A Temperature for Life From Ice to Fire: The Incredible Science of Temperature

Everything around us exists somewhere on a vast scale,

from cold to hot.

The tiniest insect, all of us, the Earth, the stars,

even the universe itself, everything has a temperature.

I'm Dr Helen Czerski.

In this series, I'm going to unlock temperature's deepest mysteries.

Across three programmes, I'm going to explore the extremes of the

temperature scale.

From some of the coldest temperatures,

to the very hottest, and everything in between.

I'm a physicist, so my treasure map is woven from the fundamental

physical laws of the universe, and temperature is an essential part of that.

It's the hidden energy contained within matter...

..and the way that energy endlessly shifts and flows.

It's the architect that has shaped our planet...

..and the universe.

It's not often that I get up at 5am to watch a pond,

but this one is worth watching.

This time, I'm going to explore the narrow band of temperature that has

led to life.

From the origins of life in a dramatic place

where hot meets cold...

You're bringing together these chemical ingredients that could

start producing some of the building blocks of life.

..to the latest surgery that's using temperature to push the human

body to the very limits of survival.

Temperature is in every single story that nature has to tell,

and in this series, I'll be exploring why, what temperature means,

how it works, and just how deep its influence on our lives

and our world really is.

This is a Painted Lady butterfly and it's been kept cool,

at around six degrees, but as it sits in the sun,

it's warming itself up, fluttering its flight muscles...

..and getting ready to fly.

These insects can't control their own body temperature, so they're

reliant on heat from the sun.

And there he goes.

The butterfly's survival depends on its delicate relationship with temperature.

And that's true of every living thing on our planet,

plant or animal, large or small.

Everything depends on temperature for its existence.

And that relationship is as complex as it's profound.

On the road to all of this, all this colour and smell and

movement that's alive, there's a story,

and it's the story of the intricate dance of life along a tightrope

stretching from hot to cold.

There's only one place in the universe where we're absolutely

sure that life exists, and that is here on Earth,

but it's hard not to look up into the night sky and wonder what else

might be living out there.

And as astronomers started to learn about our solar system,

they looked at the other planets and wondered what might be living there.

Perhaps there are monsters on Venus or an entire civilisation on Mars.

Because after all, those planets seem to be in the same sort of

position as us - not so close to the sun that they got fried

and not so far away that they were frozen.

And that led to the concept of a habitable zone,

a distance from the sun that was just right for life.

But it turned out not to be that simple.

Our nearest neighbour, Venus, just a little closer to the sun,

has a surface temperature of over 450 degrees Celsius.

While on Mars, the next planet out, it's minus 60.

Temperatures far more extreme than Earth, making life impossible.

But even Earth's own temperature isn't what you might expect.

If you average out the temperatures across the planet,

you get a rather pleasant 14 degrees Celsius.

But that's around 30 degrees warmer than might be expected,

given the Earth's distance from the sun.

At 30 degrees colder, you'd expect Earth to be completely different.

A barren, desolate world.

So why is our planet warmer than it appears it should be?

The answer lies in one of the most intriguing substances to be found

anywhere in the universe.

SWELLING MUSIC PLAYS

This is the Skogafoss waterfall in Iceland.

Every day here, hundreds of millions of litres of water tumble down

towards the sea.

MUSIC PLAYS

More than 70% of Earth's surface is covered with water,

but that wasn't always the case.

Early in our planet's history,

when the surface was far too hot for liquid water,

this planet was shrouded in a thick atmosphere of carbon dioxide and

water and all you would've seen from space was the white cloud tops.

But as the planet cooled, the rains began and a deluge shifted most of

that water from the atmosphere to the oceans.

And then when the rain finished and the clouds cleared,

the liquid of our blue planet was on show to the universe for the first time.

Ever since, the sheer physical power of water has been carving and

shaping the surface of our planet.

And crucially for our story, all this water has had huge consequences

for the Earth's temperature.

To understand why, we need to delve into the strange

world of water at the molecular scale.

And that journey begins with a chance discovery that revealed for

the first time what water is actually made of.

In 1766, a reclusive scientist called Henry Cavendish,

added various metals to a liquid called spirits of salt,

now known as hydrochloric acid.

And what he saw was something that he called inflammable air,

but today we know as hydrogen.

And Cavendish was the first person to recognise its significance and

to do experiments on it to test its properties.

Cavendish collected the gas given off by his experiment.

When he had enough, he took a flaming splint and put it next to the opening...

BOOM

..with explosive results!

Afterwards, Cavendish noticed something intriguing.

