 0:02:06.200 --> 0:02:07.560 so much, Jacob for joining us. 0:02:07.600 --> 0:02:09.360 You are welcome, glad to be here. 0:02:09.520 --> 0:02:12.000 So my first question for you is how many fingers 0:02:12.080 --> 0:02:12.799 am I holding up? 0:02:13.320 --> 0:02:13.639 Five? 0:02:15.080 --> 0:02:19.160 Good? Although you can't see my other hands, it was 0:02:19.240 --> 0:02:20.320 kind of a trick question. 0:02:20.280 --> 0:02:21.120 That's true. 0:02:22.040 --> 0:02:24.160 Well, to get us started, I was wondering if you 0:02:24.160 --> 0:02:28.079 could tell us where did fingers and toes come from? 0:02:28.360 --> 0:02:31.880 Yeah, so all vertebrates with limbs that live on land, 0:02:32.160 --> 0:02:36.440 they ultimately evolved from something much more fish like or 0:02:36.560 --> 0:02:37.400 a fish itself. 0:02:38.400 --> 0:02:42.320 Okay, so a quick refresher here about evolution. All vertebrates 0:02:42.320 --> 0:02:46.800 with limbs meaning all animals with bones and arms and legs, 0:02:47.080 --> 0:02:51.480 which include all mammals like us, dogs and horses, and 0:02:51.639 --> 0:02:56.160 awesome birds, reptiles, and amphibians all came from a common 0:02:56.320 --> 0:03:00.440 ancestor which evolved from fish and which had fingers on 0:03:00.520 --> 0:03:05.160 its feet. But here's the shocker. That animal probably had 0:03:05.440 --> 0:03:08.120 more than five fingers on each limb. 0:03:09.480 --> 0:03:12.880 They didn't have five fingers, they had somewhere between six 0:03:12.919 --> 0:03:15.560 and eight. We've got fossil showing some of these like 0:03:15.639 --> 0:03:19.600 sort of fish like tetrapods with six digits on their limbs, 0:03:19.600 --> 0:03:21.640 with seven digits on their limbs, and with eight, so 0:03:21.960 --> 0:03:23.480 we think it was more than five. 0:03:23.600 --> 0:03:27.680 Originally what we used to have more than five fingers. Yes, 0:03:28.800 --> 0:03:32.800 that's right, our long ago ancestor and the ancestor of 0:03:32.840 --> 0:03:38.440 all mammals, reptiles, birds, amphibians, et cetera, had potentially up 0:03:38.520 --> 0:03:42.240 to eight fingers on each limb. Okay, let's back up 0:03:42.240 --> 0:03:44.760 a little bit. I asked Jacob to lay out the 0:03:44.800 --> 0:03:49.160 history of all land animals starting with this common ancestor. 0:03:50.560 --> 0:03:53.040 It was a very just an ancestor that looks, you know, 0:03:53.080 --> 0:03:56.840 a lot more like a fish salamander type creature, and 0:03:56.880 --> 0:04:00.720 they evolved in these tide pools or swamps just otherwise 0:04:00.720 --> 0:04:03.600 shallow water where they kind of have the opportunity to 0:04:03.680 --> 0:04:06.920 be embracing that fish side of it, but then occasionally 0:04:07.000 --> 0:04:09.320 maybe would push up out of the water or move 0:04:09.360 --> 0:04:13.840 between different tide pools. So the first land animals were 0:04:13.920 --> 0:04:18.360 fishes that started adapting to living on dry land. They 0:04:18.360 --> 0:04:21.960 started to develop lungs, their chests and hip bones started 0:04:22.000 --> 0:04:24.440 to shift so they could push themselves out of the 0:04:24.480 --> 0:04:27.920 water and kind of waddle around, and that's when fingers 0:04:28.160 --> 0:04:33.000 or digits came into the picture. The conventional thinking right 0:04:33.040 --> 0:04:36.120 now is that digits helped kind of in that transition 0:04:36.640 --> 0:04:40.160 moving from water to land or transitioning between different shallow 0:04:40.440 --> 0:04:41.359 water areas. 