[00:22.30]Imagine a land so hostile that you'd need special equipment to breath.
[00:28.20]Where there's no shade and where the solar radiation would kill you in minutes.
[00:36.14]At night temperatures plummet to sub zero, you could freeze or fry in the same day.
[00:44.95]This is not some far away planet.
[00:47.35]Its earth as it would have been 500 million years ago,
[00:51.32]when the land was alien and uninhabitable.
[00:57.26]Back then, life was only to be found in the sea.
[01:02.00]How did life conquer this hostile world,
[01:05.21]taking an evolutionary journey that would one day lead to us?
[01:30.36]Over the millennia this harsh new world was invaded by a few pioneering life forms.
[01:38.14]These evolved into a multitude of ...animal designs able to...
[01:41.78]...to cope with the extremes ...of life on land
[01:52.39]Our family, the mammals... is just one of the results.
[02:03.20]Every living thing is linked by the branches on the tree of life...
[02:17.01]In this programme just how this extraordinary variety of animals...
[02:22.12]...arose and how we are connected to each and every one of them.
[02:34.96]It's a story full of surprises, which leads all the way from a fish... to you... and me.
[02:48.14]And how, if it wasn't for one giant twist of fate the dinosaurs might...
[02:53.11]...still rule supreme today.
[02:58.52]In the beginning, dry land was a no go... area for life.
[03:02.99]Because all life began in the sea...
[03:05.86]where the temperature hardly changes
[03:07.76]and where water protects against the pull of gravity and the burning sun.
[03:15.24]The sea was the first laboratory of life.
[03:18.67]And for more than three quarters of life's history...
[03:21.27]...it was home to every living thing.
[03:26.38]This was mostly a gentle era of soft bodied creatures like jellyfish.
[03:35.62]Jellyfish are 95% water and have no skeleton at all.
[03:41.56]Unlike us, they don't need one -
[03:44.33]Because water gives them all the buoyancy and support they need.
[03:49.07]Water suspends all life in a three dimensional world.
[03:53.31]For a diver, it's a bit like floating in space.
[04:07.95]It enables these kelp fronds to tower 30 meters upwards from the ocean bed.
[04:21.97]In fact, just the like animals, for hundreds of millions of years,
[04:25.94]plants could only exist in the sea.
[04:44.32]On land, this watery support would all be gone, so what would happen then?
[04:53.93]The kelp would collapse.
[05:00.94]And the diver would suddenly aware of the weight of his equipment
[05:04.84]because gravity is now dragging him down.
[05:13.02]How could any animal designed for life in the sea ever make it on dry land?
[05:19.76]One group of sea creatures had just the right kit to get up and go.
[05:25.37]Arthropods with their hard jointed skeleton.
[05:30.10]This living suit of armor holds it up, so gravity can't pull it down
[06:06.04]Armor gave arthropods the staying power they needed to make it on land.
[06:13.31]And its made crabs expert land grabbers
[06:33.83]In Florida, crabs have even invaded peoples gardens.
[06:38.84]And the lawns are riddled with their burrows.
[06:49.65]These blue land crabs can live up to 5 kilometers from water
[07:05.16]Crabs are actually well adapted to life on land.
[07:08.27]Their jointed, outer skeletons may have evolved in the sea...
[07:11.80]...but it also supports their weight on land making it easier for them to get around,
[07:16.94]...and it helps stop them from drying out.
[07:22.72]They roam at will, and pop up in surprising places.
[07:29.46]And they're not fussy about what they eat.
[07:42.27]On the other side of the world...
[07:43.80]In the Indian Ocean, there's a land where crabs have really made themselves at home.
[07:54.18]Christmas Island, is completely overrun with red land crabs.
[08:02.59]120 million of them live here on this small island outnumbering the human
[08:07.36]residents by 300,000 to 1
[08:12.43]Once a year, vast numbers hit the road.
[08:25.14]They're heading for the beach.
[08:26.88]Why? Because despite their armor they can't shake off their ties to the sea.
[08:42.26]There's a vital purpose to this mass maneuver.
[08:44.96]The females clasp thousands of soft eggs beneath them.
