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(STEVE LEONARD) Imagine a land so hostile
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that you'd need special equipment
just to breathe,
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Where there's no shade, and where
the solar radiation would kill you in minutes,
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At night, temperatures plummet to sub-zero,
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You could freeze or fry in the same day,
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(GASPS)
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I'm not talking about a faraway planet.
I'm talking about Earth.
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500 million years ago, this was
what the land was like to all life-forms -
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alien and uninhabitable.
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Back then, life was only to be found in the sea,
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so how did it conquer this hostile world,
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taking an evolutionary journey
that would one day lead to us?
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Over the millennia, this harsh new world
was invaded by a few pioneering life-forms,
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These evolved
into a multitude of animal designs
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able to cope
with the extremes of life on land,
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Our family - the mammals -
is just one of the results,
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Every living thing is linked
by the branches on the tree of life,
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I'm going to find out just how
this extraordinary variety of animals arose..,
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..and how we are connected
to each and every one of them,
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It's a story full of surprises
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which leads from a fish
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to you and me,
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And if it hadn't been
for one giant twist of fate,
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the dinosaurs might still rule supreme today,
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In the beginning, dry land
was a no-go area for life,
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As well as deadly temperature extremes,
there was the crushing effect of gravity,
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Setting foot on this new world
would be an enormous challenge.
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It'd be like trying to live on another planet.
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That's because all life started in the sea,
where the temperature hardly changes
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and the water protects against the pull
of gravity and the burning power of the sun,
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The sea was the first laboratory of life,
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and for more than three-quarters of life's
history, it was home to every living thing,
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This was mostly a gentle era
of soft-bodied creatures like jellyfish,
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Jellyfish are 95% water
and have no skeleton at all,
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Unlike us, they don't need one, because
being underwater is like being in space,
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Water buoys you up, suspending
all life in a three-dimensional world,
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It enables these kelp fronds to tower
30 metres upwards from the ocean bed,
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In fact, just like animals,
for hundreds of millions of years,
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plants could only exist in the sea,
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On land, this watery support would
all be gone, so what would happen then?
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Everything would collapse,
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(SPLUTTERS)
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Thrown back on to land, I'm suddenly aware
of just how heavy all this kit is,
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and my own body weight.
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It's hard work just trying to get up.
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Gravity is definitely dragging me down.
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For a jellyfish, the force of gravity is fatal,
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Once they collapse, they die,
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So how could any animal designed for life
in the sea ever get out of the water?
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There was one group of animals in the sea
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that had just the right kit
to get up and go onto the land.
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The arthropods had hard, jointed skeletons,
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This living suit of armour holds
their body up so gravity can't pull it down,
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Armour gave the arthropods the staying
power they needed to make it on land,
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Armour has made the crabs
expert land grabbers,
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Some have now evolved
to live far from the sea,
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These blue land crabs in Florida
are found up to three miles from the beach,
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Crabs are actually well adapted to life on land.
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Their jointed outer skeleton
evolved in the sea,
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but it also supports their weight on land,
making it easy for them to get around.
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And it stops them from drying out.
Hello, petal.
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Where are you going?
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On the other side of the world
in the Indian Ocean,
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there's a land where the crabs
have really made themselves at home,
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Christmas Island is completely overrun
with red land crabs,
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120 million live on this small island,
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outnumbering the human residents
by 300,000 to one,
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Once a year, vast numbers hit the road,
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They're heading for the beach..,
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..because despite their armour,
they can't shake off their ties to the sea,
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There's a vital purpose
to this mass manoeuvre,
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The females clasp thousands
of soft eggs underneath them,
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Like all land crabs, they have
to return to the water to release their eggs,
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Their young must grow up in the sea,
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Bt the first creatures ever to venture on land
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lived long before crabs
even came into existence,
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when the land was barren and lifeless.
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How do we know? Well, because like me,
they left footprints.
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Preserved in the rock
in what is now Ontario, Canada,
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are the oldest footprints anywhere on Earth,
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Just a few small steps for a bug,
but one giant leap for life,
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These prints are 500 million years old,
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Their maker is thought to have been
an armoured arthropod
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that looked like a giant woodlouse
more than a foot long,
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But what made this ancient trail blazer
drag itself ashore onto the barren land?