0:04:41.640 --> 0:04:44.040 So fingers kind of came from fins. 0:04:44.600 --> 0:04:47.520 Yeah, Ultimately, ultimately it came from fins. 0:04:47.560 --> 0:04:51.479 That's a short answer, right, Okay, here, it's worth taking 0:04:51.520 --> 0:04:55.680 a little dive into fins. Almost all fish that are 0:04:55.720 --> 0:05:00.240 alive today have what are called ray fins, like a 0:05:00.240 --> 0:05:02.800 bunch of spikes coming out of the fish's body with 0:05:03.120 --> 0:05:07.000 webbing between them, sort of like those traditional folding fans 0:05:07.000 --> 0:05:10.160 that come from Asia. But those are not the fins. 0:05:10.400 --> 0:05:15.200 Our arms and legs and Brut's wings come from about 0:05:15.240 --> 0:05:18.400 four hundred million years ago. There were other species of 0:05:18.440 --> 0:05:22.560 fish with what are called lobe fins, and these look 0:05:22.640 --> 0:05:26.000 more like paddles or stubby little tails on the side 0:05:26.040 --> 0:05:29.719 of the fish's body. They extended out with little armlike 0:05:29.839 --> 0:05:33.160 bony segments, and they ended on a group of bones 0:05:33.520 --> 0:05:36.839 which fanned out to form fins. And they just happened 0:05:36.880 --> 0:05:40.520 to have four of those fins, and those are where 0:05:40.520 --> 0:05:45.560 our arms and legs come from. Eventually, those low fin 0:05:45.680 --> 0:05:48.919 fish started to wattle out of the water and they 0:05:49.040 --> 0:05:53.280 used those paddle like fins to push themselves around. Then 0:05:53.320 --> 0:05:55.599 there's a little bit of a gap in the fossil record, 0:05:55.920 --> 0:05:58.880 but about three hundred and sixty million years ago you 0:05:58.960 --> 0:06:03.360 start to see animals like a Cantostega gunari, which looked 0:06:03.400 --> 0:06:07.880 like a cross between a salamander and the fish, or Idiostega, 0:06:08.320 --> 0:06:12.000 which looked like a cross between a crocodile and the fish. 0:06:13.120 --> 0:06:17.800 But here's the thing. Fossils of Acantostega and Ikiostega show 0:06:18.080 --> 0:06:21.960 they had more than five fingers on their limbs. Idiostega 0:06:22.080 --> 0:06:25.600 had seven toes in its back leg, and a Cantostega 0:06:25.720 --> 0:06:29.400 had eight fingers in its front legs, So it's very 0:06:29.560 --> 0:06:33.600 likely that our ancestors had a lot more than five fingers. 0:06:34.080 --> 0:06:36.960 I mean, imagine if we still had eight fingers on 0:06:37.000 --> 0:06:39.880 each of our hands. It might look weird, but think 0:06:39.880 --> 0:06:43.040 about how much faster we could type, or what kind 0:06:43.040 --> 0:06:46.919 of masterpieces we could play on the piano. Our numbering 0:06:46.960 --> 0:06:50.719 system would be different. Instead of base ten, which is 0:06:50.720 --> 0:06:53.200 based on how many fingers we have right now, we 0:06:53.279 --> 0:06:57.520 would all be using Hexa decimal numbers, which begs the 0:06:57.600 --> 0:07:02.520 question what happened. Well, according to our expert, what happened 0:07:02.640 --> 0:07:08.080 was digit loss. That is, during evolution, you can lose fingers, 0:07:08.760 --> 0:07:11.120 and actually it happens a lot. 0:07:12.640 --> 0:07:16.760 It's actually really common to evolve digit loss, especially in mammals. Well, 0:07:16.800 --> 0:07:19.440 I think the king of reducing your digits is the horse. 0:07:19.720 --> 0:07:24.120 Horses have functionally just one digit on each foot. They 0:07:24.160 --> 0:07:27.720 do have little bones from the palm, but they're fuse 0:07:27.840 --> 0:07:29.760 to that one major digit. So if you look at 0:07:29.800 --> 0:07:32.920 a horse leg, it sort of ends in just one digit, 0:07:33.120 --> 0:07:35.560 which ends in a hoof. So they're basically standing on 0:07:35.720 --> 0:07:36.600 just their middle finger. 