[09:00.15]Like all land crabs they have to return to the water to release their eggs.
[09:05.22]Their young still have to grow up in the sea.
[09:18.23]But the first creatures ever to venture on land lived long
[09:21.27]before crabs even came existed.
[09:27.24]We know because they left their footprints.
[09:34.45]Preserved in the rock in what is now Ontario Canada
[09:37.78]are the oldest footprints anywhere on earth...
[09:44.29]Just a few small steps for a bug - but one giant leap for life!
[09:51.33]These prints are 500 million years old,
[09:54.83]their maker is believed to have been an armored arthropod...
[09:58.60]...a bit like a giant wood louse more than 30 cm long...
[10:07.01]What made this ancient trailblazer drag itself ashore onto the barren land?
[10:17.16]500 million years ago,
[10:19.06]there were so many hardened hunters in the ocean that it had become a dangerous place.
[10:26.00]For the first time in the history of life
[10:28.23]there was good reason to leave the crowded seas.
[10:37.48]There's another ancient creature that re-traces those pioneering steps each year.
[10:50.49]The horseshoe crab has been around for hundreds of millions of years.
[11:00.13]And every year it still makes a dramatic pilgrimage to a beach
[11:03.70]in North America to breed.
[11:21.09]Horseshoes crabs lay their eggs on land...
[11:23.59]...to put them out of reach of marine predators.
[11:28.96]And its probably the same reason that brought those early trailblazers to shore.
[11:39.24]Job done, the horseshoe crabs turn round and crawl straight... back into the sea.
[11:45.58]Those first track makers were probably just visitors too.
[11:50.95]About 430 million years ago...
[11:53.92]...another group of arthropods abandoned the sea for good.
[12:01.09]The colonization of the land had truly begun.
[12:04.90]And it was creatures like these millipedes that lead the way.
[12:10.54]Others soon followed, spider and scorpion like creatures,
[12:14.44]almost a hundred million years before our ancestors left the sea.
[12:22.11]Today, more than 90% of land dwellers are arthropods. Most of them insects
[12:32.09]Amongst the most successful are the ants,
[12:34.96]which keep their eggs and young moist in nests underground.
[12:42.17]Driver ants have evolved sophisticated strategies to breed and feed on land.
[12:47.67]By massing in their millions around the nest,
[12:50.04]they raise its temperature and speed up the development of the young inside.
[12:58.82]Their sturdy skeletons carry them easily across the ground
[13:01.95]and stop them from drying out.
[13:14.10]These fearsome jaws have... made them deadly hunters.
[13:17.44]Thousands work together as one giant predator devouring everything in its path.
[13:25.41]And many jaws make light work of creatures far bigger than themselves.
[13:33.02]But the arthropod's jointed construction does have drawbacks.
[13:36.49]The bigger they get, the heavier the armor, and the harder it is to breathe.
[13:42.33]It would take another group of animals to overcome this hurdle.
[13:48.00]It all started in the sea about 550 million years ago
[13:51.14]With a tiny wormlike sea creature...
[13:55.31]...this is its closed living relative - a lancelet.
[14:01.25]It has a revolutionary feature vital to our evolution.
[14:05.12]A rod running through the body - called the notochord.
[14:12.19]It provided a strong yet flexible lever for the muscles to pull against.
[14:16.70]And led the evolution to whole new family of backboned animals, The vertebrates.
[14:26.01]Having your skeleton inside allows you to grow much much bigger.
[14:34.15]The backbone's evolution happened only once,
[14:37.38]but this lucky break was a crucial turning point in the journey of life.
[14:50.23]It led to all the vertebrates - that's fish, amphibians, reptiles, birds and mammals.
[14:58.97]Including you and me.
[15:05.95]But to walk you need legs. How did they evolve?
[15:11.38]Underwater mudskippers look much like other fish.
[15:15.09]But watch what they can do with their fins.
[15:25.43]A fish that walks, well almost.
[15:29.24]The mudskipper can shuffle over land using two pairs of modified fins.
[15:36.34]This is how we once thought backboned creatures left the water...
