Episode 2 Wonders of Life

Every living thing that we know to exist is found on this one rock...

..it became a home to life almost four billion years ago,

and today hosts an incredibly diverse natural world...

..in this programme, I want to show you how living things

evolved some of their most important abilities...

..and how the laws of physics govern the lives of all things -

from the very biggest...

to the very smallest.

These are the Mammoth Caves, in Kentucky.

With over 300 miles of mapped passages,

they're the longest cave system in the world...

..but this is also the place to start exploring our own senses.

We're normally dependent on our sight

but down here, in the darkness, it's a very different world

and I have to rely on my other senses

to build a picture of my environment.

Now, it's...

completely dark in this cave.

I can't see anything...at all.

You can see me because we're lighting it with infrared light

and that's a wavelength that my eyes are completely insensitive to.

So, as far as I'm concerned, it is pitch black.

And because it's so dark...

..your other senses become heightened - particularly hearing...

..it's virtually silent in here...

..but, if you listen carefully,

you can just hear the faint drop

of water, from somewhere deep in the cave system.

You'd never hear that if the cave were illuminated...

but you focus on your hearing when it's as dark as this.

Now, as well as sight and hearing,

we have, of course, a range of other senses.

There's touch, which is really a mixture of sensations,

temperature, and pressure, and pain.

And then there are chemical senses - so, smell and taste -

and we share those senses with almost every living thing

on the planet today

because they date back virtually to the beginning of life on earth.

And even here, in water that's been collected from deep within a cave,

there are organisms

that are detecting and responding to their environment

in the same way that living things have been doing

for over a billion years.

Ah...

..there it is.

That is a paramecium.

It may look like a simple animal

but, in fact, it's a member of a group of organisms called protists

and you'd have to go back around two billion years

to find a common ancestor between me and the paramecium.

Paramecia have probably changed little in the last billion years...

..and, although they appear simple,

these tiny creatures display some remarkably complex behaviour.

You can even see them responding to their environment...

..the cell swims around, powered by cohorts of cilia -

tiny hairs embedded in the cell membrane.

If it bumps into something,

the cilia change direction and it reverses away...

..they're clearly demonstrating a sense of touch.

Even though they're single-celled organisms,

they have no central nervous system,

they can still do what all life does -

they can sense their environment and they can react to it,

and they do that using electricity.

By manipulating the number of positive ions

inside and outside its membrane,

the paramecium creates a potential difference of 40 millivolts.

When it bumps into something its cell membrane deforms,

opening channels that allow positive ions to flood back

across the membrane...

..as the potential difference falls, it sets off an electrical pulse

that triggers the cilia to start beating in the opposite direction.

That electrical pulse spreads around the whole cell in a wave,

called an action potential...

..and the paramecium reverses out of trouble.

Now, this ability to precisely control flows of electric charge

across a membrane is not unique to the paramecium,

it actually lies at the heart of all animal senses.

In fact, every time I sense anything in the world -

with my eyes, with my ears or with my fingers -

at some point between that sensation and my brain

something very similar to that will happen.

Sight is our dominant sense...

..almost all animals can see.

In fact, 96% of animal species have eyes.

And what's interesting is that, at the molecular level,

every eye in the world works in the same way.

In order to form an image of the world,

then, obviously, the first thing you have to do is detect light...

and...I have a sample, here, of the molecules that do that,

that detect light in my eye.

It's actually specifically the molecule

that's in the black and white receptor cells in my eyes, the rods.

It's called rhodopsin

and the moment I expose this to light

you'll see an immediate physical change.

There you go!

Did you see that? It was very quick.

It came out very pink indeed and it immediately went yellow.

This subtle shift in colour

is caused by the rhodopsin molecule changing shape

as it absorbs the light.

In my eyes, what happens is, that change in structure

triggers an electrical signal,

which ultimately goes all the way to my brain,

which forms an image of the world.

It's this chemical reaction that's responsible

for all vision on the planet.

Closely related molecules lie at the heart of every animal eye...

..and that tells us that this must be a very ancient mechanism.

To find its origins, we must find a common ancestor

that links every organism that uses rhodopsin today.

We know that common ancestor must have lived

before all animals' evolutionary lines diverged...

..but it may have lived at any time before then.

So what is that common ancestor?

Well, here's where we approach the cutting edge of scientific research.

The answer is that we don't know for sure

but a clue might be found...here...

..in these little green blobs,

which are actually colonies of algae, algae called volvox.