0:07:38.280 --> 0:07:40.120 So whenever a horse rears up, it's giving you the 0:07:40.120 --> 0:07:41.680 middle finger basically. 0:07:41.320 --> 0:07:45.040 Exactly, it's flipping you off. So, actually, horses are a 0:07:45.120 --> 0:07:47.880 really cool example of this because we have an awesome 0:07:47.920 --> 0:07:51.440 fossil record all through their digit loss. First they lost 0:07:51.480 --> 0:07:53.640 their equivalent to their big toe. They lost it on 0:07:53.680 --> 0:07:56.080 the hind limb first, and then you can see if 0:07:56.080 --> 0:07:58.840 they would lose the pinky next, and then the pointer 0:07:58.960 --> 0:08:02.440 finger and the ring finger were slowly just reduced in 0:08:02.560 --> 0:08:04.480 length until they're basically nothing. 0:08:04.520 --> 0:08:08.760 Now, oh, we can see it in the fossil record. Yeah, yeah, 0:08:08.800 --> 0:08:12.280 I had no idea. A horse's hoof is basically its 0:08:12.360 --> 0:08:16.520 last remaining finger. Okay, as tacob to give me more 0:08:16.520 --> 0:08:19.400 examples of animals losing fingers. 0:08:19.880 --> 0:08:21.560 I'll give you all the examples I can think of. 0:08:22.040 --> 0:08:26.520 Most salamanders have lost at least one digit. Plenty of reptiles. 0:08:26.760 --> 0:08:29.760 So modern birds they have three digits on their forelim 0:08:29.840 --> 0:08:32.080 on their wing. The wing is the whole arm, and 0:08:32.080 --> 0:08:35.160 then it ends in these three digits, and then four 0:08:35.160 --> 0:08:38.160 digits on their hind limb. There's a big debate about 0:08:38.200 --> 0:08:42.400 whether the identities of those digits on the wing are 0:08:42.760 --> 0:08:46.200 some pointer middle or if they're pointer middle index. 0:08:46.559 --> 0:08:50.760 Wow, fascinating. So they're mixing it up three and four yeah. 0:08:50.520 --> 0:08:53.719 Yeah, So actually the digit loss tends to evolve more 0:08:53.800 --> 0:08:56.720 or less independently between the forelam and the hind limb, 0:08:57.120 --> 0:09:00.400 So there's plenty of species that only have digit loss 0:09:00.480 --> 0:09:02.600 in one of the limbs and not the other. You'll 0:09:02.600 --> 0:09:05.400 see different numbers in a lot of species, Like what 0:09:05.640 --> 0:09:09.520 so dogs and cats they've both lost their big toe 0:09:09.600 --> 0:09:11.440 on the hind limb. So if you go home and 0:09:11.440 --> 0:09:13.760 look at your dog or your cat, you'll see that 0:09:13.840 --> 0:09:16.720 they have, you know, full five digits on their front paws, 0:09:16.880 --> 0:09:18.000 but only four on the back. 0:09:18.400 --> 0:09:21.960 Really, yeah, well, I've never thought to count dogs or 0:09:22.000 --> 0:09:22.840 cats as fingers. 0:09:22.920 --> 0:09:25.719 It's something that I only like really noticed after I 0:09:26.080 --> 0:09:28.600 started doing this research. I was like, oh, does my 0:09:28.720 --> 0:09:31.199 dog also have you know, only four digits? And it 0:09:31.240 --> 0:09:33.840 turns out yeah, he's a dog, so he will whoa. 0:09:34.200 --> 0:09:37.679 And there's plenty of other examples, like guinea pigs have 0:09:38.480 --> 0:09:41.320 four on their front legs and only three on the back. 0:09:41.679 --> 0:09:43.960 Are there any animals with two fingers or toes? 0:09:44.280 --> 0:09:47.280 Yes, So that's the group that includes things like cow 0:09:47.480 --> 0:09:51.920 and bison and giraffes. So all three of those have 0:09:52.559 --> 0:09:56.240 at least functionally two digits, And that's in comparison to 0:09:56.280 --> 0:10:01.000 the horses, rhinos, and tape ears, which are odd toe animals, 0:10:01.000 --> 0:10:03.720 so they'll always have one or three digits. 