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Well, 500 million years ago, there were
so many tooled up and hungry predators
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that the ocean had become
a dangerous place to be,
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For the first time in the history of life, there
was good reason to leave the crowded seas,
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And another ancient creature still retraces
those first pioneering steps each year,
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The horseshoe crab has been around
for hundreds of millions of years,
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Once a year, it still makes a dramatic
pilgrimage to the beaches of North America..,
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..to breed,
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Horseshoe crabs lay their eggs on land
to put them out of reach of marine predators,
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It's likely that those early trail blazers first
took to the beaches for the same reason,
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Job done, The horseshoe crabs turn around
and crawl straight back into the sea,
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So those first track makers
were probably just visitors, too,
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About 430 million years ago, another group
of arthropods abandoned the sea for good,
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The colonisation of the land had truly begun,
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and it was creatures
like millipedes that led the way,
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Others soon followed, almost 100 million
years before our ancestors left the sea,
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Today, more than 90%
of all land dwellers are armoured arthropods,
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including spiders, millipedes and insects,
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making them the most successful group
of animals on Earth,
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The vast majority are insects like ants
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that have cracked the problems
of breeding on dry land,
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Ants keep their eggs
and young moist in nests underground
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where they're constantly cleaned
by the adults,
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Driver ants now have sophisticated strategies
to breed and feed on land,
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By massing in their millions around the nest,
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they raise its temperature and speed up
the development of the young inside,
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Their sturdy skeletons carry them easily
across the ground and stop them drying out,
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These fearsome jaws
have made them deadly hunters,
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Thousands work together
as one giant predator,
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devouring everything in its way,
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Many jaws make light work of animals
far bigger than themselves,
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But the arthropod's jointed construction
does have its drawbacks,
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Land crabs are amongst the most impressive
land-based arthropods around today.
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Thankfully, they don't get much bigger
than this - this one's already a handful -
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because the bigger they get, the heavier
their armour, and the harder it is to breathe.
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The adaptations that allowed them
to leave the water
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have now limited their size on land.
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Off you go.
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Bt there was another group of animals
built to a very different design,
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one that allowed them
to get much, much bigger.
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It all started around 550 million years ago
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with a tiny worm-like creature
that lived at the bottom of the sea.
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This is its closest living relative.
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It's the lancelet.
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It doesn't look like much, does it?
So why is it so special?
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Well, the lancelet has a revolutionary feature
that was vital to our evolution,
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It's the notochord - a stiff rod
running through the body -
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the beginnings of a backbone,
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A strong yet flexible lever
for muscles to pull against,
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Having your skeleton inside
also allows you to grow much, much bigger,
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Incredibly, the backbone evolved just once,
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but this one lucky break was
a crucial turning point in the journey of life,
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It led to a completely new family
of animals - the vertebrates,
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That's fish, amphibians,
reptiles, birds and mammals,
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And that includes you and me.
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So the backbone
which evolved to help animals swim
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would eventually help them
to colonise the land.
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But to walk, you need legs,
How did THEY evolve?
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Under water, mudskippers
look much like other fish..,
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..but watch what they can do with their fins,
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A fish that walks,
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Well, almost,
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The mudskipper can shuffle over land
using two pairs of modified fins,
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This is how we once thought
backboned creatures left the water -
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ancient fish propped themselves up onto fins,
and once ashore, the fins turned into legs,
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We now know that something
rather more surprising happened.
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Legs actually evolved for use
in the water - but how?
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The oceans are home
to other bizarre creatures
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that appear to walk across the seabed,
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Rosy-lipped batfish never leave the sea,
and they're perfectly good swimmers,
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but they also use their fins like pairs of legs,
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By stalking prey on stilt-like feet,
they don't stir up the water or sand,
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So this shrimp doesn't have a clue,
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The frogfish has also evolved
two pairs of modified fin feet
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to help it navigate the nooks and crannies
of the ocean floor,
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So fishy fin feet weren't a new idea,
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but evolution didn't really run with it
until around 370 million years ago,
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when the climate changed,
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The earth warmed up
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and became covered in shallow, weedy
swamps where normal fins got in the way,
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Over time, the paired fins started to evolve
a better shape for pushing through the weeds,
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until they ended up more like four feet,
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Those early four-footed creatures left a legacy
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which can be traced throughout
the tree of life right up to the present day,
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They were the ancestors
of all land-living backboned animals,
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The for-legged blueprint had been set,
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and all vertebrates that have ever lived
on land have followed the same basic pattern.
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And if for some reason our ancestral fish
hadn't had two sets of fins,
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I might have ended up with two legs
and no arms or even an extra set of arms.
