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GreatBritish1
  • 6/17/2025
Secrets of Bones episode 2

Category

🐳
Animals
Transcript
00:00Bones. They offer structure, support, and strength. But they have a much bigger story to tell.
00:17Vertebrates may look very different on the outside, but one crucial thing unites them all. The skeleton.
00:35I'm Ben Garrett, an evolutionary biologist with a very unusual passion.
00:42This is unbelievable. There are too many skeletons for me to look at all at once.
00:46As a child, I was fascinated by bones. Now, skeletons have become my life.
00:58And I put them together for museums and universities all over the world.
01:06I'm going to explore the natural world from the inside out
01:12to see how the skeleton has enabled animals to move, hunt, and even sense the world.
01:23I will take you on a very personal journey to discover how this one bony blueprint has shaped such massive diversity across the animal kingdom.
01:32And how it has come to dominate life on planet Earth.
01:37This time, we'll discover the way the skeleton has adapted for vertebrates to move on land.
01:43For speed on the ground.
01:44You can see all these adaptations coming into one very sleek, fast animal right here.
01:49Agility in the treetops.
01:52And for moving underground.
01:54Driving one animal to evolve possibly the oddest bone in the natural world.
01:59What you can see instantly is just the weirdness of this bone.
02:12Although bone might seem like an unchanging and hard structure, to me it isn't that at all.
02:17Instead, it's a living, flexing, ever-changing framework that makes every single species just what it is.
02:24Bones have adapted in an enormous number of ways for movement on land.
02:35Animals can swing in the highest trees, slide along the forest floor, dig through subterranean worlds, and run at speed across the savannas.
02:47This is a story of survival.
02:49Each bone telling us how animals have evolved for locomotion.
02:53Allowing them to exploit any habitat on the surface of the Earth.
02:58Whenever I build a skeleton, any skeleton, I always start with a vertebrae.
03:03These are the bones of a gorilla, which I'm assembling to form the whole skeleton.
03:08The vertebrae themselves go together to make up the spine.
03:11And it's this spinal column which is shared by every single vertebrate on Earth.
03:15To really understand movement, the spine is where it all begins.
03:22It's the central support for the body, and it's also flexible.
03:27That's mainly down to the way these individual bones work together.
03:31If you look at these ones here, you can see that they have an incredible structure, allowing each one to perfectly interlock with the one before it.
03:42More than that, it allows also to interlock perfectly with the one behind, and so on, and so on.
03:47Now this is what gives the spine, the spinal column, this flexibility.
03:54And more than that, an incredible range of movement.
04:00So the spine gives rigidity, flexibility, and provides anchor points for muscles.
04:05It also protects a whole mass of nerves that need to run the length of the body.
04:11But whilst the spine might be the constant in all vertebrates, its structure vary significantly between species.
04:18And it's that change in structure which has had a dramatic effect on how those animals are able to move.
04:26Take the fastest animal on land, the cheetah, capable of speeds of nearly 70 miles an hour over short bursts.
04:38The secret to its speed is in the spine.
04:44Here we have a cheetah, which is hunting Thompson's gazelle in the Great Rift Plains of East Africa.
04:48Now these Thompson's gazelles, or Tommies as they're known, are incredibly fast and agile animals, and can turn and change direction almost in an instant.
05:04Cheetahs have had to evolve constantly throughout the millions of years in order to stand any chance of capturing this very agile prey.
05:11It's an evolutionary arms race, with each animal adapting to move in ways that give it an advantage.
05:18A finely balanced fight for survival.
05:22The cheetah can go from zero to 60 miles an hour in only three seconds.
05:28Truly phenomenal.
05:30If I just pause it here, if we look at the Tommy here, you can see this incredibly flat, straight, inflexible back.
05:37It's almost horizontal.
05:40Now compare this to the cheetah.
05:43This is a beautiful curve there.
05:47And it curves so much that the back legs and the front legs overlap to such an extent that it creates almost a spring motion and gives the cheetah a seven metre stride.
06:00And if I press play again, you can see the spine flexing and extending, giving the cheetah that huge stride length.
06:09And that's how it can reach those extraordinary top speeds.
06:13The cheetah's spine is so flexible because the joints are simple and open, allowing for a wider range of movement.
