[00:11.50]Imagine the freedom... The excitement, of taking to the skies!
[00:20.20]To fly as free as a bird is something we humans can only dream of
[00:25.78]...but for two thirds of the animals alive today it's a reality.
[00:38.29]Flight is one of the greatest breakthroughs in the whole story of evolution.
[00:44.29]We're going to find out just how life achieved this miracle and took to the skies.
[00:54.04]A string of evolutionary accidents created wings in four very different ways
[01:03.25]Hundreds of millions of years apart.
[01:20.20]Flight is a unique weapon.
[01:25.04]Mastering the skies leaves earthbound creatures at your mercy...
[01:35.11]When there's a black eagle on the wing,
[01:37.18]rock hyrax need to be especially alert to aerial attack...
[02:20.26]But wings aren't just about extra strike power.
[02:25.03]Flying is also the fastest, most efficient way to travel...
[02:29.00]It allows you to find food, explore new territory or escape from danger.
[02:38.21]So there were lots of good reasons for life to get into the air...
[02:43.55]...but who would overcome gravity and get airborne first
[02:50.82]500 million years ago, animals took their first steps onto land.
[03:02.63]The pioneers included millipedes... scorpions... and spiders.
[03:11.38]While these creatures quickly spread over the earth...
[03:14.88]the skies remained empty and lifeless.
[03:24.42]Life stayed grounded for at least a hundred million years.
[03:28.56]Until one group of animals finally defied gravity
[03:32.16]and changed the course of evolutionary history for ever.
[03:42.41]The insects - the first animals ever to fly.
[03:48.41]But how did they sprout wings?
[03:52.75]One idea is that it's all because insects love to sunbathe...
[04:01.56]Insects are cold-blooded - they need heat from outside to warm up and move around.
[04:08.23]They often bask in the sun... and maybe in the past,
[04:12.07]one type of bug developed small flaps on its back to help soak up the rays
[04:17.44]- like mini solar panels.
[04:20.54]Purely by chance, these panels may also have helped the insect to be carried by the wind...
[04:34.12]Perhaps the panels then evolved into a more aerodynamic shape
[04:38.50]- eventually becoming wings.
[04:46.30]But there's a flaw in this theory.
[04:48.74]Insect wings have complex hinged joints, worked by muscles and tendons.
[04:55.15]Could they really have evolved from those simple fixed panels?
[05:02.89]There is another explanation as to how flight began...
[05:06.69]bizarrely it may have started in the water.
[05:16.00]Many insects spend the first part of their lives submerged...
[05:21.07]They breathe through gills, pumping them up and down to take in oxygen...
[05:26.04]Some scientists believe that in the past gills like these gradually turned into wings.
[05:34.35]What triggered such a dramatic change?
[05:38.76]One insect living in the rivers of North America holds a clue...
[05:46.03]Stonefly larvae spend almost their first year underwater,
[05:50.40]then emerge in late winter to start their adult lives.
[05:56.04]The adults live just a few days and their sole purpose is to breed...
[06:05.65]But to find a mate, they have to make a death-defying crossing to the shore...
[06:14.26]Their wings are far too weak to fly...
[06:20.03]But they do make good sails... perhaps this is how flight began...
[06:26.24]An ancient insect waved its gills to catch the breeze,
[06:29.71]and set sail, just like stoneflies do today.
[06:38.05]Gills already had the joints and muscles to flap up and down,
[06:42.09]and could then have evolved from sails to wings.
[06:48.33]And then - insects really took off!!
[07:01.14]Whatever that first flying insect was,
[07:03.54]it won the biggest evolutionary jackpot of all time...
[07:10.05]Its genes were now unstoppable...
[07:12.48]today its legacy is around 10 million flying insect species...
[07:17.55]the most successful type of animal on earth.
[07:52.92]We may never know exactly what that first aerial insect looked like,
[07:56.99]but fossils have shed light on some of its descendants
[08:06.07]Among them a design that's really stood the test of time.
[08:13.28]Dragonflies have been around for more than 300 million years.
[08:18.88]They were already on the wing when our ancestors crawled out of the swamps.
[08:34.16]Dragonflies are the fighter planes of the insect world...
[08:38.54]Fast, Agile, aerial Aces
[08:43.47]These are male emperor dragonflies, and they're fiercely territorial.
[08:48.91]When rivals meet, they test each other's flying skills to the limit
[09:08.27]These dogfights can be vicious, even deadly...
[09:20.21]Dragonflies are dazzling fliers.
[09:22.58]But the best insect stunt pilot of them all is smaller and much less glamorous.
