(STEVE LEONARD) Imagine a land so hostile
that you'd need special equipment just to breathe,
Where there's no shade, and where the solar radiation would kill you in minutes,
At night, temperatures plummet to sub-zero,
You could freeze or fry in the same day,
(GASPS)
I'm not talking about a faraway planet. I'm talking about Earth.
500 million years ago, this was what the land was like to all life-forms -
alien and uninhabitable.
Back then, life was only to be found in the sea,
so how did it conquer this hostile world,
taking an evolutionary journey that would one day lead to us?
Over the millennia, this harsh new world was invaded by a few pioneering life-forms,
These evolved into a multitude of animal designs
able to cope with the extremes of life on land,
Our family - the mammals - is just one of the results,
Every living thing is linked by the branches on the tree of life,
I'm going to find out just how this extraordinary variety of animals arose..,
..and how we are connected to each and every one of them,
It's a story full of surprises
which leads from a fish
to you and me,
And if it hadn't been for one giant twist of fate,
the dinosaurs might still rule supreme today,
In the beginning, dry land was a no-go area for life,
As well as deadly temperature extremes, there was the crushing effect of gravity,
Setting foot on this new world would be an enormous challenge.
It'd be like trying to live on another planet.
That's because all life started in the sea, where the temperature hardly changes
and the water protects against the pull of gravity and the burning power of the sun,
The sea was the first laboratory of life,
and for more than three-quarters of life's history, it was home to every living thing,
This was mostly a gentle era of soft-bodied creatures like jellyfish,
Jellyfish are 95% water and have no skeleton at all,
Unlike us, they don't need one, because being underwater is like being in space,
Water buoys you up, suspending all life in a three-dimensional world,
It enables these kelp fronds to tower 30 metres upwards from the ocean bed,
In fact, just like animals, for hundreds of millions of years,
plants could only exist in the sea,
On land, this watery support would all be gone, so what would happen then?
Everything would collapse,
(SPLUTTERS)
Thrown back on to land, I'm suddenly aware of just how heavy all this kit is,
and my own body weight.
It's hard work just trying to get up.
Gravity is definitely dragging me down.
For a jellyfish, the force of gravity is fatal,
Once they collapse, they die,
So how could any animal designed for life in the sea ever get out of the water?
There was one group of animals in the sea
that had just the right kit to get up and go onto the land.
The arthropods had hard, jointed skeletons,
This living suit of armour holds their body up so gravity can't pull it down,
Armour gave the arthropods the staying power they needed to make it on land,
Armour has made the crabs expert land grabbers,
Some have now evolved to live far from the sea,
These blue land crabs in Florida are found up to three miles from the beach,
Crabs are actually well adapted to life on land.
Their jointed outer skeleton evolved in the sea,
but it also supports their weight on land, making it easy for them to get around.
And it stops them from drying out. Hello, petal.
Where are you going?
On the other side of the world in the Indian Ocean,
there's a land where the crabs have really made themselves at home,
Christmas Island is completely overrun with red land crabs,
120 million live on this small island,
outnumbering the human residents by 300,000 to one,
Once a year, vast numbers hit the road,
They're heading for the beach..,
..because despite their armour, they can't shake off their ties to the sea,
There's a vital purpose to this mass manoeuvre,
The females clasp thousands of soft eggs underneath them,
Like all land crabs, they have to return to the water to release their eggs,
Their young must grow up in the sea,
Bt the first creatures ever to venture on land
lived long before crabs even came into existence,
when the land was barren and lifeless.
How do we know? Well, because like me, they left footprints.
Preserved in the rock in what is now Ontario, Canada,
are the oldest footprints anywhere on Earth,
Just a few small steps for a bug, but one giant leap for life,
These prints are 500 million years old,
Their maker is thought to have been an armoured arthropod
that looked like a giant woodlouse more than a foot long,
But what made this ancient trail blazer drag itself ashore onto the barren land?