0:10:03.960 --> 0:10:09.120 It's like two big branches in evolution. Yep, exactly, all right, 0:10:09.360 --> 0:10:12.760 So it seems like the common ancestor to all animals 0:10:12.760 --> 0:10:16.640 with fingers started off with six to eight fingers in 0:10:16.679 --> 0:10:19.640 their limbs. But now there are plenty of animals that 0:10:19.720 --> 0:10:23.280 have much less than that we have five. Others have 0:10:23.360 --> 0:10:27.840 evolved to have four, three, two, and even one finger 0:10:28.080 --> 0:10:31.320 or toe per hand or foot. So my next question 0:10:31.480 --> 0:10:35.480 was what causes this species to lose a finger or 0:10:35.520 --> 0:10:39.040 a toe? All right, when we come back, we're gonna 0:10:39.080 --> 0:10:43.840 put our finger on this mystery, which honestly sticks out 0:10:44.120 --> 0:10:47.280 like a sore thumb. So stay with us. We'll be 0:10:47.440 --> 0:10:59.880 right back. And we're back all right. We are counting 0:10:59.880 --> 0:11:03.040 the number of fingers in our hand and asking why 0:11:03.080 --> 0:11:07.520 do we have five fingers? Why not four or six? 0:11:08.400 --> 0:11:11.040 In the last segment, we learned that the ancestor of 0:11:11.080 --> 0:11:17.680 all four limb vertebrates, that is, the ancestor of mammals us, birds, reptiles, 0:11:17.679 --> 0:11:21.840 and amphibians, lived about three hundred and sixty million years ago, 0:11:22.320 --> 0:11:26.280 and fossils show it probably had six to eight fingers 0:11:26.520 --> 0:11:30.520 or toes on each limb, but we have five fingers 0:11:30.520 --> 0:11:35.480 and toes on each extremity. So what happened Well, according 0:11:35.520 --> 0:11:38.920 to our expert scientists think one of the main reasons 0:11:39.360 --> 0:11:40.319 is locomotion. 0:11:42.280 --> 0:11:46.080 For most animals, that's the primary use of their limbs 0:11:46.120 --> 0:11:49.800 and their digits is getting around. Usually, the cause of 0:11:49.880 --> 0:11:53.600 loss tends to be associated with the method of locomotion. 0:11:54.440 --> 0:11:57.439 So a lot of animals that are really specialized for 0:11:57.559 --> 0:12:02.440 running will lose digits. So think again, cats, horses, bison, 0:12:02.880 --> 0:12:06.480 All of these animals that basically hunted, chase for food, 0:12:06.640 --> 0:12:09.720 or try to get away from predators tend to have 0:12:10.360 --> 0:12:11.280 reduced digits. 0:12:11.800 --> 0:12:15.199 Generally, why does losing your finger help you run? 0:12:15.880 --> 0:12:18.319 I can't give you a super confident answer on that, 0:12:18.600 --> 0:12:22.640 but I can give you some associated evidence. One of 0:12:22.640 --> 0:12:25.920 the thoughts on why horses were able to lose so 0:12:25.960 --> 0:12:29.080 many digits was because they evolved that trait during an 0:12:29.080 --> 0:12:31.719 ice age when the ground was more compact. If you're 0:12:31.720 --> 0:12:34.840 trying to walk around on like marshy, swampy, muddy ground, 0:12:35.120 --> 0:12:37.680 you kind of want more digits because you can disperse 0:12:37.720 --> 0:12:40.600 your weight a little bit more evenly and not sink right, right, 0:12:40.760 --> 0:12:42.640 But when the ground is harder, you can kind of 0:12:42.720 --> 0:12:45.559 just slam your whole weight down on one point really 0:12:45.559 --> 0:12:48.120 efficiently without the ground breaking around you. 0:12:48.600 --> 0:12:50.839 It's sort of like, I guess if you're walking on snow, 0:12:50.840 --> 0:12:52.400 it's better to have a snowshoe. 0:12:52.559 --> 0:12:53.959 Yes, exactly, that's exactly right. 