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Which might have been quite useful
round about now!
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Whoa. Not only that, we could have ended up
with more fingers and toes as well.
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Having five might be just a fluke of nature.
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Fossils show that some early four-legged
creatures that crawled through the swamps
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had six, seven or even eight fingers,
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But the one successful species that became
the ancestor of all the later vertebrates
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just happened to have five fingers and toes,
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So having five fingers and four limbs
is an ancient blueprint
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all land-living animals inherited,
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Bt what about horses, you might ask.
Surely they don't have five fingers?
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Well, they don't have today,
but their ancestors used to.
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They started off with five fingers.
Gradually, over millions of years,
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they lost the use of four of them,
leaving them with a central digit.
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The nail turned into the horse's hoof,
and these three bones that are in the finger
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are present in the horse's leg today,
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and this big long bone is the same bone
that's hidden within my hand.
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Surprisingly, my wrist is exactly
the same joint as the horse's knee.
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OK, big fella.
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Come on, then. Good lad.
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And some of those lost digits are still present
at the start of every horse's life,
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For the first months inside the womb,
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a foal still has two extra toes,
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but in a replay of its evolutionary past,
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they fold back into the central cannon bone
before it's born,
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We now know how our underwater ancestors
evolved to walk,
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but there were other challenges they had
to face in leaving the water for the hostile land,
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To start with, how would
they manage to breathe?
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I can only breathe down here
because of this - an aqualung.
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In the same way that I'd drown down here
without this diving gear,
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life on land for fish is equally suffocating.
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Remarkably, though, there are some fish
that can breathe air just like us
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as they have done for 400 million years,
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Lungfish - still found
in Africa and Australia today,
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For those first sea creatures that came ashore,
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it took the winning formula of lungs..,
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..and legs to make the quantum leap
from ocean to land,
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Enter the first four-legged air breathers -
our ancestors,
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But even these groundbreaking creatures
couldn't conquer dry land just yet,
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First the land itself would have to change,
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The drying and burning effects of the sun
are potentially lethal,
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especially when you’re exposed like this...
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..with no shade.
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I have to drink about two litres of water
a day just to compensate...
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..and I've got a relatively waterproof skin.
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For the first four-legged creatures
with their delicate skins,
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this was an even bigger problem than it is for us,
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They needed moisture and they needed shade,
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They had to wait
for another invasion of the land -
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that of woody plants.
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Early plants were tiny algae, and like
every other form of life, confined to water,
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Just like animals, to make it on the land,
they had to combat gravity,
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They evolved a skeleton made up
of rings of lignin -
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the plants' equivalent of bone,
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Now they could leave water and stand alone,
228
00:26:57,430 --> 00:27:03,062
Lignin gave the plant cells strength
to keep on growing up and up..,
229
00:27:05,430 --> 00:27:08,581
..reaching towards the sunlight,
230
00:27:28,750 --> 00:27:33,744
Lignin eventually expanded
to fill up the cell walls and form wood,
231
00:27:38,750 --> 00:27:42,140
And wood allowed plants to really make it big,
232
00:27:42,310 --> 00:27:47,748
They evolved into the incredible hulks
of the land - the trees,
233
00:28:05,550 --> 00:28:11,739
Eventually, the battle for light
led to lush forests, cool, shady and moist -
234
00:28:11,910 --> 00:28:16,028
ideal conditions for delicate creatures
like or ancestors.