[09:34.12]The fly. We all know how hard it is to swat one.
[09:48.00]Their maneuverability is unrivalled and their reactions are 12 times
[09:52.34]faster than our own...
[09:58.28]So what's their secret?
[10:00.98]Well, bizarrely, it's because the flies ancestors gave up a pair of wings.
[10:07.36]Like dragonflies, all early insects had two pairs.
[10:19.27]In one type though, the hind wings shrank, evolving into tiny rod
[10:23.81]and ball structures called halteres.
[10:27.71]They're high tech precision instruments...
[10:30.35]and work a bit like gyroscopic stabilizers on a modern helicopter or plane.
[10:38.96]Flies come in a great variety of shapes and sizes -
[10:42.53]they include the hoverfly, crane fly, mosquito and our friendly housefly.
[10:49.17]And they all have halteres.
[10:53.70]Halteres give the fly precise feedback on its position,
[10:57.14]helping it make split second adjustments without losing control...
[11:01.61]So it can dodge a swipe and instantly regain perfect poise...
[11:22.03]There are now more than a hundred thousand kinds of fly.
[11:29.31]Their duck & dive design has let them take over the world
[11:33.34]and though they're small,
[11:34.74]they have a massive impact on other life, including us.
[11:41.45]In fact, flies are the most dangerous animals on earth,
[11:44.75]responsible for more human deaths than any other creature.
[11:51.90]They carry and spread disease...
[11:54.46]and because they're so small and maneuverable they're impossible to avoid.
[12:15.39]But being a successful insect isn't just about having the top technology for flight.
[12:22.59]It's also about when to fly.
[12:25.09]It's hard to believe that this...
[12:27.66]...and this are the same kind of animal.
[12:37.01]Why does such a gorgeous creature have such weird and wingless offspring?
[12:46.22]Caterpillars are munching machines, their only purpose is to eat and grow...
[12:51.79]At this stage, having wings would just get in the way.
[13:02.13]A caterpillar can balloon to an incredible 2,000 times its starting weight.
[13:07.87]That's like a human baby gaining eight tons in a month -
[13:11.64]and becoming the size of a bus!
[13:17.35]Once it's finished gorging, though,
[13:19.05]the caterpillar uses all those calories to make a stunning transformation...
[13:31.06]From ugly bug...
[13:38.94]To winged beauty...
[13:43.77]The adult has a very different mission... to attract a mate and lay eggs far and wide.
[13:51.81]Now wings are a big help and one butterfly has taken air travel to extremes.
[14:00.36]This is the monarch butterfly, long haul operator of the insect world.
[14:09.10]It's late summer in Canada
[14:10.93]and these are great, great grandchildren of monarchs
[14:13.84]that had flown north in spring.
[14:20.88]Now they're ready to embark on one of nature's most miraculous journeys.
[14:36.06]Triggered by shorter, cooler nights, they head south,
[14:39.63]flying the entire length of North America, and beyond...
[14:59.02]Finally, up to five thousand kilometers away
[15:02.32]the monarchs reach their winter destination in the mountains of central Mexico.
[15:12.00]Somehow they find their way back to the same trees used by previous generations...
[15:17.53]Millions cluster together for safety and warmth, creating an amazing spectacle.
[15:50.37]Insects were the first creatures into the air,
[15:53.14]and flew unchallenged for about a hundred million years.
[15:56.94]Then another twist of evolution brought a second wave of flight...
[16:04.58]Down on the ground was the ancestor of a different group of animals
[16:07.98]that produced some of the most impressive flyers ever -
[16:32.54]Sadly no pterosaurs are left to help us tell their story.
[16:38.75]What we do know is that they evolved from an ancient kind of reptile.
[16:44.72]How did a four-legged reptile...
[16:46.89]turn into one of nature's most magnificent flyers.
[16:51.63]Today, tree dwelling reptiles offer a clue.
[16:57.03]For a green iguana cornered in the treetops, there's just one way out...
[17:04.27]They generally hang out over water, for a safer landing.
[17:17.22]A simple free fall isn't very stylish - but it's a start...
[17:30.73]If you can add some control to your descent then you're a step closer to taking flight.
[17:38.31]And in the forests of South East Asia lives an animal
[17:41.38]that's taken free fall that one stage further.
[18:02.33]This gecko has evolved a kind of onboard parachute.
[18:08.77]Webbed toes and flaps of skin along its sides act like air brakes,
[18:12.94]giving it some control over its fall.
[18:23.09]It can't go far but it's a leap in the right direction.
[18:31.63]And if you think that's impressive, watch what its neighbor can do!