Well, 500 million years ago, there were so many tooled up and hungry predators
that the ocean had become a dangerous place to be,
For the first time in the history of life, there was good reason to leave the crowded seas,
And another ancient creature still retraces those first pioneering steps each year,
The horseshoe crab has been around for hundreds of millions of years,
Once a year, it still makes a dramatic pilgrimage to the beaches of North America..,
..to breed,
Horseshoe crabs lay their eggs on land to put them out of reach of marine predators,
It's likely that those early trail blazers first took to the beaches for the same reason,
Job done, The horseshoe crabs turn around and crawl straight back into the sea,
So those first track makers were probably just visitors, too,
About 430 million years ago, another group of arthropods abandoned the sea for good,
The colonisation of the land had truly begun,
and it was creatures like millipedes that led the way,
Others soon followed, almost 100 million years before our ancestors left the sea,
Today, more than 90% of all land dwellers are armoured arthropods,
including spiders, millipedes and insects,
making them the most successful group of animals on Earth,
The vast majority are insects like ants
that have cracked the problems of breeding on dry land,
Ants keep their eggs and young moist in nests underground
where they're constantly cleaned by the adults,
Driver ants now have sophisticated strategies to breed and feed on land,
By massing in their millions around the nest,
they raise its temperature and speed up the development of the young inside,
Their sturdy skeletons carry them easily across the ground and stop them drying out,
These fearsome jaws have made them deadly hunters,
Thousands work together as one giant predator,
devouring everything in its way,
Many jaws make light work of animals far bigger than themselves,
But the arthropod's jointed construction does have its drawbacks,
Land crabs are amongst the most impressive land-based arthropods around today.
Thankfully, they don't get much bigger than this - this one's already a handful -
because the bigger they get, the heavier their armour, and the harder it is to breathe.
The adaptations that allowed them to leave the water
have now limited their size on land.
Off you go.
Bt there was another group of animals built to a very different design,
one that allowed them to get much, much bigger.
It all started around 550 million years ago
with a tiny worm-like creature that lived at the bottom of the sea.
This is its closest living relative.
It's the lancelet.
It doesn't look like much, does it? So why is it so special?
Well, the lancelet has a revolutionary feature that was vital to our evolution,
It's the notochord - a stiff rod running through the body -
the beginnings of a backbone,
A strong yet flexible lever for muscles to pull against,
Having your skeleton inside also allows you to grow much, much bigger,
Incredibly, the backbone evolved just once,
but this one lucky break was a crucial turning point in the journey of life,
It led to a completely new family of animals - the vertebrates,
That's fish, amphibians, reptiles, birds and mammals,
And that includes you and me.
So the backbone which evolved to help animals swim
would eventually help them to colonise the land.
But to walk, you need legs, How did THEY evolve?
Under water, mudskippers look much like other fish..,
..but watch what they can do with their fins,
A fish that walks,
Well, almost,
The mudskipper can shuffle over land using two pairs of modified fins,
This is how we once thought backboned creatures left the water -
ancient fish propped themselves up onto fins, and once ashore, the fins turned into legs,
We now know that something rather more surprising happened.
Legs actually evolved for use in the water - but how?
The oceans are home to other bizarre creatures
that appear to walk across the seabed,
Rosy-lipped batfish never leave the sea, and they're perfectly good swimmers,
but they also use their fins like pairs of legs,
By stalking prey on stilt-like feet, they don't stir up the water or sand,
So this shrimp doesn't have a clue,
The frogfish has also evolved two pairs of modified fin feet
to help it navigate the nooks and crannies of the ocean floor,
So fishy fin feet weren't a new idea,
but evolution didn't really run with it until around 370 million years ago,
when the climate changed,
The earth warmed up
and became covered in shallow, weedy swamps where normal fins got in the way,
Over time, the paired fins started to evolve a better shape for pushing through the weeds,
until they ended up more like four feet,
Those early four-footed creatures left a legacy
which can be traced throughout the tree of life right up to the present day,
They were the ancestors of all land-living backboned animals,
The for-legged blueprint had been set,
and all vertebrates that have ever lived on land have followed the same basic pattern.
And if for some reason our ancestral fish hadn't had two sets of fins,
I might have ended up with two legs and no arms or even an extra set of arms.
Which might have been quite useful round about now!
Whoa. Not only that, we could have ended up with more fingers and toes as well.
Having five might be just a fluke of nature.
Fossils show that some early four-legged creatures that crawled through the swamps
had six, seven or even eight fingers,
But the one successful species that became the ancestor of all the later vertebrates
just happened to have five fingers and toes,
So having five fingers and four limbs is an ancient blueprint
all land-living animals inherited,
Bt what about horses, you might ask. Surely they don't have five fingers?