235
00:28:20,150 --> 00:28:22,141
So lignin created wood,
236
00:28:22,310 --> 00:28:24,221
and wood created forests,
237
00:28:24,390 --> 00:28:28,019
without which we wouldn't exist,
238
00:28:42,150 --> 00:28:44,220
Those early four-legged animals
239
00:28:44,390 --> 00:28:50,101
had a whole new world to explore,
with all the moisture and shade they needed,
240
00:29:00,550 --> 00:29:04,941
Damp forests everywhere
are still inhabited by their descendants -
241
00:29:05,110 --> 00:29:08,420
the soft-skinned amphibians,
242
00:29:12,150 --> 00:29:14,584
There are now more than 4,000 species,
243
00:29:14,750 --> 00:29:18,789
but even today, some 350 million years later,
244
00:29:18,950 --> 00:29:23,785
amphibians can't escape
the pull of their aquatic roots,
245
00:29:29,070 --> 00:29:31,504
Every spring, just after the snow melts,
246
00:29:31,670 --> 00:29:37,381
there's an extraordinary mass migration
through the forests of north-east America,
247
00:29:42,150 --> 00:29:47,144
Thousands of spotted salamanders that have
spent the past year hidden underground
248
00:29:47,310 --> 00:29:49,460
emerge with one mission in mind -
249
00:29:51,510 --> 00:29:55,389
to get to water for a single night of passion,
250
00:30:20,550 --> 00:30:23,747
Then, the females spawn,
251
00:30:31,990 --> 00:30:36,302
Amphibian eggs are soft
and dry out quickly in the air,
252
00:30:36,470 --> 00:30:41,339
which is why spotted salamanders
must still return to the water to breed,
253
00:30:45,550 --> 00:30:49,623
But ever since their ancestors
first crawled out of the swamps,
254
00:30:49,790 --> 00:30:56,059
amphibians have found all kinds of weird and
wonderful ways to break free from the water,
255
00:31:09,350 --> 00:31:14,219
The mountains of northern Spain -
a rocky, barren landscape,
256
00:31:19,350 --> 00:31:26,984
But the midwife toad is perfectly at home,
It has evolved to live and breed on dry land,
257
00:31:28,670 --> 00:31:32,709
So what do midwife toads do
that salamanders don't?
258
00:31:34,590 --> 00:31:37,388
(PIPING CALL)
259
00:31:38,430 --> 00:31:41,900
A pair meet up and start to mate,
260
00:31:45,710 --> 00:31:50,181
The male's embrace squeezes the female
until she releases her eggs,
261
00:31:57,790 --> 00:32:02,261
She catches them in her back legs,
where he fertilises them,
262
00:32:05,430 --> 00:32:09,503
But there's no water here,
How will the eggs survive?
263
00:32:11,230 --> 00:32:15,018
The male midwife toad
is one of nature's new men,
264
00:32:15,190 --> 00:32:20,344
He hoists the eggs up his legs as if
putting on a pair of shorts with no hands,
265
00:32:29,350 --> 00:32:32,865
From now on, the eggs are his responsibility,
266
00:32:33,030 --> 00:32:37,626
He carries them away and burrows
under a rock to keep them moist,
267
00:32:40,550 --> 00:32:43,622
The male holes up for several weeks,
268
00:32:46,350 --> 00:32:50,309
Then he has to go off to search for a pond,
269
00:32:54,430 --> 00:32:59,106
He must return to water, where the tadpoles
wriggle free and grow up,
270
00:32:59,270 --> 00:33:01,386
just like other toads,
271
00:33:18,030 --> 00:33:19,748
(CHIRPING, CROAKING)
272
00:33:19,910 --> 00:33:26,099
In this forest in Costa Rica, I can hear frogs
calling to their mates all around me,
273
00:33:26,270 --> 00:33:29,342
but there are no pools or streams
that I can see,
274
00:33:29,510 --> 00:33:32,582
How do they manage to breed here?
275
00:33:35,550 --> 00:33:41,386
This is the strawberry poison arrow frog,
and on its back is a tadpole
276
00:33:41,550 --> 00:33:45,065
It hatched on the ground,
but now gets a piggyback from Mum
277
00:33:45,230 --> 00:33:48,540
as she begins a mother of a climb,
278
00:33:59,270 --> 00:34:03,024
She carries her tadpole
all the way up into the trees..,
279
00:34:09,110 --> 00:34:12,864
..to its own cradle in the canopy,
280
00:34:13,910 --> 00:34:17,869
In these hanging gardens
up to 20 feet above the forest floor,
281
00:34:18,030 --> 00:34:21,864
and egg-cup-sized pond
is all this mother needs,
282
00:34:34,590 --> 00:34:38,299
And after all this effort,
her work still isn't finished,
283
00:34:41,430 --> 00:34:45,742
A few days later, she has
to make the same climb all over again,
284
00:34:45,910 --> 00:34:52,019
because although her tadpole has water
in its treetop nursery, it has no food,
285
00:34:52,190 --> 00:34:56,069
so it waits for her like a chick in its nest,
286
00:35:00,430 --> 00:35:05,868
As she lowers herself in, it headbutts her,
even gives her a little nip..,
287
00:35:07,910 --> 00:35:10,743
..until she gives it what it wants,
288
00:35:10,910 --> 00:35:15,700
She lays an infertile egg for it to eat,
289
00:35:18,350 --> 00:35:20,386
With regular meals from Mum,
290
00:35:20,550 --> 00:35:25,624
the tadpole has all it needs
in its tiny world to grow into a frog,
291
00:35:32,150 --> 00:35:37,304
Strawberry poison arrow frogs may be
the hardest-working frog mothers on Earth,
292
00:35:37,470 --> 00:35:41,986
but another frog takes the award
for the most devoted frog father.