0:12:54.200 --> 0:12:56.680 But if you're running on a track like at the Olympics, 0:12:56.800 --> 0:12:58.440 you don't want to be wearing snowshoes. 0:13:00.080 --> 0:13:01.360 That's a good way to think about it. 0:13:02.120 --> 0:13:04.200 You almost want sort of like a pogo stick on 0:13:04.240 --> 0:13:06.480 your feet, right, Yeah, it's kind of what it is. 0:13:07.480 --> 0:13:10.480 So one reason so many species of lost fingers or 0:13:10.559 --> 0:13:15.360 toes is low commotion. Animals need to move and run fast, 0:13:15.640 --> 0:13:19.400 and sometimes having a lot of toes can be a 0:13:19.440 --> 0:13:21.679 bad thing. If you're trying to go as fast as 0:13:21.679 --> 0:13:24.560 you can, think having a bunch of toes flapping around 0:13:24.559 --> 0:13:27.439 when you hit the ground could actually slow you down. 0:13:28.040 --> 0:13:31.680 I mean, think about why us humans wear shoes to 0:13:31.800 --> 0:13:35.760 walk and run. It essentially reduces our number of toes 0:13:36.200 --> 0:13:39.600 to one. And this idea doesn't just apply to feet 0:13:39.679 --> 0:13:43.000 and toes, It can apply to fingers and hands. 0:13:43.760 --> 0:13:47.000 There's some animals like spider monkeys that they swing from 0:13:47.080 --> 0:13:50.199 vine to vine. They've lost their thumb because it's more 0:13:50.200 --> 0:13:53.040 efficient to swing just kind of using your other forge 0:13:53.200 --> 0:13:54.880 just as a hook, when the thumb would just get 0:13:54.920 --> 0:13:57.600 in the way there. So that's the current thinking at least. 0:13:58.200 --> 0:14:02.079 Yeah, there are monkeys out there without opposable thumbs. If 0:14:02.080 --> 0:14:05.240 you look up spider monkeys, their hands don't have thumbs. 0:14:05.760 --> 0:14:08.840 So there are lots of examples in nature of animal 0:14:08.880 --> 0:14:13.400 species that have loss fingers or toes. Our ancestors started 0:14:13.440 --> 0:14:16.640 with six to eight. At some point that became five, 0:14:17.280 --> 0:14:21.560 and since then some have gone down to four, three, two, 0:14:22.000 --> 0:14:25.600 and even one finger or toe. This means the history 0:14:25.600 --> 0:14:28.640 of fingers and toes in animals. It's not that some 0:14:28.760 --> 0:14:33.200 animals evolved two digits and others four or five from scratch. 0:14:33.760 --> 0:14:37.800 We all started with eight and progressively lost fingers and 0:14:37.880 --> 0:14:41.880 toes over millions of years. But there is something odd 0:14:41.920 --> 0:14:44.960 about this pattern, which is that you rarely see a 0:14:45.040 --> 0:14:50.680 species evolved to gain a finger or a toe. Now, 0:14:50.840 --> 0:14:54.480 sometimes an individual animal of a species might be born 0:14:54.560 --> 0:14:58.200 with an extra finger or toe, but that's not quite 0:14:58.240 --> 0:14:58.680 the same. 0:15:00.720 --> 0:15:03.200 Like in labs, we can knock out genes and it 0:15:03.240 --> 0:15:06.160 will cause mice to develop extra digits. We know that 0:15:06.160 --> 0:15:09.560 there are human congenital diseases where people will be born 0:15:09.600 --> 0:15:12.920 with extra digits, or there's certain dog breeds that tend 0:15:13.000 --> 0:15:16.120 to have extra digits, there's certain populations of cats, But 0:15:16.160 --> 0:15:19.840 in terms of like a whole species, nothing has evolved 0:15:20.160 --> 0:15:23.720 more digits stably, like a brand new digit with its 0:15:23.720 --> 0:15:27.400 own identity being formed. It's very, very very uncommon, like 0:15:27.440 --> 0:15:29.760 to the point where I can't think of a single 0:15:29.800 --> 0:15:30.840 example offhand. 