BBC Secrets of Bones s01e02 Episode Script

Down to Earth

Bones They offer structure support and strength.
But they have a much bigger story to tell.
Vertebrates may look very different on the outside, but one crucial thing unites them all the skeleton.
I'm Ben Garrod, an evolutionary biologist, with a very unusual passion.
This is unbelievable.
There are too many skeletons for me to look at all at once.
As a child I was fascinated by bones.
And now skeletons have become my life.
And I put them together for museums and universities all over the world.
I'm going to explore the natural world from the inside out to see how the skeleton has enabled animals to move .
.
hunt and even sense the world.
I will take you on a very personal journey to discover how this one bony blueprint has shaped such massive diversity across the animal kingdom and how it's come to dominate life on planet Earth This time, we'll discover the way the skeleton has adapted for vertebrates to move on land.
For speed on the ground You can see all these adaptations coming into one very sleek, fast animal right here.
Agility in the treetops And for moving underground, driving one animal to evolve, possibly, the oddest bone in the natural world.
What you can see, instantly, is just the weirdness of this bone.
Although bone might seem like an unchanging and hard structure, to me it isn't that at all.
Instead it's a living, flexing, ever-changing framework that makes every single species just what it is.
Bones have adapted in an enormous number of ways for movement on land.
Animals can swing in the highest trees, slide on the forest floor, dig through subterranean worlds and run at speed across the savannahs.
This is a story of survival.
Each bone telling us how animals have evolved for locomotion, allowing them to exploit any habitat on the surface of the earth.
Whenever I build a skeleton, any skeleton, I always start with the vertebrae.
These are the bones of a gorilla which I'm assembling to form the whole skeleton.
The vertebrae themselves go together to make up the spine and it's this spinal column which is shared by every single vertebrate on earth.
To really understand movement, the spine is where it all begins.
It's the central support for the body and it's also flexible.
That's mainly down to the way these individual bones work together.
If 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.
More than that, it allows also to interlock, perfectly, with the one behind and so on and so on.
This is what gives the spine, the spinal column, this flexibility.
And more than that, an incredible range of movement.
So the spine gives rigidity, flexibility, and provides anchor points for muscles.
It also protects a whole mass of nerves that need to run the length of the body.
But whilst the spine might be the constant in all vertebrates, its structure varies significantly between species.
And it's that change in structure which has had a dramatic effect on how those animals are able to move.
Take the fastest animal on land - the cheetah, capable of speeds of nearly 70mph over short bursts.
The secret to its speed is in the spine.
Here we have a cheetah which is hunting Thomson's gazelle in the Great Rift Plains of East Africa.
These Thomson's gazelles or tommies, as they're known, are incredibly fast and agile animals and can turn and change direction, almost in an instant.
Cheetahs have had to evolve constantly throughout the millions of years in order to stand any chance of capturing this very agile prey.
It's an evolutionary arms race with each animal adapting to move in ways that give it an advantage.
A finely balanced fight for survival.
The cheetah can go from 0 to 60mph in only three seconds.
Truly phenomenal! If I just pause it here, if we look at the tommie here, you can see this incredibly flat, straight, inflexible back.
It's almost horizontal.
Now compare this to the cheetah.
This is a beautiful curve there.
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.
And if I press play again, you can see the spine flexing and extending, giving the cheetah that huge stride length.
And that's how it can reach those extraordinary top speeds.
The cheetah's spine is so flexible because the joints are simple and open, allowing for a wider range of movement.
And 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.
In some animals, the vertebrae have adapted to the extreme .
.
and the spine is practically the only thing left to generate movement.
This is a milk snake, and like all snakes, it has one of the simplest skeletons in the Animal Kingdom.
If I put him down now Then if he decides to move! Come on.
Then you can see straight away that beautiful, S-curve here as the snake moves.
Now this is seven-time movement or undulatory locomotion.
But how is the snake just so flexible? Snakes lost their limbs over 100 million years ago and now they're essentially one long, very flexible spine with ribs.