293
00:35:48,790 --> 00:35:52,339
Guess where the Darwin's frog
keeps his tadpoles!
294
00:36:02,350 --> 00:36:06,229
The male's throat is the nursery
where the tadpoles grow,
295
00:36:06,390 --> 00:36:11,180
He gives birth by spitting out
the froglets fully formed,
296
00:36:16,910 --> 00:36:20,585
So the Darwin's frog
has reduced its need for water
297
00:36:20,750 --> 00:36:23,503
by carrying its own pond around inside it,
298
00:36:24,750 --> 00:36:27,867
But despite all these extraordinary solutions,
299
00:36:28,030 --> 00:36:31,864
almost all amphibians
are still basically tied to water,
300
00:36:32,030 --> 00:36:34,669
just as their early ancestors were,
301
00:36:35,630 --> 00:36:40,943
So how did the first land-living animals
colonise the deserts and dry lands?
302
00:36:42,150 --> 00:36:45,745
The entire spawning process
had to be transformed,
303
00:36:50,430 --> 00:36:53,900
Eggs evolved
their own internal life support system,
304
00:36:54,070 --> 00:36:58,860
and at the same time, shells developed
that could hold water,
305
00:37:04,030 --> 00:37:10,185
The arrival of the hard-shelled egg was
one giant step forward on the journey of life,
306
00:37:13,070 --> 00:37:17,143
Each egg provided the growing youngster
with its own portable pond,
307
00:37:17,310 --> 00:37:23,385
Eggs could now be laid on dry land, and
this led to a whole new group of animals -
308
00:37:24,350 --> 00:37:27,422
the reptiles,
309
00:37:36,190 --> 00:37:40,741
Reptile eggs contain more food reserves
than those of amphibians,
310
00:37:40,910 --> 00:37:45,062
giving their babies a head start in life,
311
00:37:59,750 --> 00:38:05,666
Australian bearded dragons hatch out
fully formed and ready to take on the world,
312
00:38:05,830 --> 00:38:08,822
just as they've done for millions of years,
313
00:38:20,750 --> 00:38:23,423
The reptiles evolved into many families,
314
00:38:23,590 --> 00:38:28,903
including one of the most impressive
dynasties ever to dominate the land,
315
00:38:36,510 --> 00:38:43,382
The smash hit that was the hard-shelled egg
led to the age of the dinosaurs,
316
00:38:50,550 --> 00:38:53,348
(ROARS)
317
00:38:55,350 --> 00:38:58,626
(SQUAWKS)
318
00:39:06,350 --> 00:39:11,470
Dinosaurs ruled the earth
for about 180 million years,
319
00:39:16,750 --> 00:39:19,787
There were as many as 700 species,
320
00:39:19,950 --> 00:39:24,228
from the size of a rat to that of a whale,
321
00:39:26,150 --> 00:39:29,347
But the egg wasn't the reptile's only strength,
322
00:39:29,510 --> 00:39:32,149
They had another trick up their sleeve -
323
00:39:32,310 --> 00:39:34,062
their skin,
324
00:39:35,270 --> 00:39:40,663
A tough scaly armour that could
withstand the driest conditions on Earth,
325
00:39:42,550 --> 00:39:47,340
And you don't get much drier
than the sand dunes of Namibia,
326
00:39:53,350 --> 00:39:58,504
But even here, reptiles have solved
the problem of drying out,
327
00:40:00,750 --> 00:40:04,709
And they can turn the power of the sun
to their advantage,
328
00:40:07,350 --> 00:40:09,068
First thing in the morning,
329
00:40:09,230 --> 00:40:14,623
the cold-blooded chameleon
heats up its side by facing it to the sun,
330
00:40:23,790 --> 00:40:28,818
Once it's recharged its solar batteries,
it's ready to go hunting,
331
00:40:36,870 --> 00:40:40,021
Here, armoured arthropods
have met their match,
332
00:40:40,190 --> 00:40:43,421
Despite arriving
more than 100 million years later,
333
00:40:43,590 --> 00:40:46,263
reptiles now appear to have the upper claw,
334
00:41:03,150 --> 00:41:06,062
But even reptiles have their limitations,
335
00:41:06,230 --> 00:41:10,223
because they slow down
as the temperature drops,
336
00:41:10,390 --> 00:41:14,941
and when the sun is gone,
it can get very cold,
337
00:41:17,710 --> 00:41:22,306
Bt for us and other mammals,
temperature does not limit where we can live.