[15:40.05]Ancient fish propped themselves up on fins, and once ashore the fins turned into legs.
[15:59.30]We now know that something even more surprising happened.
[16:04.44]Legs actually evolved for use in the water.
[16:12.21]Deep in the ocean other bizarre sea creatures seem to walk across the sea bed.
[16:21.12]Batfish never leave the sea, and they can swim perfectly well.
[16:25.69]But they too use their fins like pairs of legs.
[16:39.10]By stalking prey on stilt-like feet they don't stir up water or sand.
[16:51.32]So this shrimp doesn't have a clue.
[17:05.03]The frogfish has also evolved two... pairs of modified fin feet
[17:10.00]to help navigate the nooks and crannies of the ocean floor.
[17:24.05]So fishy fin feet weren't a new idea.
[17:27.39]But evolution didn't really run with it until around 370 million years ago
[17:32.76]when the climate changed
[17:35.26]The earth warmed up and became covered in shallow weedy swamps...
[17:39.16]...where normal fins got in the way
[17:48.11]Over time the paired fins began to develop a better shape for pushing through the weeds.
[17:53.75]Until they ended up more like four feet.
[18:00.15]And those early four-footed creatures left a legacy
[18:03.52]which can be traced throughout the tree of life right up to the present day.
[18:16.14]They were the ancestors of all land-living, back-boned animals...
[18:26.31]The four-legged blueprint had been set and all vertebrates
[18:29.98]that have ever lived on land have followed the same basic pattern.
[18:34.85]But this was just a fluke of nature,
[18:37.32]if our ancestral fish had had more fins,
[18:39.93]we might have... ended up with more set of arms.
[18:43.33]And we now know that we might easily have had many more fingers and toes.
[18:56.51]Fossils show that some of those early four-legged creatures
[19:00.15]that crawled through the swamps had six... seven... or even eight fingers.
[19:07.85]But the one successful species that became the ancestor of
[19:10.99]all the later vertebrates just happened to have five fingers and toes.
[19:20.67]So having five fingers and four limbs is an ancient blueprint all land
[19:25.14]- living animals inherited.
[19:32.48]But horses don't have five toes
[19:38.08]Well they don't have today, but their ancestors used to.
[19:42.62]And you can find some of those lost toes at the beginning of every horse's life.
[19:47.69]In the unborn foal, there are two extra toes,
[19:51.46]only present in the first months of development.
[19:53.53]In a replay of the horses' evolutionary past
[19:57.14]they fold back into the central cannon bone before the foal is born.
[20:15.09]We now know how our underwater ancestors evolved to walk...
[20:19.22]but there were other challenges they had to face in leaving
[20:21.76]the water for the hostile land.
[20:26.97]To start with, how would they manage to breathe?
[20:32.34]Down here we need special breathing equipment and in the same way...
[20:36.58]...that we'd drown without this diving gear, most fish would suffocate on land.
[20:47.72]Remarkably, though, there are some fish that can breathe air just like us...
[20:51.42]as they have done for 400 million years.
[20:57.96]Lungfish - still alive and well in Africa, Australia and South America today.
[21:22.32]For those first sea creatures that came ashore it took the winning formula of lungs...
[21:30.33]and legs... to make the quantum leap from ocean to land.
[21:37.07]Enter the first four-legged air-breathers... our ancestors.
[21:45.91]But even these groundbreaking creatures couldn't conquer dry land just yet.
[21:53.65]They needed shade and moisture, so they had to wait until another group invaded first.
[21:59.42]The woody plants
[22:07.10]Early plants were tiny algae and like every other form of life, confined to water.
[22:14.07]Just like animals to make it on the land they had to combat gravity.
[22:18.94]They evolved a skeleton, made up of rings of lignin
[22:22.35]- the plants' equivalent of bone.
[22:25.98]Now they could leave water and stand alone...
[22:33.19]Lignin gave plants the strength to grow up and up reaching towards the sunlight
[22:55.75]Lignin eventually expanded to fill up the cell walls and form wood
[23:03.86]And wood allowed plants to really make it big...
[23:07.46]...they evolved into the incredible hulks of the land, the trees.