06:20And a flexible spine also means the cheetah can change direction suddenly, helping to make it one of the most successful hunters of all the big cats.
06:34In some animals, the vertebrae have adapted to the extreme.
06:46And the spine is practically the only thing left to generate movement.
06:52This is a milk snake, and like all snakes, it has one of the simplest skeletons in the animal kingdom.
07:02If I put him down now, then he decides to move.
07:10Come on.
07:11Then you can see straight away that beautiful S curve here as the snake moves.
07:22Now this is serpentine movement or undulatory locomotion, but how is the snake just so flexible?
07:29Snakes lost their limbs over 100 million years ago, and now they're essentially one long, very flexible spine with ribs.
07:39But unlike our vertebrae, which work together to allow movement backwards and forwards, the snake's vertebrae have evolved to work in a very different way altogether.
07:52Professor Susan Evans from University College London works on snake vertebrae.
07:58If we look at a couple of vertebrae here, you can see that what you've actually got is a ball at one end and a cup at the other.
08:06So you've effectively got a ball and socket joint.
08:09In the same way that we've got one in our shoulder and in our hips.
08:12Exactly.
08:13So is this what gives the snake the flexibility?
08:14Well, you'd think so, wouldn't you? Because you can rotate them very well.
08:18Mm-hm.
08:19But if the snake did that, with its spinal cord going through the middle, it would damage its spinal cord.
08:25Not a good idea.
08:26Not a good idea.
08:28To stop this happening, the snake vertebrae have evolved a double set of joints, which allow lateral or side-to-side movement.
08:35But stop the twisting so the snake can move with no harm to the spinal cord.
08:43These individual joints allow for some flexibility, but not as much as you might imagine when you see a snake move.
08:50What gives the snake its flexibility is not so much the individual joints between the vertebrae, but the fact that you've got so many vertebrae.
08:58So that if you take a little bit of a snake's spine like this, for example, you can see that although you just get a small amount of movement between individual vertebrae, when you multiply that by a length of several vertebrae, then you're getting that flexibility.
09:16So the key thing here is the repetition of the vertebrae, the increase in the number of vertebrae, up to 500 in a snake.
09:24Mm-hm.
09:25It's an amazing adaptation.
09:26It may seem like an obvious, maybe silly question, but why have snakes lost their legs?
09:31It may seem an obvious question, but it's actually one of the ones that's debated quite a lot.
09:35It's clearly a very adaptive shape, particularly if you're moving in small spaces, in confined spaces, or if you want to burrow.
09:44You can keep your body size relatively large, but it's now become very thin so that it can get into small spaces or it can burrow.
09:57By burrowing, or at least being able to crawl into tight spaces, snakes were able to avoid predators and exploit new sources of food underground.
10:09Their spine really is a fantastic adaptation, allowing them to travel practically everywhere.
10:17They can slither up trees, inch themselves along in a straight line, glide, one can even jump.
10:31But being limbless does have its limitations.
10:34Snakes can't move in the numerous, highly specialised ways of vertebrates with arms and legs.
10:47It's those limbs that really do allow animals to exploit every environment on land to its full potential.
10:55And all vertebrate limbs are based on the same ancestral blueprint.
10:59You can see here with the gorilla's forelimb, or its arm, that it's made up of several parts.
11:05You've got the one large bone here, the humerus, followed down to the two smaller bones, the radius and ulna.
11:11And in the hand you've got a group of bones here, the carpals.
11:14This then leads down into the five very distinct digits.
11:18It's called the pentadactyl limb, because each one ends in five digits.
11:23And it's the same basic pattern in the hind limb or leg.
11:28This time it's the femur, the tibia and fibula, bones in the feet and five toes.
11:34Any of my limbs, such as my arm, are exactly the same.
11:38The one bone, the two bones, the collection of little bones and then the five digits.
11:43As animals have evolved to move through every environment on earth, so this basic pentadactyl limb has adapted and specialised.
11:56Up in the trees, one animal has a limb that sets it apart from all other canopy dwellers.
12:15I think this is one of the most spectacular locomotors of them all.
12:18The gibbon.
12:22Acrobats of the primate world, perfectly adapted to life in the trees.
12:26First off, they've got these incredibly specialised hands with these very elongated fingers.