0:15:32.200 --> 0:15:34.720 Okay, what Jacob is saying is that while you might 0:15:34.800 --> 0:15:38.000 get a person or an individual animal here and there 0:15:38.120 --> 0:15:43.960 to grow an extra finger or toe, it usually doesn't stick. Overall. 0:15:44.120 --> 0:15:47.520 It's extremely rare for a species to gain a finger 0:15:47.680 --> 0:15:51.120 or toe, and yet, as you heard before, losing a 0:15:51.160 --> 0:15:55.720 toe or losing multiple toes is extremely common in evolution. 0:15:56.360 --> 0:15:59.040 What this means is that in nature there are no 0:15:59.160 --> 0:16:03.000 species of anim animals with more than five fingers or toes. 0:16:03.440 --> 0:16:06.320 We started with six or eight when fishes first of 0:16:06.320 --> 0:16:08.760 all to walk on land, and then there spent a 0:16:08.880 --> 0:16:13.960 gradual decline overall species, our branch of the evolutionary tree 0:16:13.960 --> 0:16:17.160 of life got down to five fingers or toes, and 0:16:17.400 --> 0:16:21.000 at least for us stop there. But other species kept 0:16:21.040 --> 0:16:25.440 losing more digits, and no species alive today hasn't gained any. 0:16:26.120 --> 0:16:28.480 So this answer is part of the question of why 0:16:28.520 --> 0:16:32.560 we have five fingers. We don't have six fingers because 0:16:32.680 --> 0:16:36.360 our ancestors lost that six finger a long time ago, 0:16:36.800 --> 0:16:40.600 and it's super rare for any animal to gain a digit, 0:16:41.120 --> 0:16:44.680 so we're left with five. But this brings up two 0:16:44.800 --> 0:16:50.560 very important questions. Number one, why is finger evolution so biased? 0:16:51.160 --> 0:16:54.000 Meaning why is it easier to lose a finger during 0:16:54.000 --> 0:16:58.080 evolution but so hard to gain one? And number two, 0:16:58.360 --> 0:17:01.040 we need to answer the second half of today's question, 0:17:01.520 --> 0:17:04.040 which is why do we have five fingers and toes 0:17:04.840 --> 0:17:08.600 not four or three? I mean, when's the last time 0:17:08.640 --> 0:17:11.840 you use your pinky toe for anything? We'll answer both 0:17:11.880 --> 0:17:15.320 those questions when we come back, so don't twiddle your thumbs. 0:17:15.520 --> 0:17:30.720 Stay with us, we'll be right back. Welcome back. 0:17:32.119 --> 0:17:32.520 All right. 0:17:32.720 --> 0:17:36.120 We're asking why we have five fingers, and so far 0:17:36.520 --> 0:17:39.960 we've learned that the ancestor of all animals with fingers 0:17:39.960 --> 0:17:43.720 and toes had somewhere between six and eight fingers on 0:17:43.800 --> 0:17:48.040 each limb, and we learned that basically it's all been 0:17:48.520 --> 0:17:52.080 downhill from there. The history of the evolution of digits 0:17:52.440 --> 0:17:57.639 is dominated by animal species losing fingers and toes and 0:17:57.760 --> 0:18:00.520 almost never gaining them. 0:18:00.680 --> 0:18:01.480 It seems to be a. 0:18:01.440 --> 0:18:07.000 Pretty prevalent rule of thumb unintended. So another question is 0:18:07.440 --> 0:18:10.320 why is that? Why is it easier to lose a 0:18:10.359 --> 0:18:13.919 finger than to gain one? And also why haven't we 0:18:13.960 --> 0:18:17.359 lost more fingers or toes? I mean, it's not like 0:18:17.400 --> 0:18:20.679 our pinkies or our pinky toes or any toe for 0:18:20.760 --> 0:18:24.639 that matter, are that essential for survival. Why do we 0:18:24.680 --> 0:18:27.080 still have them? The point of finger At the answer, 0:18:27.480 --> 0:18:35.280 I reached out to another finger evolution expert, doctor Friedson Gallis. Well, 0:18:35.280 --> 0:18:38.000 thank you Tolis for joining us. Can please tell us 0:18:38.000 --> 0:18:39.280 who you are and what you do. 0:18:39.520 --> 0:18:43.480 I'm an evolutionary biologist