We have very little in common with algae -

we've been separated, in evolutionary terms,

for over a billion years...

..but we do share one surprising similarity.

These volvox have light-sensitive cells that control their movement...

..and the active ingredient in those cells is a form of rhodopsin,

so similar to our own

that it's thought they may share a common origin.

'What does that mean?'

Does it mean that we share a common ancestor with the algae

and in that common ancestor the seeds of vision can be found?

To find a source that may have passed this ability to detect light

to both us and the algae,

we need to go much further back down the evolutionary tree...

..to organisms like cyanobacteria...

..they were among the first living things to evolve on the planet

and it's thought that the original rhodopsins

may have developed in these ancient photosynthetic cells.

So, the origin of my ability to see

may have been well over a billion years ago,

in an organism as seemingly simple as a cyanobacterium.

The basic chemistry of vision

may have been established for a long time

but it's a long way from that chemical reaction

to a fully functioning eye that can create an image of the world.

'The eye is a tremendously complex piece of machinery'

built from lots of interdependent parts

and it seems very difficult to imagine how that could have evolved

in a series of small steps

but, actually, we understand that process very well indeed.

I can show you by building an eye.

The first step in building an eye

would be to take some kind of light-sensitive pigment -

rhodopsin, for example -

and build it onto a membrane.

So, imagine this is such a membrane with the pigment cells attached.

Then immediately you have something that can detect the difference

between dark and light.

But the disadvantage, as you can see,

is that there's no image formed, at all -

it just allows you to tell the difference between light and dark.

But you can improve that a lot by adding...

an aperture, a small hole in front of the retina.

So this is a movable aperture,

just like the type of thing you've got in your camera.

Now, you'll see that the image gets sharper...

..but the problem is that, in order to make it sharper,

you have to narrow down the aperture,

and that means that you get less and less light.

So this eye becomes less and less sensitive.

So there's one more improvement that nature made,

which is to replace the pinhole, the simple aperture...

..with a lens.

Look at that!

A beautifully sharp image.

The lens is the crowning glory of the evolution of the eye...

..by bending light onto the retina, it allows the aperture to be opened,

letting more light into the eye,

and a bright, detailed image is formed.

Our brain's ability to process the information in that image

completes our visual system...

..and allows us to respond to the world around us.

As well as sight, we use another sense - hearing -

to build up a picture of the world,

and it too has an ancient evolutionary past.

The story of how we developed our ability to hear

is one of the great examples of evolution in action...

..because the first animals to crawl out of the water onto the land

would have had great difficulty hearing anything

in their new environment.

These are the Everglades...

a vast area of swamps and wetlands

that has covered the southern tip of Florida for over 4,000 years.

Through the creatures we find here,

like the American alligator, a member of the crocodile family,

we can trace the story of how our hearing developed

as we emerged onto the land.

These are the smallest three bones in the human body.

They're called the malleus, the incus and the stapes,

and they sit between the eardrum and the entrance to your inner ear,

so the place where the fluid sits.

The bones help to channel sound into the ear through two mechanisms...

..first, they act as a series of levers,

magnifying the movement of the eardrum...

..and second, because the surface area of the eardrum

is 17 times greater than the footprint of the stapes,

the vibrations are passed into the inner ear with much greater force.

And that has a dramatic effect.

Rather than 99.9% of the sound energy being reflected away,

it turns out that with this arrangement, 60% of the sound energy

is passed from the eardrum into the inner ear.

Now, this set up is so intricate and so efficient

that it almost looks as if these bones

could only ever have been for this purpose

but, in fact, you can see their origin

if you look way back in our evolutionary history.

Back around 530 million years,

to when the oceans were populated with jawless fish called agnathans -

they're similar to the modern lamprey.

Now, they didn't have a jaw

but they had gills, supported by gill arches.

Now, over a period of around 50 million years

the most forward of those gill arches

migrated forward in the head...

..to form jaws.

And you see fish like these, the first jawed fish,

in the fossil record around 460 million years ago.

And there, at the back of the jaw, there is that bone,

the hyomandibula, supporting the rear of the jaw.

Then, around 400 million years ago,

the first vertebrates made the journey from the sea to the land.

Their fins became legs.

But in their skull and throat other changes were happening -

the gills were no longer needed

to breathe the oxygen in the atmosphere and so they faded away,

and became different structures in the head and throat,

and that bone, the hyomandibular,

became smaller and smaller...

until its function changed.