[18:36.43]The draco looks like any other lizard - until it jumps...
[18:46.08]This simple wing is actually skin membrane stretched over its ribs.
[18:53.18]It lets this tiny lizard glide up to a hundred meters -
[18:57.92]still not strictly speaking flying -
[19:00.42]but it saves an awful lot of climbing up and down!
[19:06.43]At some point an ancient gliding reptile took the next crucial step.
[19:11.77]Turned its arms to wings and developed true powered flight.
[19:22.11]This second airborne breakthrough heralded the arrival of the pterosaurs.
[19:52.98]Fossils suggest that many lived like some birds do today.
[19:59.32]One had a mouth with a built -in sieve - to filter out small creatures from the water...
[20:05.66]much as a modern flamingo does.
[20:18.27]Another had sharp hooked teeth good for snagging slippery prey.
[20:23.61]It used its beak to trap fish just below the surface,
[20:27.48]like a skimmer does today.
[20:33.08]Another pterosaur had an expandable throat pouch,
[20:36.35]a built-in fishing net... it was a kind of prehistoric pelican.
[21:00.11]But there's one thing about the pterosaurs no modern bird can match -
[21:09.09]Some got as big as a hang glider
[21:13.16]How did these giants take off and stay airborne?
[21:18.49]As for a hang glider a head wind would be vital to provide enough lift to get off the ground.
[21:29.04]The largest pterosaur, Quetzalcoatlus measured 12 meters across
[21:35.54]But how did the world's biggest ever flying animal stay up?
[21:43.92]Pterosaurs were built a bit like this glider - with massive long wings and a small body.
[21:51.16]Like the glider's hollow aluminum tubes
[21:53.76]their bones were also partly hollow to minimize weight.
[21:58.80]Once up in the air they'd have soared using updrafts
[22:02.04]and thermals just as a glider pilot does.
[22:28.30]Pterosaurs dominated the skies for 150 million years
[22:32.47]but they vanished along with the dinosaurs.
[22:37.47]By then a third airborne animal had arrived.
[22:41.41]Its success was down to a brand new structure - the feather.
[22:47.45]It led to perhaps the most beautiful of all flying animals.
[23:26.29]The surprising thing about feathers though is that
[23:29.33]they didn't even evolve for flight...
[23:33.06]in fact feathers didn't begin with birds at all.
[23:36.30]The birds inherited them.
[23:47.28]Birds are descended from small dinosaurs,
[23:49.91]a bit like this one preserved in rock from a 124 million years ago.
[23:57.22]Look closely and you'll notice some mysterious impressions - feathers.
[24:13.10]So what made these scaly hunters go all fluffy?
[24:21.34]Feathers are in fact modified scales...
[24:24.31]Over countless generations, some scales must have begun to thin and fray,
[24:29.32]evolving into fluffy down.
[24:35.16]But why did dinosaurs need feathers?
[24:42.23]Well they'd be perfect for keeping warm like those found on young birds
[24:48.50]In the Antarctic albatross chicks survive sub-zero temperatures for months on end.
[24:53.68]Thanks to their thick down.
[24:57.58]Downy feathers probably evolved to insulate small dinosaurs...
[25:01.68]But they're not much good for making a wing.
[25:06.32]So how did fluffy feathers turn into stiff ones like these used for flight.
[25:21.17]The latest theory is that evolution hijacked the feather
[25:24.51]for a completely different job long before it was used for flight...
[25:30.28]It's now thought that feathers might have been used for display,
[25:34.02]just like many birds use them today.
[25:53.07]And stiff display feathers are far more spectacular than fluffy down.
[26:00.38]Male birds flaunt themselves to win a female, and perhaps it was the same for dinosaurs.
[26:06.25]By choosing the most striking males,
[26:08.35]the females helped feathers evolve in a completely different way.
[26:15.29]As soft feathers became longer and stronger,
[26:18.13]they were much more suitable for building wings, as well as showing off.
[26:28.14]But how did that first leap into the skies actually happen?
[26:45.09]The most popular theory is that flight started up in the trees.
[26:51.26]Feathered arms would come in very handy for a tree-dwelling dinosaur.
[26:56.60]For a start they'd break a fall.
[27:00.24]And it's easy to imagine how a leap could then become a glide.
[27:05.94]Arms were slowly reshaped into wings...
[27:08.94]strong enough to flap their way into the air...
[27:14.72]Gliding became true powered flight.
[27:20.42]Enter the birds!
[27:38.04]The evolution of birds was a massive breakthrough.
[27:41.61]Their superb flying ability unlocked a way of life that has proved very successful.