[23:29.98]So lignin created wood and wood created forests
[23:34.35]- without which we wouldn't exist.
[23:48.97]A new damp shady world opened up for those early four-legged animals to explore.
[24:13.83]Even today, some 350 million years later,
[24:17.53]amphibians still can't escape the pull of their aquatic roots.
[24:28.14]Every spring, just after the snow melts,
[24:30.88]there's an extraordinary mass migration through the forests of North East America.
[24:39.28]Thousands of spotted salamanders that have
[24:41.62]spent the past year hidden underground emerge with one mission in mind...
[24:50.10]To get to water for a single night of passion.
[25:16.19]The following night the females spawn...
[25:30.04]Amphibian eggs are soft and dry out quickly in the air
[25:34.14]- which is why spotted salamanders still have to return to the water to breed.
[25:42.11]Ever since their ancestors first crawled out of the swamps,
[25:45.28]amphibians have come up with all kinds of weird
[25:47.85]and wonderful ways to break free from the water.
[26:05.40]The mountains of Northern Spain... A rocky barren landscape...
[26:15.51]But the midwife toad is perfectly at home.
[26:18.64]It has evolved to live and breed on dry land...
[26:23.48]So what do midwife toads do that salamanders don't?
[26:29.25]A pair meet up and start to mate...
[26:41.30]The male's embrace squeezes the female until she releases her eggs.
[26:53.04]She catches them in her back legs where he fertilizes them.
[27:01.62]But how will the eggs survive without water?
[27:10.29]The male midwife toad is one of nature's most devoted dads.
[27:14.36]He hoists the eggs up his legs, as if wriggling into a pair of shorts.
[27:28.01]From now on the eggs are his responsibility.
[27:31.28]And for the first few weeks he holes up under a rock to keep them moist.
[27:42.03]After that however, he has to emerge to find a pond.
[27:46.30]And there the tadpoles wriggle free and grow up just like other toads.
[28:07.25]What other solutions has evolution come up with so that frogs
[28:10.45]and toads can live further away from water?
[28:15.29]This forest in Costa Rica is full of frogs calling to their mates.
[28:21.83]There don't seem to be many pools and streams around here.
[28:25.13]So how do they breed?
[28:31.31]The strawberry poison arrow frog carries its tadpole on its back.
[28:36.28]The tadpole hatched out on the ground,
[28:38.45]but now it gets a piggyback- as mum begins a mother of a climb!
[28:54.16]One at a time, she carries her four or five tadpoles all the way up into the trees
[29:13.05]Each to its own cradle in the canopy.
[29:16.25]An egg cup sized pond in these hanging gardens up to seven meters above the forest floor.
[29:26.13]And after all this effort, mum's still got more work to do!
[29:33.00]A few days later she must make the same climb all over again...
[29:36.77]because although her tadpole has water in its treetop nursery it has no food,
[29:42.31]so it waits... like a chick in its nest.
[29:52.36]As she lowers herself in, the tadpole head-butts her, even gives her a nip.
[29:59.86]Until she lays an infertile egg for it to eat
[30:07.04]With these regular food parcels the tadpole has all
[30:10.07]it needs in its tiny world to grow into a frog
[30:16.51]Strawberry poison arrow frogs may be the hardest working...
[30:19.52]...frog mothers on earth but another frog is the most devoted frog father.
[30:30.16]Guess where the Darwin's frog keeps his tadpoles!
[30:38.20]The male's throat is the nursery where they grow... he gives birth,
[30:43.04]as it were - by spitting out he froglets fully formed!
[31:01.99]So the Darwin's frog has reduced its need for water by carrying its own pond around inside.
[31:10.43]But despite all these extraordinary solutions
[31:13.24]almost all amphibians are still basically tied to water
[31:17.01]- just as their early ancestors were.
[31:23.18]So how did the first land-lovers colonize the deserts and dry lands?
[31:33.02]The entire spawning process had to be transformed...
[31:40.16]Eggs evolved their own internal life support system...
[31:43.77]And at the same time shells developed that could hold water.
[31:57.21]The arrival of the hard-shelled egg was one giant step forward on the journey of life.