338
00:41:22,470 --> 00:41:24,938
We can be active whether it's hot or cold.
339
00:41:27,630 --> 00:41:31,225
We do it by heating ourselves
from the inside,
340
00:41:31,390 --> 00:41:34,587
and we do that by burning up food,
341
00:41:37,630 --> 00:41:38,949
Ah!
342
00:41:41,230 --> 00:41:45,667
The reason my face is white hot
and hers is a cool red
343
00:41:45,830 --> 00:41:48,549
is because I am generating heat inside,
344
00:41:48,710 --> 00:41:50,746
whereas she is cooling down
345
00:41:50,910 --> 00:41:56,268
as she loses the heat that she's absorbed
from the sun during the day,
346
00:42:00,910 --> 00:42:04,141
Bt it's no good having
an internal heating system
347
00:42:04,310 --> 00:42:09,020
if all that heat is simply lost,
which is what's happening to me right now.
348
00:42:09,190 --> 00:42:12,307
You’d have to eat non-stop just to stay alive.
349
00:42:20,150 --> 00:42:21,947
What you need...
350
00:42:22,870 --> 00:42:25,623
..is good insulation.
351
00:42:38,350 --> 00:42:41,342
So there would have to be another breakthrough
352
00:42:41,510 --> 00:42:45,583
before land animals could reach
the coldest corners of the world,
353
00:42:47,750 --> 00:42:52,983
About 200 million years ago,
a reptile-like ancestor of mammals
354
00:42:53,150 --> 00:42:57,302
started growing fine barbs underneath its scales,
355
00:42:58,910 --> 00:43:02,823
Over many generations,
they became finer and longer,
356
00:43:02,990 --> 00:43:07,142
until eventually they turned into hair and fur,
357
00:43:12,550 --> 00:43:18,989
The mammals had arrived, and now they
were ready to take on the elements anywhere,
358
00:43:20,270 --> 00:43:23,228
Musk oxen have the ultimate fur coats,
359
00:43:23,390 --> 00:43:29,943
Their insulation is so good, they can survive
at temperatures of minus 40 degrees Celsius,
360
00:43:32,470 --> 00:43:37,305
Warmed by their central heating
and stoked up by regular meals,
361
00:43:37,470 --> 00:43:40,303
musk oxen have managed to beat the elements,
362
00:43:40,470 --> 00:43:44,702
and eat their way north
to the fringes of the Arctic Ocean,
363
00:44:09,350 --> 00:44:16,779
Central heating and fur coats let mammals
range even to the icy reaches of the poles,
364
00:44:27,350 --> 00:44:31,104
And this must be the ultimate test,
365
00:44:36,430 --> 00:44:40,901
Dense fur allows a baby seal to undergo
the biggest temperature shock
366
00:44:41,070 --> 00:44:44,301
experienced by any animal on Earth,
367
00:44:45,750 --> 00:44:49,106
That's from around 40 degrees
inside its mother's body
368
00:44:49,270 --> 00:44:54,981
to as low as minus 30
when it's born and it hits the ice,
369
00:44:55,710 --> 00:44:59,862
The miracle of this baby's survival here
depends not just on fur,
370
00:45:00,030 --> 00:45:02,624
but on its mother's dedication,
371
00:45:02,790 --> 00:45:06,465
She provides the fuel that keeps it warm inside,
372
00:45:27,430 --> 00:45:33,665
But the mammal that really takes
parenting to the extreme is the elephant,
373
00:45:39,910 --> 00:45:44,028
It takes nearly 22 months
to make a baby elephant,
374
00:45:44,190 --> 00:45:47,227
longer than any other animal,
375
00:46:00,150 --> 00:46:05,019
During that time, the embryo
floats in its own centrally-heated world,
376
00:46:05,190 --> 00:46:09,024
an echo of life's earliest existence
in warm primeval seas,
377
00:46:11,950 --> 00:46:18,423
And it's safer than inside an egg,
Mum has total control over the baby's growth,
378
00:46:20,350 --> 00:46:23,660
After investing all that effort
even before birth,
379
00:46:23,830 --> 00:46:28,745
it's not surprising mammals
take exceptional care of their young,
380
00:46:41,430 --> 00:46:47,869
For a mammal mother, birth
is just the start of a long and demanding job,
381
00:46:59,350 --> 00:47:01,659
A mammal mother is a mobile milk bar,
382
00:47:01,830 --> 00:47:05,425
and delivering enough of the white stuff
to build up baby
383
00:47:05,590 --> 00:47:09,060
is an even greater drain on her
than pregnancy,
384
00:47:10,510 --> 00:47:16,107
Milk is highly nutritious, and at this stage,
it's the only food that baby needs,
385
00:47:18,150 --> 00:47:20,539
As long as no one tries to nick it!