On the inside of the glass vessel there were tiny droplets of a clear

liquid and he wondered what that was, he tasted it, he smelt it

and he came to the conclusion that it was water.

And so Cavendish was the first person to realise that water was a

combination of hydrogen and oxygen and today we know that the chemical

formula is H2O, two hydrogens and one oxygen.

And that sounds beautifully simple but still, water is one of the most

fascinating molecules we know of.

The molecular structure of water is the key to why Earth's temperature

is warmer than you might expect.

Yet it's in a cold place that I can begin to uncover why that is.

This is Jokulsarlon Lagoon in Iceland.

Isn't this all stunning?

All these bits of glacier that have just fallen off from up there.

We take scenes like this for granted.

This is our impression of the Arctic and the Antarctic,

floating icebergs,

but from a material science point of view this, that thing,

is really weird because it's floating.

With almost everything else, when you cool things down and freeze them,

the solid will sink to the bottom of the liquid but water is different.

It floats.

As a liquid, the molecules of water are constantly sliding past each

other, always on the move, but as it freezes,

their positions become fixed in a regular hexagonal lattice.

Ice floats because the molecules in the lattice are taking up more space

than in the liquid, which makes ice less dense than water.

This happens because of the forces holding the molecules in position.

Something I can more easily show you with water in its liquid state.

I've got some plastic pipe here and a proper Icelandic woolly jumper,

because it's made of wool and therefore it's good at charging up

the plastic.

So this pipe now has an electric charge and what I'm going to do...

..is put it near a stream of water.

And you can see that it bends the stream really strongly...

..and all the water is doing is falling but it's being pulled

towards the electric field.

The reason for this phenomenon lies within the water molecules themselves.

This is the water molecule, so we've got two Hs,

that's the H2 and then O is the oxygen at the top

and the charge on the molecule isn't evenly distributed,

so it's more positive around here and it's more negative up there.

So when the stream of water comes down,

it's got all these molecules moving round inside it.

When you bring the electrical field close, some of those molecules

will flip around so that their opposite charge is attracted in to

the electric field, so the whole stream of water moves.

It's such a simple demo but it shows you that the water molecule

itself has uneven charge distribution.

And this has a huge effect on how water behaves.

Within the liquid, the negatively charged oxygen atom from one

molecule is pulled towards the

positively charged hydrogen atoms of

another, creating a strong

attraction known as a hydrogen bond.

And these bonds are key to water's

influence on Earth's temperature.

Hydrogen bonds are so strong that it takes a lot of energy to break them.

And that means that the water in the Earth's oceans can absorb a huge

amount of heat energy from the sun without changing from a liquid to a gas.

The oceans act like a huge store of energy, and as they move they

distribute heat from the equator to cooler latitudes north and south.

But it's not only in the oceans that water plays a part in Earth's temperature.

The bonds between water molecules in liquid water are very strong but

provide enough energy and they'll break apart and then you get what's

all around me in the air here - water vapour.

And in this form, as vapour in the atmosphere,

water has perhaps its greatest influence.

The atmosphere traps the sun's heat,

a process known as the greenhouse effect.

But although we tend to associate this with carbon dioxide,

it's actually water vapour that accounts for much of the trapped heat.

I've got a thermal camera here,

and if I point it at the sand and the pebbles, what you can see is

that they're bright, they're radiating away energy.

And you can see it's just the surface because if I dig down a little way,

down in the hole, everything is very dark blue.

The red areas are warmer and what's happening is that they're emitting

infrared radiation.

So the sun heats up the surface and then because the surface is warm,

infrared radiation travels back up into the atmosphere.

Now, here's the thing.

The visible light went straight through the atmosphere,

but the infrared doesn't.

And one of the main things that stops it is water vapour.

The water molecules are able to bend and stretch in three different ways,

which allows them to absorb a lot of energy.

So as the infrared gets up into the atmosphere,

hits all those water molecules, some of it's absorbed,

and once it's been absorbed,

the important point is it isn't going straight up to space any more.

It then gets scattered in lots of different directions

and some of it comes back down to Earth.

It's a huge difference.

That invisible water vapour in the air is playing a huge role

in keeping us nice and warm.

Were it not for the water in the oceans

and the atmosphere keeping Earth's temperature warm and stable,

our planet would be as inhospitable as Venus or Mars.

But the influence of temperature on life goes far deeper

because the story of how life itself began is a story of temperature.

And it starts with Earth's complex geology.

This is the Gunnuhver vent in Iceland

and it's impossible to come here

and not wonder what's causing all of this.