Well, they don't have today, but their ancestors used to.
They started off with five fingers. Gradually, over millions of years,
they lost the use of four of them, leaving them with a central digit.
The nail turned into the horse's hoof, and these three bones that are in the finger
are present in the horse's leg today,
and this big long bone is the same bone that's hidden within my hand.
Surprisingly, my wrist is exactly the same joint as the horse's knee.
OK, big fella.
Come on, then. Good lad.
And some of those lost digits are still present at the start of every horse's life,
For the first months inside the womb,
a foal still has two extra toes,
but in a replay of its evolutionary past,
they fold back into the central cannon bone before it's born,
We now know how our underwater ancestors evolved to walk,
but there were other challenges they had to face in leaving the water for the hostile land,
To start with, how would they manage to breathe?
I can only breathe down here because of this - an aqualung.
In the same way that I'd drown down here without this diving gear,
life on land for fish is equally suffocating.
Remarkably, though, there are some fish that can breathe air just like us
as they have done for 400 million years,
Lungfish - still found in Africa and Australia today,
For those first sea creatures that came ashore,
it took the winning formula of lungs..,
..and legs to make the quantum leap from ocean to land,
Enter the first four-legged air breathers - our ancestors,
But even these groundbreaking creatures couldn't conquer dry land just yet,
First the land itself would have to change,
The drying and burning effects of the sun are potentially lethal,
especially when you’re exposed like this...
..with no shade.
I have to drink about two litres of water a day just to compensate...
..and I've got a relatively waterproof skin.
For the first four-legged creatures with their delicate skins,
this was an even bigger problem than it is for us,
They needed moisture and they needed shade,
They had to wait for another invasion of the land -
that of woody plants.
Early plants were tiny algae, and like every other form of life, confined to water,
Just like animals, to make it on the land, they had to combat gravity,
They evolved a skeleton made up of rings of lignin -
the plants' equivalent of bone,
Now they could leave water and stand alone,
Lignin gave the plant cells strength to keep on growing up and up..,
..reaching towards the sunlight,
Lignin eventually expanded to fill up the cell walls and form wood,
And wood allowed plants to really make it big,
They evolved into the incredible hulks of the land - the trees,
Eventually, the battle for light led to lush forests, cool, shady and moist -
ideal conditions for delicate creatures like or ancestors.
So lignin created wood,
and wood created forests,
without which we wouldn't exist,
Those early four-legged animals
had a whole new world to explore, with all the moisture and shade they needed,
Damp forests everywhere are still inhabited by their descendants -
the soft-skinned amphibians,
There are now more than 4,000 species,
but even today, some 350 million years later,
amphibians can't escape the pull of their aquatic roots,
Every spring, just after the snow melts,
there's an extraordinary mass migration through the forests of north-east America,
Thousands of spotted salamanders that have spent the past year hidden underground
emerge with one mission in mind -
to get to water for a single night of passion,
Then, the females spawn,
Amphibian eggs are soft and dry out quickly in the air,
which is why spotted salamanders must still return to the water to breed,
But ever since their ancestors first crawled out of the swamps,
amphibians have found all kinds of weird and wonderful ways to break free from the water,
The mountains of northern Spain - a rocky, barren landscape,
But the midwife toad is perfectly at home, It has evolved to live and breed on dry land,
So what do midwife toads do that salamanders don't?
(PIPING CALL)
A pair meet up and start to mate,
The male's embrace squeezes the female until she releases her eggs,
She catches them in her back legs, where he fertilises them,
But there's no water here, How will the eggs survive?
The male midwife toad is one of nature's new men,
He hoists the eggs up his legs as if putting on a pair of shorts with no hands,
From now on, the eggs are his responsibility,
He carries them away and burrows under a rock to keep them moist,
The male holes up for several weeks,
Then he has to go off to search for a pond,
He must return to water, where the tadpoles wriggle free and grow up,
just like other toads,
(CHIRPING, CROAKING)
In this forest in Costa Rica, I can hear frogs calling to their mates all around me,
but there are no pools or streams that I can see,
How do they manage to breed here?