386
00:47:25,910 --> 00:47:30,347
Like all mammals,
baby elephants learn through play,
387
00:47:30,510 --> 00:47:33,183
and it's a good excuse to mess around,
388
00:48:03,030 --> 00:48:05,339
It's been an extraordinary journey,
389
00:48:05,510 --> 00:48:10,948
We've evolved a backbone,
lungs and four limbs..,
390
00:48:13,590 --> 00:48:16,866
..with five fingers and toes,
391
00:48:19,430 --> 00:48:20,829
We've grown fur
392
00:48:20,990 --> 00:48:26,110
and developed central heating to cope
with extremes of temperature on land,
393
00:48:26,910 --> 00:48:30,346
Bt the mammals needed
other changes to a body design
394
00:48:30,510 --> 00:48:33,422
which originally evolved in the sea.
395
00:48:34,430 --> 00:48:36,739
Our eyes work fine underwater,
396
00:48:36,910 --> 00:48:39,140
we can even taste there,
397
00:48:39,310 --> 00:48:42,143
but hearing is a different matter,
398
00:48:45,670 --> 00:48:51,381
Sound travels very differently
because water is 800 times as dense as air,
399
00:48:52,990 --> 00:48:58,223
So animals living on land
had to develop a totally new way of hearing.
400
00:49:13,430 --> 00:49:19,505
And when it comes to ears, few mammals
are more advanced than the bat-eared fox,
401
00:49:24,230 --> 00:49:30,544
It lives in the South African bush,
where it scans the ground listening for a meal,
402
00:49:32,470 --> 00:49:37,749
It hears in the same way
as all mammals do - it's just more sensitive,
403
00:49:40,150 --> 00:49:43,460
Broad outer ears capture
the soundwaves in the air
404
00:49:43,630 --> 00:49:48,385
and funnel them through to the eardrum,
making it vibrate,
405
00:49:50,230 --> 00:49:55,065
Tiny bones in the middle ear
transmit and amplify these vibrations,
406
00:49:58,270 --> 00:50:03,867
The sound waves are then converted
into nerve impulses and sent to the brain,
407
00:50:11,990 --> 00:50:17,542
Bat-eared foxes can hear grubs and termites
moving nearly a foot below the ground,
408
00:50:20,510 --> 00:50:22,546
Bad news for arthropods,
409
00:50:28,350 --> 00:50:31,865
A mammal's ear is a hi-tech triumph,
410
00:50:32,030 --> 00:50:36,546
How did the chance workings of evolution
create such a remarkable device?
411
00:50:37,550 --> 00:50:43,261
Well, believe it or not, it all began in
the solid jaws of prehistoric reptilian animals.
412
00:50:44,270 --> 00:50:48,741
Over millions of years, two of the jaw bones
became detached, shrank,
413
00:50:48,910 --> 00:50:53,062
and moved upwards to become
the key components of the mammal ear,
414
00:50:53,230 --> 00:50:57,428
They were hijacked by evolution
for an entirely different use,
415
00:50:58,750 --> 00:51:01,867
It's amazing to think
that these delicate ear bones
416
00:51:02,030 --> 00:51:03,543
have evolved from bones
417
00:51:03,710 --> 00:51:08,704
that were originally designed so or
reptile-like ancestors cold bite their prey.
418
00:51:08,870 --> 00:51:11,589
If you don't believe me, try eating an apple.
419
00:51:13,790 --> 00:51:18,545
We can sense echoes of our evolutionary past
every time we chew.
420
00:51:18,710 --> 00:51:22,020
We hear every crunch through or jaw bone.
421
00:51:24,150 --> 00:51:28,109
Not only mammals' senses
have been perfected through evolution.