What there is beneath my feet is a magma pool

and seawater is seeping in through cracks and fissures

and when it hits the hot rock, it boils.

And all of this is just the spout of a gigantic natural kettle.

This is a thermal vent.

It gives us a rare glimpse of the heat at the Earth's core.

But here, at the surface of the planet,

isn't the only place where such vents exist.

Similar vents can be found deep on the ocean floor

and even in this dark, inhospitable place,

many of them are teeming with life,

a profusion of organisms found in few other places on Earth.

It's a spectacle that Dr John Copley from the University of Southampton

has seen first-hand.

When you get a moment to pause and think,

you're struck by how you are next to

a truly awesome force of nature.

John is part of a research project

exploring the life that exists around these deep sea vents.

The stuff that is gushing out, what's in it?

That is a very hot mineral-rich fluid.

How hot? Well, these vents, 401 degrees C.

-Which is enormous! Enormous!

-Yeah, and it's still liquid.

It doesn't boil into steam because of the pressure.

Because we're at 500 times atmospheric pressure,

it's still liquid,

and it's mineral rich because that hot fluid is the end product

of seawater percolating down into the ocean crust.

There it's reacting with the surrounding rocks

and it's leaching a lot of minerals and elements from those rocks,

so we've got microbes that can use some of those dissolved minerals

as an energy source.

There's some thinking that these sorts of places

might have been where life originated.

What makes them so good for that?

When we're making a temperature measurement

at the throat of one of these vents and we're reading 401 degrees,

if we move that temperature probe

a few centimetres in that flow coming out of the top

of that, what we call chimney, it will drop off by 120 degrees.

And then the chemistry is changing over that distance as well,

from being really rich in these dissolved minerals

to being much more influenced by,

you know, normal seawater, and that's mixing, so we've got changes

in chemistry and in temperature over very, very small spaces.

And that means you can get very exciting reactions.

Reactions will run more rapidly at higher temperatures

and you're bringing together these chemical ingredients

that could start producing some of the building blocks of life.

Even looking at the pictures feels like you're looking at something

very primitive, that there was one moment at some point

that might have happened in an environment like this

that just tipped chemistry into biology

and it's a huge thought.

When we explore these today, we become aware that, you know,

there are several thousand of these out there

dotted around the world's oceans

and they are roiling away all the time.

Give yourself millions of years and at some point it was enough

and it tipped things over to give us life

from just physics and chemistry.

If life did begin at these vents,

then to move beyond them,

it would require a different source of energy altogether,

one that wasn't limited to these rare pockets of heat

from the Earth's core.

And that source was revealed by a chance discovery

in the 18th century,

by a scientist who wasn't even looking for it.

In the 1770s, there was a Dutch physician called Jan Ingenhousz

and he was a medical doctor

who had become famous for smallpox inoculations.

But he had a lively mind.

He paid attention to the science of his day,

and that decade he turned his mind to leaves.

Ingenhousz had recently read of an experiment involving plant leaves

submerged in water and how this had resulted in bubbles

containing a mystery gas.

Some scientists of the day thought that the bubbles were attracted by the leaves

from the water, but Ingenhousz wasn't convinced

and he did his own experiments.

The first observation that he made was that the bubbles didn't form

when the leaves were in shadow,

but they did form when you put them in the sunlight

and he checked very carefully

that it wasn't just the warmth of the sun,

it was actually the light itself.

And the gas wasn't coming from the water.

It seemed to be coming from the leaves.

Ingenhousz tested the gas and discovered that it was pure oxygen.

He had uncovered one of the most fundamental processes

in all of nature.

Photosynthesis.

Plants absorb energy from the sun

and use it to break molecules of water into hydrogen and oxygen.

The oxygen is released, as Ingenhousz observed.

And just as important is what happens to the hydrogen.

It combines with carbon dioxide to form carbohydrates,

specifically sugars,

making the plants a store of energy.

By tapping into the energy from sunlight,

life could now move away from the thermal vents

and spread across the globe,

first in the oceans and eventually onto land.

Endlessly harvesting energy from the sun

and locking it into the chemical bonds of sugar molecules,

a process that's crucial to almost all life on Earth today.

That stored energy is important, because when it's stored

it can be released as required,

and that's what powers almost all life on Earth.

The sugars formed in photosynthesis

are the beginning of almost every food chain.

Further up the chain, complex life forms unlock that energy,

using it as the fuel that powers the thousands of chemical reactions

that take place in their cells to keep them alive.

But here, temperature poses an intriguing problem.

At everyday temperatures,

most biochemical reactions happen too slowly to sustain life.