[27:51.29]The dinosaurs died out, yet birds survived and more than that,
[27:55.56]went on to dominate the skies...
[28:13.68]All this, thanks to the fabulous feather!
[28:29.32]The conquest of the air began with insects then the pterosaurs came and went,
[28:35.56]but the arrival of the birds marked a new era in the history of life...
[28:39.80]and the beginning of an evolutionary battle between birds and insects
[28:43.97]that would push flight to extremes.
[28:48.18]Each spring mayflies have just a few hours to leave the water, find a mate, and breed.
[28:56.59]Many never make it...
[29:44.30]The mayfly feast only lasts a few days
[29:47.44]but there's lots more insect food on offer - if you can get to it.
[29:55.38]In an average British summer month,
[29:57.31]around three billion insects fly over each square kilometer,
[30:02.48]that's nearly three tons of insects.
[30:07.49]We can't see them up here because they're so small.
[30:10.49]But there are birds that can...
[30:12.53]that have evolved to reach this aerial banquet.
[30:16.83]And none are better at it than the swift.
[30:21.10]The ultimate high flyer.
[30:31.65]They fly to and from South Africa each year for summer's insect feast,
[30:36.02]a return trip of around 23 thousand kilometers.
[30:46.16]A long history of aerial pursuit has given swifts a streamlined body
[30:50.33]with long narrow wings to slice through the sky.
[31:03.21]Only at normal speed can you really appreciate how well they live up to their name...
[31:20.16]Swifts are so specialized for living on the wing, they can't even land on the ground.
[31:25.70]Their tiny legs and long wings would make it impossible for them to take off again.
[31:32.34]That's why they always nest up high...
[31:37.25]One mouthful like this may contain up to 500 insects,
[31:40.85]and when food is plentiful, the chick may get 10 meals -
[31:44.32]5000 insects - in a day.
[31:54.23]But in bad weather, most insects are grounded,
[31:57.23]which can mean the parents have a hard job finding food.
[32:06.88]Sometimes the adults have no choice
[32:08.78]but to desert their chicks and keep on flying ahead of the storm.
[32:25.13]A British swift will even cross the sea to Continental Europe...
[32:35.24]A trip to Holland is no big deal when you can cover nearly 300 kilometers in a day.
[32:48.25]Back at the nest,
[32:49.32]the helpless chick can only wait whilst adults feed in fair Continental skies...
[33:04.17]Another day and still the chick waits, living on its fat reserves...
[33:13.94]Swift chicks are tough...
[33:15.78]They can lose half their bodyweight, surviving even weeks without a meal.
[33:20.68]But now this chick needs to eat, and soon...
[33:28.12]The parent is in sight of home,
[33:30.09]after a round trip of perhaps sixteen hundred kilometers or more...
[33:37.27]Just in time to deliver a life-saving meal...
[33:51.21]Millions of years of chasing tiny winged insects have shaped swifts
[33:55.78]into true aerial aces...
[33:59.49]Amazing to think they started as feathered dinosaurs
[34:02.52]that clambered around in the trees.
[34:12.03]Central America is home to the swifts' closest living relatives -
[34:16.57]a group of birds that have pushed flight along a very different evolutionary track...
[34:30.59]Hummingbirds can fly forwards, up and down and backwards.
[34:35.62]And all with unbelievable precision.
[34:40.60]These aerobatics are unrivalled among birds.
[34:51.31]And it's all because of flowers.
[34:57.35]But how did flowers lead to such fancy flight?
[35:02.88]Over millions of years
[35:04.19]hummingbirds and plants have struck an evolutionary deal.
[35:08.06]The hummingbirds get to drink nectar in return for pollinating plants.
[35:14.30]And hovering has proved the most efficient way to get a meal...
[35:20.27]It's a relationship that's been perfected
[35:22.34]until bird and flower fit together like a key in a lock.
[35:28.18]Plants have forced hummingbirds to become more like feathered insects...
[35:32.28]which is why, at first glance, it's hard to tell a hummingbird...
[35:35.78]from a hummingbird hawkmoth.
[35:43.36]It's a great example of what's known as convergent evolution...
[35:47.03]where two entirely unrelated animals
[35:49.46]come up with almost identical solutions to the same problem.
[36:06.38]But the hummingbird's supreme flying ability comes at a great cost.
[36:14.19]Their high energy lifestyle means they need to eat
[36:17.29]half their weight in nectar every day.
[36:23.13]And they can only achieve such precision flying by staying very small.
[36:33.41]Swans have the opposite problem -
[36:37.08]they're so heavy that just getting into the air is a struggle.