422
00:51:28,270 --> 00:51:34,106
Mammals - and I include myself here - have
also become the cleverest animals on Earth.
423
00:51:37,750 --> 00:51:41,743
And there aren't many more intelligent
than baboons,
424
00:51:41,910 --> 00:51:44,299
They lead a complex social life,
425
00:51:44,470 --> 00:51:49,305
It's not just who you know, but what
you do with that knowledge that counts,
426
00:51:58,510 --> 00:52:03,061
Male baboons use brains
as well as brawn to get their way,
427
00:52:06,070 --> 00:52:11,588
If one male is threatened by another,
he'll pick up a baby as a living shield,
428
00:52:11,750 --> 00:52:16,460
He knows that no baboon will risk
hurting the youngster by attacking him,
429
00:52:28,350 --> 00:52:33,743
As well as being devious, baboons use
their intelligence to find new things to eat,
430
00:52:33,910 --> 00:52:36,299
(BUZZING INSECTS)
431
00:52:45,870 --> 00:52:50,785
Some have learned the art of ambush
to get much larger meals,
432
00:53:44,470 --> 00:53:47,826
Mammals are one of the most
successful animal groups.
433
00:53:47,990 --> 00:53:51,266
They can cope with the most
extreme climates on Earth,
434
00:53:51,430 --> 00:53:55,628
from the scorching heat of barren deserts
to the steaming rainforest,
435
00:53:55,790 --> 00:53:59,703
and from the deepest seas to great cities.
436
00:54:01,950 --> 00:54:04,418
But it could have been very different.
437
00:54:04,590 --> 00:54:07,229
(ROARING)
438
00:54:18,710 --> 00:54:23,101
More than 100 million years
after our mammal ancestors had arrived,
439
00:54:23,270 --> 00:54:26,740
the land was still ruled by dinosaurs,
440
00:54:28,470 --> 00:54:33,908
Most of the early mammals were shrew-like
creatures that ventured out mainly at night,
441
00:54:34,430 --> 00:54:39,550
The dramas of their lives took place
entirely in the shadow of the giants,
442
00:55:24,390 --> 00:55:30,340
But one event 65 million years ago
played right into the mammals' hands,
443
00:55:47,390 --> 00:55:50,939
A massive meteorite collided with the earth,
444
00:55:53,310 --> 00:55:58,782
85% of all land-dwelling animals
were snuffed out,
445
00:55:58,950 --> 00:56:04,468
The dinosaurs became extinct
over just a few thousand years,
446
00:56:06,590 --> 00:56:10,469
But among the animals
that did survive were mammals,
447
00:56:15,150 --> 00:56:19,985
Perhaps that was because they fed
on the other major group of survivors -
448
00:56:20,150 --> 00:56:22,345
the armoured arthropods,
449
00:56:23,750 --> 00:56:27,504
With the dinosaurs gone,
the mammals seized their chance,
450
00:56:27,670 --> 00:56:31,504
diversifying and expanding to fill the world,
451
00:56:43,310 --> 00:56:47,747
The mammal line exploded
into the wonderful variety we see today,
452
00:56:57,030 --> 00:56:59,544
Ultimately, that led to us.
453
00:56:59,710 --> 00:57:04,181
If that meteorite had missed the earth,
we probably wouldn’t be here.
454
00:57:06,270 --> 00:57:10,149
We've come a long way since or ancestors
crawled out of the sea.
455
00:57:10,310 --> 00:57:13,905
We've coped with everything
this hostile new world throws at s.
456
00:57:17,310 --> 00:57:20,939
But can we call ourselves
the true inheritors of the earth?
457
00:57:24,310 --> 00:57:27,427
No, It's the armoured arthropods,
458
00:57:27,590 --> 00:57:32,345
They colonised the land 100 million years
before we vertebrates,
459
00:57:36,070 --> 00:57:40,746
And today they outnumber
all other animals combined,
460
00:57:42,030 --> 00:57:46,626
Many have hardly changed
in over 200 million years,
461
00:57:48,510 --> 00:57:51,422
On their journey of life, land-grabbing arthropods
462
00:57:51,590 --> 00:57:54,388
developed an almost bomb-proof design,
463
00:57:57,430 --> 00:58:02,299
And no doubt they'll still be here
long after we've gone.
464
00:58:16,150 --> 00:58:18,380
Next week on the “Journey of Life”,
465
00:58:18,550 --> 00:58:23,305
I'll find out how life grew wings
and conquered the skies,