To make them happen fast enough requires a special kind of molecule,

one that itself can only exist

within the tiniest band of temperature.

And there's an easy way to show you.

I've got two glasses here,

both of them have a little bit of corn-starch in water

and a little bit of iodine, which is what's made them purple,

and I'm going to add some of my own saliva using one of these,

and a cheek swab, just to one of them. Here we go.

Lovely.

I'm going to stir it into that one.

Starch is present in foods like bread and potatoes.

It's a complex carbohydrate with long-chain molecules.

And over five minutes we can see that adding saliva

to our starch mixture has caused an obvious change.

You can see that the one with the spit in

has definitely changed colour.

A chemical reaction's happened

and it's actually one that happens all the time in all of us,

both in our mouths and further down our digestive system.

What's going on is that there's an enzyme,

a biological catalyst in my saliva

which is breaking that carbohydrate down into simple sugars.

And enzymes like this are the root of all biology

because they speed reactions up.

They don't change what happens, but they make them happen faster.

There are 3,000 different types of enzymes in our body.

Each one speeds up a specific reaction,

sometimes more than a million times.

Behind every process in our body - breathing, moving, thinking -

lies a series of very precise reactions

powered by particular enzymes.

Enzymes are fabulous little biological machines

but they've got a limitation connected to temperature.

Like most chemical reactions, if you increase the temperature,

an enzyme will work a little bit faster,

until you increase the temperature past a certain point,

and at that point, everything stops happening.

And there's a simple reason why.

An egg white is made of protein molecules.

The reason its colour and texture change when cooked

is that those protein molecules change in structure

when they get hot.

Enzymes are also proteins.

Like the egg white,

if they get too hot their structure changes permanently

and they're no longer able to perform their specialised function.

So keeping them at precisely the right temperature is crucial.

Just think about all the places you find life,

very cold places in the bottom of the ocean,

very hot places in deserts,

all these different environments that life can survive,

they've all got one challenge in common,

and that's to keep their enzymes functioning

and the first way to achieve that challenge

is to keep your enzymes at the right temperature.

And that's the critical link between life and temperature.

Plants and animals that live in the oceans have it relatively easy

thanks to the water providing a stable temperature environment.

But living on land has always presented

much more of a temperature challenge,

one that I can fully appreciate in a most unlikely place.

Budleigh Salterton, on the English south coast,

has long been a popular holiday spot.

And two centuries ago, tourists went mad

for a souvenir they couldn't get elsewhere.

Quite a few of the locals made a bit of extra cash

by selling the strange stones they found on the beach.

They didn't know what they were but they gave them names, snake stones,

vertiberries and devil's fingers.

It was only when the fossilised remains of much larger organisms

were discovered here that people realised what these trinkets were.

Ancient animals, long since extinct.

This stretch of the British coast is an extraordinary record

of how life on Earth has evolved down the ages.

And what I am interested in is how it's been affected by temperature.

Perhaps the most striking example is these distinctive red cliffs,

a clue to a period more than 240 million years ago,

that was probably one of the hottest times the world has known.

Helping me decipher the landscape is geologist Nicky Hewitt.

This is called a ventifact,

a stone that's been sandblasted by the wind.

This comes from the top layer just underneath the yellow layer

that you see there. The bottom is rough where it's stayed flat.

With the wind sandblasting it

and the back side away from the wind direction,

it's just a little bit rougher than the other two.

We're looking at the same sort of rocks

that you find in the Sahara today.

So even though we're on a beach now and you cannot imagine

an environment that is more different to the Sahara, and yet,

240 million years ago, that's what that was, the middle of a desert.

Exactly. The middle of a much bigger desert.

These cliffs are a relic of what is thought to be

one of the hottest deserts ever to exist on Earth.

A desert that formed part of a vast supercontinent.

If we were here 240 million years ago,

the Earth would have looked very different and this is it?

This is all of the continents of the world,

mashed together into one great big continent that was called Pangaea.

The UK is sitting about here.

You've got to imagine the equator coming across here at an angle

and so the UK is in the northern arid zone,

where the Sahara Desert lies today.

-So we'd have been very hot and dry in there.

-Absolutely.

The weather in the centre of this continent

would have been much, much more extreme

than it could ever be on any of the continents today,

just because you can get so much further away from the sea.

So it sounds horribly hostile.

The rocks that we're looking at here show no vegetation,

no fossils, nothing.

But then as time progresses,

the continents were moving very, very gently further north

and the climate was getting a little bit gentler.

A little bit more rain was falling, a little bit less heat.