This is the strawberry poison arrow frog, and on its back is a tadpole
It hatched on the ground, but now gets a piggyback from Mum
as she begins a mother of a climb,
She carries her tadpole all the way up into the trees..,
..to its own cradle in the canopy,
In these hanging gardens up to 20 feet above the forest floor,
and egg-cup-sized pond is all this mother needs,
And after all this effort, her work still isn't finished,
A few days later, she has to make the same climb all over again,
because although her tadpole has water in its treetop nursery, it has no food,
so it waits for her like a chick in its nest,
As she lowers herself in, it headbutts her, even gives her a little nip..,
..until she gives it what it wants,
She lays an infertile egg for it to eat,
With regular meals from Mum,
the tadpole has all it needs in its tiny world to grow into a frog,
Strawberry poison arrow frogs may be the hardest-working frog mothers on Earth,
but another frog takes the award for the most devoted frog father.
Guess where the Darwin's frog keeps his tadpoles!
The male's throat is the nursery where the tadpoles grow,
He gives birth by spitting out the froglets fully formed,
So the Darwin's frog has reduced its need for water
by carrying its own pond around inside it,
But despite all these extraordinary solutions,
almost all amphibians are still basically tied to water,
just as their early ancestors were,
So how did the first land-living animals colonise the deserts and dry lands?
The entire spawning process had to be transformed,
Eggs evolved their own internal life support system,
and at the same time, shells developed that could hold water,
The arrival of the hard-shelled egg was one giant step forward on the journey of life,
Each egg provided the growing youngster with its own portable pond,
Eggs could now be laid on dry land, and this led to a whole new group of animals -
the reptiles,
Reptile eggs contain more food reserves than those of amphibians,
giving their babies a head start in life,
Australian bearded dragons hatch out fully formed and ready to take on the world,
just as they've done for millions of years,
The reptiles evolved into many families,
including one of the most impressive dynasties ever to dominate the land,
The smash hit that was the hard-shelled egg led to the age of the dinosaurs,
(ROARS)
(SQUAWKS)
Dinosaurs ruled the earth for about 180 million years,
There were as many as 700 species,
from the size of a rat to that of a whale,
But the egg wasn't the reptile's only strength,
They had another trick up their sleeve -
their skin,
A tough scaly armour that could withstand the driest conditions on Earth,
And you don't get much drier than the sand dunes of Namibia,
But even here, reptiles have solved the problem of drying out,
And they can turn the power of the sun to their advantage,
First thing in the morning,
the cold-blooded chameleon heats up its side by facing it to the sun,
Once it's recharged its solar batteries, it's ready to go hunting,
Here, armoured arthropods have met their match,
Despite arriving more than 100 million years later,
reptiles now appear to have the upper claw,
But even reptiles have their limitations,
because they slow down as the temperature drops,
and when the sun is gone, it can get very cold,
Bt for us and other mammals, temperature does not limit where we can live.
We can be active whether it's hot or cold.
We do it by heating ourselves from the inside,
and we do that by burning up food,
Ah!
The reason my face is white hot and hers is a cool red
is because I am generating heat inside,
whereas she is cooling down
as she loses the heat that she's absorbed from the sun during the day,
Bt it's no good having an internal heating system
if all that heat is simply lost, which is what's happening to me right now.
You’d have to eat non-stop just to stay alive.
What you need...
..is good insulation.
So there would have to be another breakthrough
before land animals could reach the coldest corners of the world,
About 200 million years ago, a reptile-like ancestor of mammals
started growing fine barbs underneath its scales,
Over many generations, they became finer and longer,
until eventually they turned into hair and fur,
The mammals had arrived, and now they were ready to take on the elements anywhere,
Musk oxen have the ultimate fur coats,
Their insulation is so good, they can survive at temperatures of minus 40 degrees Celsius,
Warmed by their central heating and stoked up by regular meals,
musk oxen have managed to beat the elements,
and eat their way north to the fringes of the Arctic Ocean,
Central heating and fur coats let mammals range even to the icy reaches of the poles,
And this must be the ultimate test,
Dense fur allows a baby seal to undergo the biggest temperature shock
experienced by any animal on Earth,
That's from around 40 degrees inside its mother's body
to as low as minus 30 when it's born and it hits the ice,
The miracle of this baby's survival here depends not just on fur,
but on its mother's dedication,
She provides the fuel that keeps it warm inside,
But the mammal that really takes parenting to the extreme is the elephant,
It takes nearly 22 months to make a baby elephant,
longer than any other animal,
During that time, the embryo floats in its own centrally-heated world,
an echo of life's earliest existence in warm primeval seas,
And it's safer than inside an egg, Mum has total control over the baby's growth,
After investing all that effort even before birth,
it's not surprising mammals take exceptional care of their young,
For a mammal mother, birth is just the start of a long and demanding job,
A mammal mother is a mobile milk bar,
and delivering enough of the white stuff to build up baby
is an even greater drain on her than pregnancy,
Milk is highly nutritious, and at this stage, it's the only food that baby needs,
As long as no one tries to nick it!