But unlike our vertebrae, which work together to allow movement backwards and forwards, the snakes vertebrae have evolved to work in a very different way altogether.
Professor Susan Evans from University College London works on snake vertebrae.
If 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.
We've effectively got a ball and a socket joint.
In the same way that we've got one in our shoulder and our hips? Exactly.
So is this what gives the snake the flexibility? You would think so because you can rotate them very well.
But if the snake did that, with its spinal cord going through the middle, it would damage its spinal cord.
Not a good idea.
Not a good idea.
To stop this happening, the snake vertebrae have evolved a double set of joints which allow lateral or side-to-side movement, but stop the twisting so the snake can move with no harm to the spinal cord.
These individual joints allow for some flexibility but not as much as you might imagine when you see a snake move.
What 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.
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 the individual vertebrae, when you multiply that by a length of several vertebrae, then you're getting that flexibility.
The key thing here is the repetition of the vertebrae, the increase in number of vertebrae, up to 500 in a snake.
Mm-hm.
It's an amazing adaptation.
It may seem like an obvious, maybe silly question, but why have snakes lost their legs? It may seem an obvious question but it's actually one of the ones that's debated quite a lot.
It's clearly a very adaptive shape, particularly if you're moving in small spaces, in confined spaces, or if you want to burrow, you 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.
By burrowing, or at least being able to crawl into tight spaces, snakes were able to avoid predators and exploit new sources of food underground.
Their spine really is a fantastic adaptation - allowing them to travel practically everywhere.
They can slither up trees .
.
inch themselves along in a straight line .
.
glide.
One can even jump.
But being limbless does have its limitations.
Snakes can't move in the numerous, highly specialised ways of vertebrates with arms and legs.
It's those limbs that really do allow animals to exploit every environment on land to its full potential.
And all vertebrate limbs are based on the same ancestral blueprint.
You can see here with the gorilla's fore limb, or its arm, that it's made up of several parts.
You've got the one large bone here, the humerus, follow down to the two smaller bones, the radius and ulna and in the hand you've got a group of bones here, the carpals.
This then leads down into the five very distinct digits.
It's called the pentadactyl limb because each one ends in five digits.
And it's the same basic pattern in the hind limb or leg, this time it's the femur, the tibia and fibula, bones in the feet and five toes.
Any of my limbs, such as my arm, are exactly the same.
The one bone, the two bones, the collection of little bones and the five digits.
As animals have evolved to move through every environment on earth, so this basic pentadactyl limb has adapted and specialised.
Up in the trees, one animal has a limb that sets it apart from all other canopy dwellers.
I think this is one of the most spectacular locomotors of them all.
The gibbon.
Acrobats of the primate world, perfectly adapted to life in the trees.
First off, they've got these incredibly specialised hands with elongated fingers.
They've got the same sort of thing in their feet and this, effectively, makes the hands and feet really long, grasping hooks, which is perfect if you're swinging through the canopy.
They've also got these incredibly long arms.
They're so long that they are 1.
5 times the length of their own legs.
This is actually not that unusual for an animal which is arboreal.
What really sets them apart is their special way of moving called brachiation, using just their arms to swing through the canopy.
In this way, they can reach speeds of 35mph.
One of the reasons the gibbon can do this is down to a particular part of the pentadactyl limb, the wrist.
We can't rotate our hands at the wrist joint at all.
Any twisting comes from movement in our forearms.
But the gibbon has a ball and socket-like joint allowing it to rotate its hand at the wrist joint by 80 degrees.
This 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.
Having 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.
By 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.
So it's the specialised wrist joint of the gibbon which gives us the clue that it's such a remarkable locomotor.
And 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.
I've got three bones here from three very different animals.
These are actually all the same bone, they're the humerus, the largest home in the upper limb and what these bones really tell me is everything about the animals' locomotion, so how they move, how they get about.
The first one is this thing here.
This is from a cow.
As you would expect, it's very large, robust, heavy and stocky.
Cows can weigh up to 500kg, that's a lot of animal.
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.