Then you start to see the animals, the reptiles,

and the plants coming in.

It's estimated that the Pangaean desert would have reached

upwards of 50 degrees Celsius, making life here almost impossible.

But just as the continents had come together,

so they drifted apart and life returned to this part of the land.

Yet even in less extreme conditions,

life on land faces a temperature challenge.

The fluctuations between night and day and from season to season

mean that animals need to be able to control their body temperature.

Throughout most of the history of animal life,

there's one method that's endured.

I've come to Colchester Zoo

to meet an animal that's perfected it.

-Right, have to ask you to wait there.

-All right.

His keeper, Glen Fairweather,

is taking me behind the scenes to meet him.

Telu!

Telu!

Come on. That's it.

Good boy. Here he comes.

This is Telu, an adult male Komodo dragon.

That is a lot of lizard. He's enormous!

-He is big. Yeah.

-A slightly clumsy lizard.

Komodo dragons like Telu

are the largest lizard to be found anywhere on earth.

-OK, well, I'm just going to give Telu a little snack.

-OK.

Oh, didn't notice it.

He's... He's having a good look around there.

Oh.

Fantastic. In the wild, dragons will eat 10 to 12 meals a year, maybe.

12 meals a year sounds like almost nothing.

They have a very slow metabolism, so it would take Telu several weeks

to digest a large meal of 10, 15, 20 kilos.

The reason Telu eats so little is that he's cold-blooded.

Instead of using energy from his food to warm himself up,

he takes in heat from his surroundings.

In his natural habitat in Indonesia, he'd do that by basking in the sun.

In captivity, he has special lamps to provide both heat

and ultraviolet light.

This unique footage filmed at Chester Zoo

shows how rapidly a Komodo dragon can alter its body temperature.

In just 90 minutes, this animal's body warms

from its night-time temperature of 22 degrees to 35 degrees.

To stay active for the rest of the day,

it must now keep its body temperature within a narrow range,

between 34 and 36 degrees.

Observing Telu, palaeontologist Dr Darren Naish

can tell me how they do it.

So he's in front of his heat lamp

and he's done something quite distinctive,

which is sort of spread himself out flat.

Why has that happened?

Yeah, in order to basically be the best shape

to absorb as much heat as possible from the environment,

a lot of reptiles adopt specific poses

and the most obvious thing they do

is they do spread out and flatten the rib cage,

so they're presenting a larger surface area to the sun.

What Telu here is doing is absorbing heat from his heat lamp.

He's also receiving heat from the ground,

which has obviously been warmed by the heat lamp,

and through his own behaviour

he's very good at controlling his temperature,

keeping it quite high and in a very specific band.

Being cold-blooded does come with an obvious limitation.

You need enough heat in your environment.

We definitely do see a massive drop-off in the diversity

of cold-blooded reptiles like lizards

once you get away from the equator,

once you get further towards the north,

so clearly they are disadvantaged in cooler environments.

Today, we mostly associate cold-blooded animals

with places where there are warm conditions year round.

So for cold-blooded animals, the challenge of keeping warm enough

tends to limit them to the hotter regions of the planet.

To thrive in the cooler places,

you need a different way to keep your body temperature

warm and stable.

And evidence for this comes

from perhaps the last group of animals you'd expect.

When I was a kid, I spent a couple of years being dinosaur mad

and I remember the excitement of being taken to an exhibition

of Chinese dinosaur fossils

and I remember loving the picture that painted

of a different version of Earth, these giant lizards,

cold-blooded slow animals roaming the swamps.

Well, here today there is another exhibition of Chinese dinosaurs

but the specimens here

paint a completely different picture of that world.

Dr Adam Smith is a curator

at Wollaton Hall Natural History Museum.

When I was a kid growing up,

the picture of the environment that dinosaurs lived in

was a swampy environment surrounded by volcanoes but we now know

that dinosaurs were much more diverse than that

and the environments that they occupied

were much more diverse than that as well.

Some of them were adapted for living in forests,

some were adapted for living in open landscapes,

some lived on the shore,

even quite snowy areas would have been occupied by dinosaurs.

For decades, the spread of dinosaurs into cooler regions

away from the tropics posed the question...

..how could cold-blooded creatures survive in colder climates?

And then in 1996, a fossil was discovered in China

that changed everything.

So this specimen is obviously beautifully preserved. What is it?

This is a genuine fossil of a sinosauropteryx dinosaur.

It was living in a climate that was similar to northern Europe

and so you would have had warm seasons and cold seasons

and the special thing about it is that,

in addition to the bones being preserved,

we have evidence of the soft tissues as well.