[32:04.42]Each egg provided the growing youngster with its own private pond.
[32:08.19]Eggs could now be laid on dry land
[32:11.06]and this led to a whole new group of animals... the reptiles.
[32:25.98]Reptile eggs contain more food reserves than those of the amphibians,
[32:30.11]giving their babies a head start in life.
[32:45.33]Australian bearded dragons hatch out fully formed
[32:48.56]and ready to take on the world - just as they've done for millions of years.
[33:10.12]The reptiles evolved into a multitude of families,
[33:13.56]including one of the most impressive dynasties ever to dominate the land.
[33:27.30]The smash hit that was the hard-shelled egg led to the Age of the Dinosaurs.
[34:04.24]Dinosaurs ruled the earth for about 180 million years.
[34:08.24]There were as many as 700 species, from the size of a rat, to that of a whale.
[34:19.36]And the egg wasn't the reptiles' only strength.
[34:22.26]They also developed tough scaly armor...
[34:24.69]...that could withstand the toughest... conditions on earth.
[34:32.97]And you don't get much drier than the sand dunes of Namibia.
[34:43.25]But even here reptiles have solved the problem of drying out...
[34:47.18]...and they can turn the power of the sun to their advantage.
[34:55.96]First thing in the morning the cold-blooded chameleon heats up
[34:59.20]along the side facing the sun...
[35:14.08]Once it's recharged its solar batteries it's ready to go hunting.
[35:24.25]Here, the armored arthropods have met their match...
[35:27.46]Despite arriving more than 100 million years later,
[35:30.63]reptiles now appear to have the upper claw!
[35:57.99]But even reptiles have their limitations
[36:00.29]because they slow down as the temperature drops,
[36:03.89]and when the sun is gone it can get very cold.
[36:10.80]We mammals on the other hand, can remain active whether its hot or its cold
[36:17.04]That's because we heat ourselves from the inside by burning food as fuel.
[36:28.48]A human face shows white hot, while this iguana's is a cool red.
[36:33.62]Because while we stay warm while we generate heat inside,
[36:37.53]the iguana is cooling down as it looses the heat it absorbed from the sun during the day.
[36:51.21]But we need to keep the heat we generate.
[36:53.71]Otherwise we'd have to eat non-stop just to stay alive.
[36:57.61]What's required is effective insulation.
[37:05.92]So there would have to be another breakthrough to allow
[37:08.76]land animals to reach the coldest corners of the world.
[37:18.37]About 200 million years ago a reptile-like ancestor of mammals
[37:22.64]started growing fine barbs underneath its scales.
[37:29.95]Over many generations, they became finer and longer,
[37:34.05]until eventually they turned into hair and fur.
[37:42.22]The mammals had arrived... and now they were ready to take on the elements anywhere.
[37:51.33]Musk oxen have the ultimate fur coats.
[37:54.27]Their insulation is so good they can survive at temperatures of minus 30 degrees Celsius.
[38:16.29]Warmed by their central heating and stoked up by regular meals...
[38:19.93]musk oxen have managed to beat the elements
[38:22.53]and eat their way north to the fringes of the Arctic ocean.
[38:46.16]Central heating and fur coats...
[38:48.29]coats let mammals range even to the icy reaches of the poles...
[38:57.10]And this must be the ultimate test.
[39:09.11]Dense fur allows a baby seal to undergo the biggest temperature shock experienced
[39:14.25]by any animal on earth... from around 40 Celsius degrees inside
[39:19.05]its mother's body to as low as minus 30 when it's born and it hits the ice.
[39:30.00]The miracle of this baby's survival here depends not just on fur
[39:34.00]but on its mother's dedication.
[39:36.24]She provides milk, the fuel that keeps it warm inside.
[39:51.12]But the ultimate mammal mother lives in Africa.
[40:15.91]It takes nearly 22 months to make a baby elephant, longer than any other animal.
[40:23.09]During that time the embryo floats in its own centrally heated world...
[40:26.99]an echo of life's earliest existence in warm primeval seas.
[40:40.20]And it's safer than inside an egg
[40:42.84]- mum has total control over the baby's growth.