12:31They've got the same sort of thing in the feet as well.
12:33And this effectively makes the hands and feet really long grasping hooks,
12:38which is perfect if you're swinging through the canopy.
12:43They've also got these incredibly long arms.
12:44Now, they're so long that they're one and a half times the length of their own legs.
12:49This is actually not that unusual for an animal which is arboreal.
12:53What really sets them apart is their special way of moving, called brachiation,
12:58using just their arms to swing through the canopy.
13:01In this way, they can reach speeds of 35 miles an hour.
13:06And one of the reasons the gibbon can do this is down to a particular part of the pentadactyl limb, the wrist.
13:14We can't rotate our hands at the wrist joint at all.
13:17Any twisting comes from movement in our forearms.
13:21But the gibbon has a ball and socket-like joint, allowing it to rotate its hand at the wrist joint by 80 degrees.
13:28This adaptation means the gibbon can turn its body as it swings, building up momentum to propel it through the trees without losing its grip on the branches.
13:37Having this specialised type of joint allows the gibbon not only to save loads of energy, it makes it incredibly flexible and ultimately makes it almost limitlessly agile.
13:47By moving in this fast, efficient way, the gibbon can cover a huge territory, a great advantage to an animal whose food is usually dispersed over a wide area.
13:59So it's the specialised wrist joint of the gibbon which gives us the clue that it's such a remarkable locomotor.
14:13And every individual bone of the pentadactyl limb, its shape, its size, its weight, can tell us so much about how that animal has evolved and in particular how it moves.
14:30I've got three bones here from three very different animals.
14:36Now these are actually all the same bone, they're the humerus, the largest bone in the upper limb.
14:41And what these bones really tell me is everything about the animal's locomotion, so how they move, how they get about.
14:48The first one is this thing here. Now this is from a cow, and as you'd expect, it's very large, robust, heavy and stocky.
14:55Now cows can weigh up to about 500 kilos, that's a lot of animal.
15:00And because you don't see cows gracefully running down the street, instead they're heavy, bulky things and they need big, heavyweight bones in order to support this weight.
15:11On the opposite end of the scale you've got something like this.
15:14Now this is long, slender, thin, gracile, graceful humerus.
15:19Now this is actually from a human and fits around here somewhere.
15:23And unlike the cow, we're not four-legged, so we don't weight-bear on our forelimbs.
15:29And then we get this little thing. Now this is the humerus of a mole.
15:32And it doesn't actually look like a humerus at all, it looks like a tooth.
15:35Now, because it's quite hard to see, I've actually scaled one up.
15:39I've had a 3D print made, which is ten times the size of the real mole humerus, so that this is now comparable to the human and cow bone.
15:48And what you can see instantly is just the weirdness of this bone.
15:53And that's because there are so many special adaptations for the underground lifestyle that the mole has.
15:57The bone is very squat, very short, very flat, what we call spatulate.
16:01And this allows the whole forelimb to act like a paddle.
16:05But more importantly than that, you can see these incredible projections here all over the side of the humerus.
16:10Now, having a larger surface area and having all these little projections and grooves and flanges and holes really allows for much larger muscle attachments and ultimately much stronger muscle attachments as well.
16:23And this is a perfect adaptation for a mole, which spends its entire life tunnelling underground.
16:34This European mole is able to move its own body weight in soil every minute, searching for worms, beta larvae and slugs to eat.
16:44Each mole has its own tunnel network, sometimes over 100 metres long.
16:50They really are super-powered burrowers.
16:55But their ability to dig isn't just down to their oddly shaped humerus.
17:00There's an adaptation to the hand of the mole, which has been puzzling scientists for years.
17:06I absolutely love mole hands. They're very personal to me, actually.
17:11I was given one when I was about three by my granddad. He used to be a mole catcher.
17:16And I kept it with me for ages in a matchbox.
17:19But there's something I know now that I didn't know 30 odd years ago.
17:23And that's that they have something that resembles an extra digit.
17:28That's strange, because as far as we know, no living species normally has more than the five digits of the pentadactyl limb.
17:37When you look at an x-ray of the mole hand, it starts to become clear what's going on.
17:42Now you can see really clearly they've got these five distinct digits.
17:47Each one made up of lots of little bones, just like my hand.