and I work at the Naturalist 0:18:43.520 --> 0:18:48.520 Biodiversity Center in Liden, Than, Netherlands, and I am interested 0:18:48.680 --> 0:18:53.280 in how body plans can change, what kind of mechanisms 0:18:53.400 --> 0:18:58.640 make it difficult for evolutionary changes to occur, like why 0:18:58.760 --> 0:19:01.080 we have two eyes two years and. 0:19:01.160 --> 0:19:03.960 Things like that. Great, I think that makes you the 0:19:04.000 --> 0:19:08.000 perfect expert for our question here today we're trying to 0:19:08.040 --> 0:19:13.560 find out why finger evolution is so one directional. 0:19:13.880 --> 0:19:17.679 Yeah, Well, what you see is the losses of digits 0:19:17.720 --> 0:19:21.719 happen very often, but there are never gains of digits. 0:19:21.880 --> 0:19:24.760 And I can explain why that is if you want 0:19:24.800 --> 0:19:25.199 to know that. 0:19:26.600 --> 0:19:30.920 Okay, Doctor Gallas says. The secret to understanding why it's 0:19:31.040 --> 0:19:34.640 easier in evolution to lose a finger than to gain 0:19:34.680 --> 0:19:38.879 one is to understand how fingers grow in the body, 0:19:39.280 --> 0:19:42.680 especially when we're just tiny embryos in the womb during 0:19:42.680 --> 0:19:44.560 what's called development. 0:19:46.000 --> 0:19:50.960 So the number of digits is determined very early during development. 0:19:51.400 --> 0:19:55.800 You have a very small embryo, and in this embryo, 0:19:56.359 --> 0:20:01.240 to get development, you need to have interactions between different parts. 0:20:01.880 --> 0:20:04.800 And because the embryo is so small at the stage, 0:20:05.000 --> 0:20:09.119 these interactions there are in the very interactive stage, and 0:20:09.600 --> 0:20:13.760 all parts are in communication with all other parts. 0:20:15.000 --> 0:20:18.320 Okay, what doctor Gals is saying is that during development, 0:20:18.680 --> 0:20:22.160 when we're just a tiny clump of generic cells multiplying, 0:20:22.480 --> 0:20:25.879 the way the cells know what to turn into is 0:20:25.920 --> 0:20:30.040 by talking to each other. There's a complex network of 0:20:30.160 --> 0:20:34.120 signals between all the cells that basically tell each other 0:20:34.960 --> 0:20:37.680 you should be where the arm is, and you should 0:20:37.680 --> 0:20:39.600 be where the brain is going to be, and you 0:20:39.640 --> 0:20:42.600 should be where the lungs are. And this keeps going 0:20:42.800 --> 0:20:46.520 until all of the body parts form and the shapeless 0:20:46.560 --> 0:20:50.000 blob of cells that you were starts to look more 0:20:50.119 --> 0:20:54.920 like an embryo and eventually like a baby animal. It's 0:20:55.000 --> 0:20:59.480 all a very complex, coordinated process, and that's when fingers 0:20:59.680 --> 0:21:02.119 start to get formed. 0:21:03.520 --> 0:21:06.920 So this number of digits is determined, then the number 0:21:06.960 --> 0:21:10.480 of eyes and the number of little structures are determined. Then, 0:21:10.760 --> 0:21:13.919 because there are all these many interactions with all the 0:21:14.000 --> 0:21:18.920 other parts, if you make a change, you get more changes. 0:21:19.359 --> 0:21:22.760 So maybe what is easy to imagine is the eye 0:21:22.800 --> 0:21:26.800 development is very early and there are many interactions with 0:21:27.040 --> 0:21:30.240 their brain. Now, if you get an extra eye or 0:21:30.280 --> 0:21:34.520 you have one less eye, that has incredible effect on 0:21:34.560 --> 0:21:35.600 the brain development. 