You can see it most clearly running along the back of the tail here,

this dark line,

and especially at the very tip of the tail, it looks very tuft-like.

The dark line on this 125-million- year-old sinosauropteryx fossil

is only faint, but it's tantalising evidence

for something you wouldn't expect on a dinosaur.

It's very similar to the downy material

that you find on a newly hatched chick.

And that's why this has been interpreted as feathers.

So this is a dinosaur and it's got feathers?

They're not true feathers as you would think of as a bird's feathers

but they were the structures that led to true feathers.

They're fuzzy feathers,

so they've been given the name proto-feathers.

For palaeontologists, these fuzzy feathers

were a spectacular revelation.

The fuzz in the dinosaur suggests that they were using it

for insulation and in that case you would expect the dinosaurs

to be generating their own heat rather than basking in the sun

to get warm from the outside environment.

Cold-blooded animals tend not to have feathers,

in part because their skin needs to absorb heat from the environment.

So this animal, that suggests, was not cold-blooded?

It's very likely, based on the evidence from the feathers,

that this particular dinosaur was warm-blooded.

This discovery is helping scientists to reimagine the world of dinosaurs.

In the case of these dinosaurs,

we know that they were very active animals,

very agile dinosaurs, very intelligent animals as well.

It's now thought that many dinosaurs

may have been at least partly warm-blooded.

This would have made them less reliant on the sun

and allowed them to thrive in cooler habitats.

Had an asteroid impact not contributed to their extinction,

some of them might still exist today.

The dinosaurs that did survive evolved into modern birds,

which are warm-blooded.

And alongside them grew the rapidly expanding class

of warm-blooded mammals.

Birds and mammals use the energy from food

to generate their own body heat,

and one area that is particularly sensitive to temperature

is the brain.

This powerful but fragile organ generates intense heat of its own,

so animals need a way to keep it at precisely the right temperature.

On a cold, rainy day like this in Colchester...

..nobody here is thinking that they're too warm.

But these animals have some amazing adaptations to keep them cool

in the really hot weather in Africa.

I am meeting Dr Chris Basu from the Royal Veterinary College,

who's an expert in giraffe physiology.

The brain itself is an organ which produces a lot of heat, so even when

they're not doing particularly anything taxing, the brain is very

metabolically active, so it's producing loads of heat.

A giraffe's natural habitat is hot,

sometimes well above 30 degrees Celsius,

so these animals have had to adapt to dissipate heat.

Their distinctive markings have long been thought to

play a role in camouflage,

but take a look at them with a thermal imaging camera and

something more is revealed.

You might expect the giraffe's body to be all the same temperature

and therefore a uniform colour,

but instead the dark patches are still visible,

and the colour shows they are actually warmer

than the surrounding skin.

Beneath those spots is actually

quite an intricate network of blood vessels.

So, blood vessels bring blood to the surface, to the skin,

and they can actually radiate heat through those blood vessels.

So when you look at those spots,

you can almost think of those spots as thermal windows -

they're getting rid of heat through those spots.

These markings are crucial to keeping the giraffe's body cool.

But keeping the brain at the right temperature is so important

that it needs its own cooling system,

one of the most sophisticated adaptations in the animal kingdom.

And it all starts with its nose.

When the giraffe breathes in,

the air helps water in the moist lining of its nostrils to evaporate,

which in turn cools the blood in the underlying blood vessels,

much like when we sweat.

And the next crucial step is what happens when this cooler blood

heads back towards the heart.

At the base of the brain is a structure called the carotid rete,

where heat can be transferred from the warmer blood

travelling to the brain to the cooler blood from the nose.

It's thought that this helps cool the blood arriving from the heart

before it reaches the brain,

preventing the brain from overheating.

We can think of it as like a heat exchanger.

It drops the temperature by about two degrees,

but the really clever thing is they can actually adapt this mechanism

based on the environmental conditions.

It sounds an amazingly practical, efficient way of losing heat.

It means that they can just respond to their environment,

they're really quite responsive.

Giraffes have evolved in this very distinctive way to cope with heat.

But if there's one animal that's found a way to live in pretty much

every temperature environment on Earth, from deserts to poles...

..it's us.

So how does the human body cope with extremes of temperature?

To show you, I first need to generate a lot of heat.

So I've come to my badminton club to train with my coach, Stuart.

We are used to the idea that our body temperature is 37 degrees,

but we don't often think about just how hard our system has to work

to make sure that's true.

I do a lot of sport, so I run around all the time,

and that sort of exercise puts a lot of stress on the system,

and the body has a challenge to get rid of that heat.