17:51But then stuck on the end is this whacking great bone here.
17:55It's a solid piece of bone that sits on the side of the hand.
17:58And whereas these five are true digits, this thing here looks like an imposter.
18:06Scientists recently found out that this imposter grows from a sesamoid bone in the mole's wrist.
18:16Sesamoid bones are found where a tendon passes over a joint.
18:21The kneecap, for instance, is a sesamoid bone.
18:24They both protect the joint and increase tension in the tendon, making movement much more effective.
18:32This sesamoid bone has evolved to massively increase the surface area of the mole's hand, allowing it to dig through the soil much more effectively.
18:44The mole is not alone in using a sesamoid bone for other purposes.
18:51The elephant has also co-opted one to act like an extra toe in its foot.
18:57By studying the fossil evidence, scientists have worked out that this evolved when elephants were getting larger, becoming more land-based and needing the additional support.
19:09All these pentadacta limbs have been modified for movement on land.
19:17But there's one animal which has taken that adaptation to the extreme.
19:22Here we've got a horse's forelimb.
19:30This equates to being the same series of bones that I have in my arm here.
19:34When you have a look at it, you think, yeah, I probably know where most of these bones are.
19:38It sounds reasonable to say this is probably the shoulder area.
19:41It's pretty much there, isn't it?
19:42And then you look down, this is probably the elbow.
19:45I guess this must be the wrist.
19:48But you're wrong.
19:49If you have a good look, you can actually see that this is the shoulder area.
19:52It means this is the elbow.
19:54And then this is the wrist.
19:56Now that means that from here on down, this is all hand and digit.
20:01But the bones haven't just become longer.
20:04Below the elbow, they've reduced in number as well.
20:07If you look at an area such as the radius and ulna, you can see it's got a very large, prominent radius here.
20:14But when you look for the ulna, it's this little projection that sticks on the back.
20:18It's still functional, but it actually fuses into the body of the radius.
20:23Now you've also got the same sort of thing happening in this area here.
20:26This is the cannon bone, which is the equivalent of this little bone that sits in the middle of my hand here.
20:32It's technically called metacarpal number three.
20:35Rolls off the tongue, doesn't it?
20:36So where are the others?
20:37Well, metacarpals two and four are actually here.
20:40As for metacarpals one and five, well, they've actually gone.
20:43The horse has evolved to lose these.
20:45And as you follow the cannon bone right to its end, you can see that the end of the limb itself finishes in this one digit.
20:52The rest have gone.
20:53So effectively, the horse is walking around on one toe or one finger on each leg.
20:59All of this reduction in the numbers of bones really serves to make the whole horse limb incredibly lightweight.
21:07Only the horse and its closest relatives, including the zebra and the donkey, have this adaptation with just one digit at the end of each limb.
21:16Lengthening and lightening the limb has meant horses can reach speeds of over 40 miles an hour.
21:26To really appreciate this wonder of evolution, I want to see the horse's limbs in action close up.
21:36Here, in the structure and motion laboratory at the Royal Veterinary College outside London, Professor John Hutchinson has been studying horse locomotion to understand more about how horse bones are adapted for speed.
21:51Why has a horse evolved to run just so quickly?
21:58Well, a horse has evolved as prey animals and certainly a prey animal needs to be fast to escape predators.
22:04So, a horse has just taken that to an extreme.
22:08You can see all these adaptations coming into one very sleek, fast animal right here.
22:14You absolutely can.
22:15That leg length is coming into play, the length in the stride and the lightning in the limbs,
22:20and the lightning in the limb enables the horse to swing that limb really, really fast and achieve a high stride rate.
22:27An animal's speed is the product of its stride length multiplied by its stride rate.
22:34To run faster, you need to increase one or the other.
22:39In most animals, if one of these elements is increased, the other one is compromised.
22:46The giraffe has a long stride length, but not a high stride rate.
22:54The horse has managed to increase both, with its long and light limbs, a combination which is thought to boost speed and efficiency.
23:09But these elongated limbs also have to cope with immense forces.
23:16When galloping, a horse often has just one hoof in contact with the ground.
23:22This effectively exerts around 600 kilograms of force on that one digit.
23:31John has been studying how the bones have adapted to deal with such forces.