It now was responsible for picking up vibrations in the jaw

and transmitting them to the inner ear of the reptiles.

And that is still true today, of our friends...over there.

But, even then, the process continued.

Around 210 million years ago, the first mammals evolved,

and, unlike our friends, the reptiles, here,

mammals have a jaw that's made of only one bone.

A reptile's jaw is made of several bones fused together.

So that freed up two bones...

which moved...and shrank...

..and eventually...

became the malleus, the incus and the stapes.

So, this is the origin of those three tiny bones

that are so important to mammalian hearing.

He's quite big, isn't he?

So, the evolution of our senses

can be closely linked to changing environments.

However, life is not only shaped by its surroundings...

..there are limitations to form and function..

imposed by the fundamental forces of nature.

..and you can clearly see them at work

when you examine the size of life.

Our world is covered in giants...

..the largest things that ever lived on this planet

weren't the dinosaurs.

They're not even blue whales - they're trees.

These are mountain ash,

they're the largest flowering plant in the world.

They grow about a metre a year, and these trees are 60, 70,

even 80 metres high.

But to get this big you need to face some very significant

physical challenges.

These giants can live to well over 300 years old...

..but they don't keep growing for ever...

..there are limits to how big each tree can get.

As with all living things,

the structure, form and function of these trees has been shaped

by the process of evolution, through natural selection

but evolution doesn't have a free hand.

It is constrained by the universal laws of physics.

Each tree has to support its mass

against the downward force of Earth's gravity...

..at the same time, the trees rely on the strength

of the interactions between molecules

to raise a column of water from the ground

up to the leaves in the canopy.

And it's these fundamental properties of nature

that act together to limit the maximum height of a tree,

which, theoretically, lies somewhere in the region of 130 metres.

Gravity doesn't just influence how tall a plant can grow...

..it also affects how big animals can get.

To show you how, I've come to track down

one of Australia's most iconic animals - the red kangaroo.

Red kangaroos are Australia's largest native land mammal.

The evolution of the ability to hop

gives kangaroos a cheap and efficient way to get around

but not everything can move like a kangaroo.

'The red kangaroo is the largest animal in the world

'that moves in this unique way -'

hopping across the landscape at high speed -

and there are reasons why there aren't, you know,

giant hopping elephants or dinosaurs,

and they're not really biological.

It's not down to the details of evolution by natural selection

or environmental pressures.

The larger an animal gets,

the more severe the restrictions on its body shape and its movement.

And it's gravity that imposes these restrictions.

To understand why this is the case,

I want to explore what happens to the mass of a body

when that body increases in size.

Take a look at this block.

Let's say it has width - one,

length - one and height - one,

and its volume is one.

Multiplied by one, multiplied by one,

which is one cubic...things,

whatever the measurement is.

Now, its mass is proportional to the volume,

so we could say that the mass of this block is one unit as well.

Let's say that we're going to double the size of this thing,

in the sense that we want to double its width,

double its length...

..and double its height.

Its volume is two, multiplied by two, multiplied by two,

equals eight cubic things.

Its volume is increased by a factor of eight

and so its mass is increased by a factor of eight as well.

So although I've only doubled the size of the blocks,

I've increased the total mass by eight.

As things get bigger,

the mass of the body goes up by the cube of the increase in size.

Because of this scaling relationship,

the larger you get, the greater the effect of gravity.

As things get bigger,

the huge increase in mass has a significant impact on the way large

animals support themselves against gravity and how they move about.

No matter how energy efficient

and advantageous it is to hop like a kangaroo,

as you get bigger it's just not physically possible.

So gravity limits how big life can get.

But it's not the physical force

that controls how small an animal can get.

And, in fact, the smaller you are, the less gravity affects your life.

This is the rhinoceros beetle, named for obvious reasons,

but, actually, it's only the males that have the distinctive

horns on their heads.

Gram for gram, these insects are among the strongest animals alive.

I can demonstrate that by just getting hold of the top of his head.

It doesn't hurt him at all...

but watch what he is able to do.

Look at that.

So he's hanging onto this branch,

which is many times his own bodyweight.

Absolutely no distress at all.

As things get smaller,

it's a rule of nature that they inevitably get stronger.

The reason is quite simple -

small things have relatively large muscles

compared to their tiny body mass...

and this makes them very powerful.

The beetles also appear to have a cavalier attitude

to the effects of gravity.

If they should fall...