Like all mammals, baby elephants learn through play,
and it's a good excuse to mess around,
It's been an extraordinary journey,
We've evolved a backbone, lungs and four limbs..,
..with five fingers and toes,
We've grown fur
and developed central heating to cope with extremes of temperature on land,
Bt the mammals needed other changes to a body design
which originally evolved in the sea.
Our eyes work fine underwater,
we can even taste there,
but hearing is a different matter,
Sound travels very differently because water is 800 times as dense as air,
So animals living on land had to develop a totally new way of hearing.
And when it comes to ears, few mammals are more advanced than the bat-eared fox,
It lives in the South African bush, where it scans the ground listening for a meal,
It hears in the same way as all mammals do - it's just more sensitive,
Broad outer ears capture the soundwaves in the air
and funnel them through to the eardrum, making it vibrate,
Tiny bones in the middle ear transmit and amplify these vibrations,
The sound waves are then converted into nerve impulses and sent to the brain,
Bat-eared foxes can hear grubs and termites moving nearly a foot below the ground,
Bad news for arthropods,
A mammal's ear is a hi-tech triumph,
How did the chance workings of evolution create such a remarkable device?
Well, believe it or not, it all began in the solid jaws of prehistoric reptilian animals.
Over millions of years, two of the jaw bones became detached, shrank,
and moved upwards to become the key components of the mammal ear,
They were hijacked by evolution for an entirely different use,
It's amazing to think that these delicate ear bones
have evolved from bones
that were originally designed so or reptile-like ancestors cold bite their prey.
If you don't believe me, try eating an apple.
We can sense echoes of our evolutionary past every time we chew.
We hear every crunch through or jaw bone.
Not only mammals' senses have been perfected through evolution.
Mammals - and I include myself here - have also become the cleverest animals on Earth.
And there aren't many more intelligent than baboons,
They lead a complex social life,
It's not just who you know, but what you do with that knowledge that counts,
Male baboons use brains as well as brawn to get their way,
If one male is threatened by another, he'll pick up a baby as a living shield,
He knows that no baboon will risk hurting the youngster by attacking him,
As well as being devious, baboons use their intelligence to find new things to eat,
(BUZZING INSECTS)
Some have learned the art of ambush to get much larger meals,
Mammals are one of the most successful animal groups.
They can cope with the most extreme climates on Earth,
from the scorching heat of barren deserts to the steaming rainforest,
and from the deepest seas to great cities.
But it could have been very different.
(ROARING)
More than 100 million years after our mammal ancestors had arrived,
the land was still ruled by dinosaurs,
Most of the early mammals were shrew-like creatures that ventured out mainly at night,
The dramas of their lives took place entirely in the shadow of the giants,
But one event 65 million years ago played right into the mammals' hands,
A massive meteorite collided with the earth,
85% of all land-dwelling animals were snuffed out,
The dinosaurs became extinct over just a few thousand years,
But among the animals that did survive were mammals,
Perhaps that was because they fed on the other major group of survivors -
the armoured arthropods,
With the dinosaurs gone, the mammals seized their chance,
diversifying and expanding to fill the world,
The mammal line exploded into the wonderful variety we see today,
Ultimately, that led to us.
If that meteorite had missed the earth, we probably wouldn’t be here.
We've come a long way since or ancestors crawled out of the sea.
We've coped with everything this hostile new world throws at s.
But can we call ourselves the true inheritors of the earth?
No, It's the armoured arthropods,
They colonised the land 100 million years before we vertebrates,
And today they outnumber all other animals combined,
Many have hardly changed in over 200 million years,
On their journey of life, land-grabbing arthropods
developed an almost bomb-proof design,
And no doubt they'll still be here long after we've gone.
Next week on the “Journey of Life”,
I'll find out how life grew wings and conquered the skies,