23:37If we look inside the foot and look at the bones, which we can see here in an x-ray that I've taken,
23:43you can see how there are lots of little bones that move together and give a lot of flexibility.
23:48And that flexibility allows the bones to move with respect to one another and deform and handle a lot of weight.
23:54So when the foot hits the ground like we see here, this foot coming down, boom, you can see that juddering that provides a lot of shock absorption.
24:04The adaptations to the horse's pentadactyl limb show us what an extraordinary material bone really is.
24:13How it can be lengthened, lightened, moulded by the evolutionary drive for animals to move.
24:23And every skeleton has adapted to allow each animal to move in particular environments.
24:29If you know what to look for, these adaptations can reveal surprising stories.
24:44And there's no better example than with my mole.
24:46Dr. Nick Crumpton, a mammal expert from Cambridge University, has brought a different mole, native to South Africa, for comparison with my European version.
24:58This is one of my favourite animals. This is a golden mole.
25:02So it's quite similar to the European mole because they both live in very similar environments.
25:07Looking at the skeletons, we can kind of see that they have a skeleton adapted to a life under the ground.
25:13So they're quite small. They have almost like a tubular shaped body.
25:17They've got much, much larger fore limbs than hind limbs, exactly the same as your mole right here.
25:24And they also have these huge elongated scapulae, like the shoulder blades.
25:28Initially, they seem similar, but a closer look reveals that each one has evolved very differently.
25:37On your mole, you have that really strange shaped humerus.
25:41Yep.
25:42But on golden moles, it's still quite strange.
25:44It's quite robust, isn't it? Yeah.
25:45But it looks not as radically peculiar as you find in European moles.
25:49Instead, we find the ulna, one of these bones in our forearms here, that actually extends a lot further back.
25:57It does, doesn't it?
25:58Now, that part there, that's called the olecranon process.
26:00Right.
26:01Now, we have those as well. That's just like, it's pretty much an elbow.
26:04Very elbow, isn't it? Yeah.
26:05The muscle attaches to that olecranon process.
26:08And so if you have a sort of bar coming out the bottom of your arm and you pull on that with a muscle,
26:13that's going to whip your arm down really fast and really powerfully.
26:17Yeah. And that's fascinating because that's a completely different way of digging to your European moles.
26:23It's these variations in the bones between the two species that helped scientists make an astonishing discovery about their evolution.
26:32For hundreds of years, people thought that these guys were really closely related.
26:36But when we started using genetic techniques and molecular techniques in the 1990s especially,
26:42we actually found that they're really not closely related at all.
26:46So whereas the European moles are more closely related to shrews and hedgehogs,
26:52the golden mole is more closely related to elephants and manatees than it is any of those sorts of mammals.
26:59And this is a fantastic example of convergent evolution.
27:05So these things are really remarkably unrelated.
27:08Natural selection has favoured certain aspects, certain shapes of their anatomy.
27:14And it just so happens that they look so similar because looking like this means you can do a really good job of digging under the ground.
27:20So the challenge of moving through the various environments on land has meant that some skeletons have adapted in very similar ways,
27:30even though they have a completely different evolutionary heritage.
27:34And the way the skeleton, this extraordinary collection of bones, has adapted to move on land is just one reason I find bones endlessly fascinating.
27:52Be that the flexible spine of the cheetah, the beautifully elegant limb of the horse, or the bulky squat frame of the European mole with its specially adapted hand.
28:05It's meant that vertebrates have been able to move into the trees, the soil, and across the land to exploit those environments to their full potential.
28:16But that's not all. Next time, we'll look at how bones have also allowed vertebrates to make the most remarkable move of all.
28:24Into the air.
28:29Oh, wow. That's absolutely amazing.
28:32The biggest pterosaurs had a wingspan of over 10 metres.
28:37This bird can travel for 15,000 kilometres from the moment it leaves the ground until the moment it lands again.
28:54The biggest pterosaurs had all better in the distance.
28:55It's coming from leaves by and the next time.
28:56What makes this changeенно, the food-centricureterrorism incident?
28:57Well, at the next time you see party on the campus, this is a welcome could you suggest childcare doing.
29:01This man's регulation should be a beautiful idea, but it's something that can change the world.
29:06Earlier this time.
29:19It's been a6call здоровzej.