[40:49.14]After investing all that effort even before birth
[40:52.11]it's not surprising mammals take exceptional care of their young.
[41:11.00]For a mammal mother, birth is just the start of a long and demanding job.
[41:29.08]Mum is a mobile milk bar.
[41:31.99]And delivering enough of the white stuff to build up baby
[41:34.96]is an even greater drain on her than pregnancy.
[41:42.36]Milk is highly nutritious and at this stage it's the only food that baby needs.
[41:50.41]No wonder others try to steal it!
[41:55.68]Like all mammals, baby elephants learn through play...
[41:59.82]and it's a good excuse to mess around.
[42:34.28]The evolution of mammals has been an extraordinary journey...
[42:38.79]our ancestors evolved... a backbone, lungs,
[42:42.29]and four limbs... ...with five fingers and toes.
[42:49.87]And we grew fur and installed central heating
[42:52.94]which could cope with the extremes of temperature on land.
[42:59.34]But mammals needed other changes to move from the water to the air
[43:06.05]For one thing, they needed a totally new way of picking up sound
[43:12.62]And when it comes to ears, few mammals are more advanced than the bat-eared fox.
[43:22.30]It lives in the South African bush where it scans the ground listening for a meal.
[43:32.11]It hears in the same way as all mammals do - it's just more sensitive.
[43:39.25]Broad outer ears capture... ...the sound waves in the air
[43:42.28]and funnel them through to the eardrum, making it vibrate.
[43:49.52]Tiny bones in the middle ear transmit and amplify these vibrations.
[43:59.27]The sound waves are then converted into nerve impulses and sent to the brain.
[44:11.25]Bat-eared foxes can hear grubs and termites moving nearly 30cm below the ground.
[44:30.63]So how did the chance workings of evolution create such a remarkable device?
[44:37.51]Believe it or not it all began in the solid jaws of prehistoric reptilians.
[44:44.15]Two of the jaw bones became detached,
[44:46.88]they shrank and moved upwards to become the key components of the mammal ear...
[44:51.82]Hijacked by evolution for a totally different use.
[44:56.56]It's amazing to think that these tiny delicate bones...
[44:59.39]...evolved from the solid biting jaws of our reptile-like ancestors
[45:06.94]Mammals have now... reached all corners... of the globe...
[45:10.37]and can live almost anywhere...
[45:18.61]But this could have been a very different story.
[45:26.25]More than 100 million years
[45:27.92]after the mammal arrived... ...the land was still ruled by the dinosaurs.
[45:45.11]Most of... the early mammals were shrew-like creatures
[45:48.44]that ventured out mainly at night.
[45:51.58]The dramas of their lives took place entirely in the... shadow of the giants.
[46:39.19]But one cataclysmic event 65 million years ago played right into the mammals' hands...
[46:50.37]A massive meteorite collided with the earth...
[47:10.96]...and 85% of all land-dwelling animals were snuffed out.
[47:15.70]Dinosaurs became extinct over just a few thousand years.
[47:22.34]But among the animals that did survive were mammals...
[47:30.31]Perhaps that was because they fed on the other major group of survivors
[47:34.12]- the armored arthropods.
[47:44.93]With the dinosaurs gone the mammals seized their chance diversifying
[47:49.76]and expanding to fill the world.
[47:58.11]The mammal line exploded... Into the wonderful variety we... see today.
[48:08.88]Ultimately, that led to us.
[48:11.89]If that meteorite had missed the Earth, we probably wouldn't be here.
[48:19.13]We've come a long way since our ancestors first crawled out of the sea
[48:23.60]and coped with everything that this hostile new world can throw at us.
[48:32.14]But can we really call ourselves the true inheritors of the earth?
[48:38.28]No - that title belongs to the armored arthropods...
[48:42.78]...they colonized the land a hundred million years... before we vertebrates.
[48:47.16]And today they outnumber all other animals combined...
[48:55.10]On their journey of life... ...the land grabbing arthropods...
[48:58.53]...developed an... apparently bombproof design.
[49:04.31]No doubt they'll still be here long after we have gone.
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