On the opposite end of the scale, you've got something like this.
This is a long, slender, thin, graceful humerus.
This is actually from a human and fits around here somewhere.
Unlike the cow, we're not four-legged so we don't weight bear on our fore limbs.
Then we get this little thing.
This is the humerus of a mole and it doesn't actually look like a humerus at all, it looks like a tooth.
Because it's quite hard to see, I've actually scaled one up.
I'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.
What you can see instantly is just the weirdness of this bone.
That's because there are so many special adaptations for the underground lifestyle the mole has.
The bone is very squat, very short, very flat, what we call spatulate.
This allows the whole fore limb to act like a paddle.
More importantly, you can see these incredible projections here, all over the side of the humerus.
Having a larger surface area, and having all these little projections and grooves and flanges and holes, really allows for much larger muscle attachment and ultimately much stronger muscle attachments, as well.
This is a perfect adaptation for a mole which spends its entire life tunnelling underground.
This European mole is able to move its own body weight in soil every minute, searching for worms, beetle larvae and slugs to eat.
Each mole has its own tunnel network, sometimes over 100 metres long.
They really are super-powered burrowers.
But their ability to dig isn't just down to their oddly shaped humerus.
There's an adaptation to the hand of the mole which has been puzzling scientists for years.
I absolutely love mole hands.
They are very personal to me, actually.
I was given one when I was about three from my granddad.
he used to be a mole catcher.
I kept her with me for ages in a matchbox There's something I know now that I didn't know, 30 odd years ago.
That's that they have something that resembles an extra digit.
And that's strange because, as far as we know, no living species normally has more than the five digits of the pentadactyl limb.
When you look at a X-ray of the mole hand it starts to become clear what's going on.
You can see really clearly they've got these five distinct digits.
Each one made up of lots of little bones, just like my hand.
But then stuck on the end, there is this whacking great bone here.
It's a solid piece of bone that sits on the side of the hand.
Whereas these five are true digits, this thing here looks like an impostor.
Scientists recently found out that this impostor grows from a sesamoid bone in the mole's wrist.
Sesamoid bones are found where a tendon passes over a joint, the kneecap, for instance, is a sesamoid bone.
They both protect the joint and increase tension in the tendon, making movement much more effective.
This 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.
The mole is not alone in using a sesamoid bone for other purposes.
The elephant has also co-opted one to act like an extra toe in its foot.
By 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.
All these pentadactyl limbs have been modified for movement on land, but there's one animal which has taken that adaptation to the extreme.
Here we've got a horse's fore limb.
This equates to being the same series of bones that I have in my arm here.
When you have a look at it, you think, yeah, I probably know where most of these bones are.
It sounds reasonable to say this is the shoulder area.
It's pretty much there, isn't it? Then you look down, this is probably the elbow.
I guess this must be the wrist.
But you're wrong.
If you have a good look, you can see that this is the shoulder area.
It means this is the elbow and this is the wrist.
That means from here on down, this is all hand and digit.
But the bones haven't just become longer.
Below the elbow, they've reduced in number as well.
If you look at an area such as the radius and ulna, you can see it has got a very large prominent radius here.
When you look for the ulna, it's this little projection that sticks on the back.
It's still functional but it actually fuses into the body of the radius.
You've also got the same sort of thing happening in this area here.
This is the cannon bone, which is the equivalent of this little bone that sits in the middle of my hand here.
It's technically called metacarpal number three - rolls off the tongue, doesn't it? So where are the others? Well, metacarpals two and four are here.
As for metacarpals one and five, they've actually gone The horse has evolved to lose these.
As you follow the cannon bone right to its end, you can see the end of the limb itself finishes in this one digit.
The rest have gone.
Effectively, the horse is walking around on one toe, or one finger, on each leg.
All of this reduction in the numbers of bones really serves to make the whole horse limb incredibly lightweight.
Only 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.
Lengthening and lightening the limb has meant horses can reach speeds of over 40mph.
To really appreciate this wonder of evolution, I want to see the horse's limbs in action, close up.