[36:57.07]Swans are one of the heaviest of all flying birds.
[37:02.27]They're like the bird equivalent of a jumbojet, so they take a lot of energy to get into the air.
[37:09.74]Once they're up in the air
[37:10.81]it's all about saving energy and that's why they fly in this formation.
[37:17.39]By sticking close together, each bird gains a little extra lift
[37:20.82]from turbulence created by the bird in front.
[37:23.96]As a result they make a 50% energy saving.
[38:03.97]When you're this big, landing is a challenge too.
[38:33.70]Thanks to that first feathered dinosaur,
[38:36.16]today more than 9000 species of bird fill the air, across every continent.
[39:49.10]Despite their success most birds share a weakness...
[39:53.27]their eyes work best in daylight.
[40:00.98]That left the night sky up for grabs and another family of animals seized their chance.
[40:16.36]Creatures of the night that can find their way
[40:18.63]around in the pitch black and live secret lives in deep, dark places.
[40:29.54]But perhaps their biggest secret is how they evolved.
[40:35.05]Bats are thought to be descended from a small nocturnal mammal,
[40:39.62]at some point their ancestors took to the trees in search of prey.
[40:51.13]They probably began to glide on skin flaps stretched between their limbs,
[40:55.34]just like some modern mammals such as flying squirrels and sugar-gliders.
[41:26.17]It's a great way to get around but still not real flight -
[41:29.87]more like free-falling with style.
[41:40.15]But the colugo may provide a missing link to show how bats evolved true wings.
[41:49.19]The difference is in its hands - they're webbed.
[41:55.40]The bats' ancestor had these too...
[41:57.87]but its fingers got longer and the skin between them stretched to turn hands into wings.
[42:08.24]It flapped these wings and took off, making bats the fourth group after insects,
[42:13.31]pterosaurs and birds, to really fly!
[42:22.19]You might think that one bat is pretty much like another
[42:25.53]but in fact they're one of the most diverse and widespread of any mammal.
[42:32.57]Flying foxes are the biggest bats of all.
[42:49.28]They spend the day crowded together in their favorite trees -
[42:52.89]there can be millions in a single colony.
[43:07.07]As dusk falls they make a mass exodus in search of fruiting trees.
[43:13.27]The power of flight means they can cover up to 50 kilometers a night
[43:17.61]in search of the ripest fruit.
[43:21.88]No other mammal puts on a show quite like this.
[43:40.40]Bats are now one of the most successful mammal families.
[43:43.97]There are nearly a thousand species each with their own special habits.
[43:50.55]Like flying foxes many are fruit lovers...
[43:56.08]Others sip nectar
[44:10.13]Some have even learned to fish
[44:24.18]But a fat juicy insect is what most bats like to eat.
[44:33.52]Bats need more than wings though to track down food in the dark.
[44:37.89]These creatures of the night had to evolve a whole new way
[44:40.73]of mapping out their world...
[44:45.33]They listen to the echoes of their own calls and create an image of their surroundings.
[44:51.77]It's called echolocation... but how did such a complex system evolve?
[45:03.65]Bats probably began by using simple clicking sounds,
[45:07.56]giving a very rough picture of what's around...
[45:11.83]Detecting even basic shapes is still better than flying blind...
[45:17.70]Then, by developing a more complex range of sounds, echolocation was improved.
[45:24.67]Bats were gradually able to pick up more details of their surroundings...
[45:34.18]and of their prey.
[45:38.39]Insects filled the night sky long before bats existed.
[45:42.46]They were a meal waiting to happen - and the bats tucked in!
[46:03.11]Of course there are more than insects on the menu.
[46:23.46]Using its finely tuned echolocation skills,
[46:26.40]a tiny Natterer's bat can even pluck a spider from its web without getting entangled.
[46:42.48]Once it has its prize, the bat goes into reverse...
[46:45.99]keeping its wings clear of the sticky threads with just a hair's breadth to spare.
[46:54.20]This has to be the ultimate refinement to seeing with sound!
[47:04.24]Bats were the last group of animals to get airborne...
[47:07.61]they've used their wings to reach almost all parts of the earth.
[47:13.05]Their sheer numbers are testament to their success.
[47:33.13]The power of flight is one of evolution's biggest triumphs.
[47:42.18]It has shaped some of the most beautiful and successful animals that ever lived.
[48:01.23]Today's winged creatures make up around two thirds of all the species on Our planet.
[48:19.11]And to think they're all descended from just four ancestors,
[48:23.28]who managed to grow wings and take to the air!
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