..they just bounce and walk off.

If I fell a similar distance, relative to my size, I'd break.

So why does size make such a difference?

Time for a bit of fundamental physics.

All things fall at the same rate under gravity.

That's because they're following geodesics through curved space time

but that's not important.

The important thing for biology

is that although everything falls at the same rate,

it doesn't meet the same fate when it hits the ground.

A grape...bounces.

A melon...

..doesn't bounce.

Now the reasons for that are quite complex, actually.

First of all, the grape has a larger surface area,

in relation to its volume and therefore its mass,

than the melon.

And so, although in a vacuum, if you took away the air,

they would both fall at the same rate,

actually, in reality, the grape falls a bit slower than the melon.

Also, the melon is more massive

and so it has more kinetic energy when it hits the ground.

Remember, from physics class, kinetic energy is a half MV squared,

so you reduce M, you reduce the energy.

The upshot of that is that the melon has a lot more energy -

when it hits the ground it has to dissipate it in some way,

and it dissipates it by exploding.

The influence of Earth's gravity on your life

becomes progressively diminished the smaller you get.

However, having a larger surface area in relation to mass

doesn't mean that life is always easy for small organisms.

In fact, it can pose a brand new challenge...

..keeping warm.

These are southern bent-wing bats...

..one of the rarest bat species in Australia.

Every evening they emerge in their thousands from this cave

in order to feed.

When fully grown,

these bats are just five and a half centimetres long,

and weigh around 18 grams.

Because of their size, they face a constant struggle to stay alive.

We're using a thermal camera here to look at the bats

and you can see that they appear as streaks across the sky.

They appear as brightly as me,

that's because they're roughly the same temperature as me.

They're known as endotherms,

they're animals that maintain their body temperature,

and that takes a lot of effort.

I mean, these bats have to eat something like

three-quarters of their own bodyweight every night,

and a lot of that energy goes into maintaining their temperature.

As with all living things,

the bats eat to provide energy to power their metabolism.

Although, like us,

they have a high body temperature when they're active,

keeping warm is a considerable challenge on account of their size.

The bats lose heat mostly through the surface of their bodies...

..but because of simple laws governing the relationship

between the surface area of a body and its volume,

being small creates a problem.

So, let's look at our blocks again

but this time for surface area to volume.

Here's a big thing, it's made of eight blocks,

so its volume is eight units,

and its surface area is two by two on each side,

so that's four, multiplied by the six faces is 24.

So the surface area to volume ratio is 24 to eight,

which is three to one.

Now, look at a smaller thing.

This is one block, so its volume is one unit.

The surface area is one by one, by one, six times, so it's six.

So this has a surface area to volume ratio of six to one.

So, as you go from big to small,

your surface area to volume ratio increases.

Small animals, like bats, have a huge surface area

compared to their volume.

As a result, they naturally lose heat at a very high rate.

To help offset the cost of losing so much energy in the form of heat,

the bats are forced to maintain a high rate of metabolism.

They breathe rapidly, their little heart races,

and they have to eat a huge amount.

So a bat's size clearly affects the speed at which it lives its life.

Right across the natural world, the size you are

has a profound effect on your metabolic rate,

or your speed of life.

'Big animals have a much smaller surface area to volume ratio'

than small animals,

and that means that their rate of heat loss is much smaller.

And that means that there's an opportunity there for large animals.

They don't have to eat as much food to stay arm

and therefore they can afford a lower metabolic rate.

And there's one last consequence of all these scaling laws

that I suspect you'll care about more than anything else

and it's this - there's a strong correlation

between the effective cellular metabolic rate of an animal

and its lifespan.

In other words, as things get bigger they tend to live longer.

So, the physical forces of nature control life in all its sizes.

Size, in turn, can determine how much energy a body needs

and how long that body will last.

Every single organism on the planet has something in common...

..they've all been influenced by the environment

and shaped by the laws of physics...

..but I think the beauty of life

is that although the same processes apply to all organisms,

no two living things are the same...

..and the tree of life, with its myriad branches,

has spread to just about every corner of the planet.

So, when you go outside tomorrow,

just take a look at a little piece of your world

in a corner of your garden, or a park,

or even the grass that's growing in a crack in the pavement

because there will be life there and it will be unique.

There'll be nowhere like that anywhere else in the universe

and that makes our tree,

from the sturdiest branch to the most fragile twig,

indescribably valuable.

Subtitles by Red Bee Media Ltd