Here 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.
Why has a horse evolved to run just so quickly? Well, horses evolved as prey animals, and certainly a prey animal needs to be fast to escape predators, so a horse has just taken that to an extreme.
You can see all these adaptations coming into one very sleek, fast animal right here.
You absolutely can.
That leg length is coming into play to lengthen the stride, and the lightening in the limb enables the horse to swing that limb really fast and achieve a high stride rate.
An animal's speed is the product of its stride length multiplied by its stride rate.
To run faster you need to increase one or the other.
In most animals, if one of these elements is increased, the other one is compromised.
The giraffe has a long stride length but not a high stride rate.
The horse has managed to increase both, with its long and light limbs, a combination which is thought to boost speed and efficiency.
But these elongated limbs also have to cope with immense forces.
When galloping, a horse often has just one hoof in contact with the ground.
This effectively exerts around 600kg of force on that one digit.
John has been studying how the bones have adapted to deal with such forces.
If we look inside the foot and look at the bones, which we can see here in an X-ray that I've taken, you can see how there are lots of little bones that move together and give a lot of flexibility, and that flexibility allows the bones to move with respect to one another and deform and handle a lot of weight, so that 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.
The adaptations to the horse's pentadactyl limb show us what an extraordinary material bone really is.
How it can be lengthened, lightened, moulded by the evolutionary drive for animals to move.
And every skeleton has adapted to allow each animal to move in particular environments.
If you know what to look for, these adaptations can reveal surprising stories.
And there's no better example than with my mole.
Dr Nick Crumpton, a mammal expert from Cambridge University, has brought a different mole, native to South Africa, for comparison with my European version.
This is one of my favourite animals, this is a golden mole.
Yeah.
It's quite similar to the European mole Mm-hm.
.
.
cos they both live in very similar environments.
Looking at their skeletons, we can kind of see that they have a skeleton adapted to a life under the ground.
So they're quite small, they have almost like a tubular-shaped body.
They've got much, much larger fore limbs than hind limbs, exactly the same as your mole right here.
Yep.
And they also have these huge, elongated scapulae, like the shoulder blades.
Initially they seem similar.
But a closer look reveals that each one has evolved very differently.
On your mole, you have that really strange-shaped humerus.
Yep.
But on golden moles, it's still fairly strange, but it looks not as radically peculiar as you find in European moles.
Instead, we find an ulna, one of these bones in our forearms here, that actually extends a lot further back.
It does, doesn't it? That part there, that's called the olecranon process.
Right.
We have those as well, that's just like It's pretty much our elbow.
Elbow isn't it, yeah.
The muscle attaches to that olecranon process, and so if you have a sort of bar coming out of the bottom of your arm, and you pull on that with a muscle, that's going to whip your arm down really fast and powerfully.
And that's fascinating because that's a completely different way of digging to your European moles.
It's these variations in the bones between the two species that helped scientists make an astonishing discovery about their evolution.
For hundreds of years, people thought that these guys were really closely related.
But when we started using genetic and molecular techniques, in the 1990s, especially, we actually found that they're really not closely related at all.
So whereas the European moles are more closely related to shrews and hedgehogs Yep.
.
.
the golden mole is more closely related to elephants and manatees than it is any of those sorts of mammals.
And this is a fantastic example of convergent evolution.
So these things are really, remarkably unrelated.
Natural selection has favoured certain aspects, certain shapes of their anatomy, and 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.
So the challenge of moving through the various environments on land has meant that some skeletons have adapted in very similar ways, even though they have a completely different evolutionary heritage.
And the way the skeleton, this extraordinary collection of bones, has adapted to move on land, is just one reason I find bones endlessly fascinating.
Be 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.
It'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.
But that's not all.
Next time we'll look at how bones have also allowed vertebrates to make the most remarkable move of all into the air.
Oh, wow, that's absolutely amazing! The biggest pterosaurs had a wingspan of over ten metres.
This bird can travel for 15,000kms from the moment it leaves the ground until the moment it lands again.