While Stuart puts me through my paces, my body has two jobs to do.

So, on court I'm thinking about what my muscles are doing,

how I'm moving, but while all that's going on,

my body has another challenge,

which is getting rid of all the heat I'm generating.

One obvious way my body does this is to sweat.

But to see what else I'm doing, I'm using the thermal imaging camera.

This will show the temperature of the interface between

my skin and the surrounding air.

The lighter and brighter the colour, the hotter the temperature.

Watching the thermal footage of me playing is fascinating

because there's so much detail.

You can see that my surface temperature is different

in different places.

My face is obviously very warm, under my arms are very warm.

All of the places where there's blood flow close to the surface,

those show up really, really brightly.

And the really interesting bit here is when you look

just after I've stopped, and you can see how hard my body

is working to get rid of that heat.

My blood vessels on my arms are just shining out

because they're so warm.

That's because when we're getting too hot, our brain tells

the blood vessels supplying our skin to widen.

This increases the flow of blood to the surface of the skin,

where it can dissipate heat.

And this shifting of blood to and from the skin's surface

is an extremely effective way to control our body temperature.

It helps prevent our bodies from overheating

and keeps them within a very narrow and safe window of temperature.

The amazing thing about this is I run around in this sports hall

all the time and I never have to think about this,

my body just takes care of it all.

But when we get cold, our bodies face the opposite challenge -

not dissipating heat, but hanging on to it.

To understand how our bodies deal with cold,

I've come to the University of Portsmouth to experience it

in a rather unusual way.

I'm pretty good with physical discomfort but I hate being cold,

and I'm a fidget and I hate sitting still,

and both of them are about to happen to me at the same time.

Putting me through this challenge is Professor Mike Tipton,

an expert in cold water survival.

He's going to immerse me in water that's 18 degrees Celsius,

nearly 20 degrees below my normal core body temperature.

And to see how the cold affects me,

Mike has got a simple manual task for me to perform,

which I'll repeat after I've spent time in the water.

Three, two, one, go.

Now come back.

-That's good.

-OK.

-Well done.

-Right, done.

You've done that, yeah, 22 seconds.

-OK.

-I will remember that.

It's time for the big plunge.

I suddenly have immense sympathy for witches in the 16th century.

Four, three, two, one, go.

Oh, it's horrible.

It's amazing how the urge to breathe is very sudden.

As soon as I am submerged, my survival mechanisms kick in.

I have started to shiver - about a minute ago, I started to shiver.

Yeah, the skin receptors are sending messages into the brain saying,

"You've got a very cold skin,"

and so that's being integrated in the centre of the brain,

the hypothalamus of the brain is saying,

"We need to start generating heat."

And that's why you have started shivering.

Shivering is my body's attempt to counteract the cold by producing

its own heat to prevent my vital organs from dropping in temperature.

But in these conditions, shivering, alone, isn't enough.

A drop in my core temperature of just two degrees Celsius

would cause hypothermia.

So after half an hour, I've reached my limit.

-I think it's probably time to bring you out.

-OK!

Ready? Here we go.

As I'm winched out of the water, the thermal imaging camera reveals

another of my body's responses to the cold.

Dark blue areas indicate where my surface temperature has dropped

dramatically as blood is diverted away from the cold water.

The body will sacrifice the extremities

in order to preserve the internal organs.

And you will have people who have got frostbite,

they are losing extremities, but to preserve

their heart and brain temperatures, because once those

temperatures fall, then it's a threat to survival.

So what we're going to do now is just ask you to do

that nut and bolt test again.

Three, two, one, go.

My wrists are very cold and I feel that's stopping me moving

-my fingers very well.

-That's it, done?

-Done.

-Oh, there we are.

-Yeah.

-Bang on a minute.

Really? Three times!

So 22 seconds before, a minute afterwards.

This experience has made me realise

just how vulnerable to cold we all are.

In fact, what enables us humans to survive and thrive

in cold temperatures isn't our in-built survival mechanisms,

it's something else.

Our physiological responses to cold really wouldn't let you

move very far away from your equatorial origins.

You know, once you start getting into zero degrees overnight,

the level of heat production, the level of heat retention

you have got will have been very limiting.

And the really important thing is that it's underpinned by intellect.

We have been using clothing for 75,000 years,

we've been using fire for a million years.

Now, as soon as you have done that, you've got a source of heat

and a source of light, you can cook food, your diet can change.

You are a tropical animal

that's taken those origins with it thermally,

so you've recreated, as I say, a microclimate next to your skin

which would be the same as if you were living naked

in the 28-degree environment from which you evolved.