0:21:36.560 --> 0:21:39.679 What doctor Gallas means is that it's such a complex 0:21:39.840 --> 0:21:43.359 coordination process when all your cells are trying to figure 0:21:43.359 --> 0:21:46.320 out how to grow into you, that if you mess 0:21:46.400 --> 0:21:49.679 with one thing, like say an eye, it's going to 0:21:49.680 --> 0:21:52.680 mess up the process and all the other parts are 0:21:52.720 --> 0:21:55.400 not going to grow the way they're supposed to grow. 0:21:56.880 --> 0:22:01.320 You get all kinds of monstrosities you have that that 0:22:01.400 --> 0:22:04.639 are not viable. And then the same happens with digit 0:22:04.920 --> 0:22:08.880 because the development of digits is connected with also all 0:22:08.960 --> 0:22:11.840 kinds of other parts of the body. So if you 0:22:12.160 --> 0:22:16.159 change the number of digits, you get other changes as well. 0:22:17.160 --> 0:22:21.400 Okay, so this explains why any mutation can be potentially 0:22:21.520 --> 0:22:27.600 disastrous for any growing embryo. Suddenly growing, say an extra finger, 0:22:28.160 --> 0:22:31.320 could mess up the complex signaling between the different parts 0:22:31.320 --> 0:22:34.680 that are growing, and it could cause your lungs, or 0:22:34.720 --> 0:22:39.240 your spinal column or your head to grow the wrong way. 0:22:40.359 --> 0:22:44.080 But this limitation applies to both gaining a finger and 0:22:44.280 --> 0:22:47.600 losing a finger. We still haven't explained why in the 0:22:47.680 --> 0:22:51.600 evolution of fingers it's way more common to lose fingers 0:22:51.920 --> 0:22:55.359 than to gain them. Well, according to doctor Gallis, the 0:22:55.440 --> 0:22:58.560 reason for that is that there's a way to fool 0:22:58.840 --> 0:23:00.440 your developing embryo. 0:23:01.600 --> 0:23:05.320 There's a way evolutionary way around it. What you get 0:23:05.600 --> 0:23:09.160 is that you have the earliest development, and this needs 0:23:09.200 --> 0:23:12.359 to happen because these interactions you need them. So what 0:23:12.520 --> 0:23:16.280 you have is that you get your digits, then development 0:23:16.359 --> 0:23:21.359 stoops after this stage, and then it gets destroyed. 0:23:22.880 --> 0:23:25.480 Okay, So the trick is that let's say you're a 0:23:25.520 --> 0:23:28.640 horse that could run faster if you had fewer toes, 0:23:29.000 --> 0:23:32.240 or a monkey that could swing easier with fewer fingers. 0:23:32.480 --> 0:23:35.280 You don't want to not grow your fingers because that 0:23:35.320 --> 0:23:39.040 would mess up the signaling in your embryo. Instead, you 0:23:39.080 --> 0:23:42.560 grow all five fingers when you're a young embryo, and 0:23:42.720 --> 0:23:46.280 once the signaling for that extra finger stops or becomes 0:23:46.359 --> 0:23:49.920 less important, you stop growing that finger and let it 0:23:50.119 --> 0:23:53.800 shrivel off. In other words, you sort of fake out 0:23:54.160 --> 0:23:56.920 your embryo. You fool it into thinking it's going to 0:23:57.000 --> 0:23:59.800 have all five fingers and toes, but then you drop 0:23:59.840 --> 0:24:03.720 the ones you don't want, which makes for some interesting 0:24:03.760 --> 0:24:06.399 surprises when the embryo is growing. 0:24:07.560 --> 0:24:12.080 Yeah, and for example, horses during development, they still have five. 0:24:11.920 --> 0:24:16.960 Digits really like a horse embryo has five toes exactly. 0:24:17.000 --> 0:24:21.119 And then what happens and then the development stops and 0:24:21.160 --> 0:24:22.160 it's broken down. 0:24:22.600 --> 0:24:27.280 Huh and only one finger remains. Yes, And this is 0:24:27.280 --> 0:24:31.080 why it's easier during evolution to lose a finger than 0:24:31.160 --> 0:24:34.639 to gain one. It's easier to fool the embryo into 0:24:34.680 --> 0:24:38.200