While all life on Earth has adapted

to survive the temperature of its habitat,

only we humans are able to create micro-habitats of our own.

We can maintain our ideal temperature wherever we go

thanks to our intelligence.

But human ingenuity hasn't just enabled us to manipulate

the temperature of our environment.

It's also allowed us, in very special circumstances,

to push the boundaries of life itself.

It's 8am and a team at Papworth Hospital are getting ready

to perform a radical type of surgery.

It involves cooling a patient's body

to a temperature that would normally be fatal,

taking them to the edge of life.

Justine has a life-threatening condition.

Clots are blocking the blood vessels in her lungs,

leaving her struggling for breath.

I've continuously got a tightness in my chest.

Just doing normal things, like going up and down the stairs,

I'm out of breath.

It's quite daunting but I know, obviously,

I've got to have this operation. If I don't,

I don't know how long I'm going to be able to continue for.

So I know that I have to do it in order to be able to

take my little girl to the park and play.

It's down to surgeon David Jenkins

to remove the clots from Justine's lungs.

But while blood is flowing through her lungs,

the operation is impossible.

Well, the main problem is the lungs usually have five litres of blood

every minute being pumped through them.

For this operation we need a completely clear field

in the small vessels in the lungs,

so the only way to do that is to drain all the blood out of the body.

Removing a patient's entire blood supply is a truly extraordinary

procedure and David is a leading specialist in the technique.

Once Justine is under anaesthetic, the first step is to divert

her blood supply to a heart-lung machine.

At this stage, her blood is still delivering fresh oxygen

to her vital organs and, crucially, to her brain.

-Running OK?

-It's running well.

At David's command,

the machine drains all Justine's blood from her body.

Now we are in this critical window where there is no blood flow

going through Justine's body,

and David is able to see

inside the pulmonary arteries to clear the blockages.

He can now begin to remove the clots from her lungs.

But he has to work against the clock because without blood circulating,

Justine no longer has a supply of oxygen.

Normally, the human brain can only survive for around four minutes

without fresh oxygen before permanent damage occurs.

But in the controlled environment of the operating theatre,

Justine is being kept alive by temperature.

Cooling to 20.

Before David began to operate,

Justine's body was slowly cooled to just 20 degrees.

This is the key to the entire procedure.

Her body is in a temporary state of stasis.

At this temperature, the function of her brain is slowed down

and it can survive 20 minutes without oxygen.

The process is being supervised by anaesthetist Dr Joe Arrowsmith.

Our body has all these mechanisms to stop us getting that cold -

why isn't she shivering?

Well, the anaesthetic I've given her

has disabled all of those mechanisms.

I've paralysed her skeletal muscle, so she physically cannot shiver.

To reduce the need for oxygen as much as possible,

the team have cooled Justine's brain still further.

We have this cap wrapped around Justine's head

and it has got a continuous flow of ice-cold water that comes

from this ice bath here with freezer ice packs in.

What we believe this does is keep the outer centimetre or two of

the brain slightly cooler than the rest of the brain,

so where the grey matter is, where all of the cell bodies

and most of the metabolism is, so we think that buys us

just a little bit of extra brain protection.

The right side is done and we managed to do that

in just under 15 minutes.

So that's good, and we're back on the heart-lung machine now.

With the clots removed, Justine's blood is returned via the machine.

It gradually warms up her blood, and in turn her body.

And after a while, her heart spontaneously restarts.

The patients who go through this procedure

live through something incredible.

They are taken to the edge of life and brought back.

And the skill and the delicacy of the process

is just amazing to watch.

And it's all made possible by control of temperature.

After six hours in surgery,

Justine has safely returned from her incredible journey

down the temperature scale.

And what's really exciting is that

our ability to manipulate temperature

is beginning to open up a whole new field of medical possibilities.

We are alive, you and I,

which means that we are directly connected to the web of life

that covers this planet and extends back through

almost all of its history.

And all of that web, in all of its variety,

only exists within a very narrow temperature range,

and we barely appreciate the temperatures of life.

But next time you hold someone's hand or give them a hug,

it's worth remembering that it's not just about the physical gesture,

you're sharing the warmth of life.

And it's a nice thought that that shows just how intimately

temperature and life are intertwined.

Next time, I'll be exploring the incredible science of heat.

What temperatures does it reach on the inside there?

100 million degrees.

That's just a ludicrous number!

I'll reveal how our ability to harness heat

lies behind some of humanity's greatest achievements...

..and promises a future of almost unlimited power.