生命之旅第二集:掠奪陸地 Land Grab

[00:00.00] [00:20.30] [00:22.30]Imagine a land so hostile that you'd need special equipment to breath. [00:27.96] [00:28.20]Where there's no shade and where the solar radiation would kill you in minutes. [00:33.10] [00:36.14]At night temperatures plummet to sub zero, you could freeze or fry in the same day. [00:42.51] [00:44.95]This is not some far away planet. [00:47.11] [00:47.35]Its earth as it would have been 500 million years ago, [00:51.12] [00:51.32]when the land was alien and uninhabitable. [00:54.35] [00:57.26]Back then, life was only to be found in the sea. [01:00.49] [01:02.00]How did life conquer this hostile world, [01:04.94] [01:05.21]taking an evolutionary journey that would one day lead to us? [01:10.07] [01:30.36]Over the millennia this harsh new world was invaded by a few pioneering life forms. [01:36.43] [01:38.14]These evolved into a multitude of ...animal designs able to... [01:41.57] [01:41.78]...to cope with the extremes ...of life on land [01:44.61] [01:52.39]Our family, the mammals... is just one of the results. [01:56.62] [02:03.20]Every living thing is linked by the branches on the tree of life... [02:07.36] [02:17.01]In this programme just how this extraordinary variety of animals... [02:21.91] [02:22.12]...arose and how we are connected to each and every one of them. [02:26.88] [02:34.96]It's a story full of surprises, which leads all the way from a fish... to you... and me. [02:44.27] [02:48.14]And how, if it wasn't for one giant twist of fate the dinosaurs might... [02:52.91] [02:53.11]...still rule supreme today. [02:55.27] [02:58.52]In the beginning, dry land was a no go... area for life. [03:02.11] [03:02.99]Because all life began in the sea... [03:05.55] [03:05.86]where the temperature hardly changes [03:07.55] [03:07.76]and where water protects against the pull of gravity and the burning sun. [03:11.99] [03:15.24]The sea was the first laboratory of life. [03:18.17] [03:18.67]And for more than three quarters of life's history... [03:21.07] [03:21.27]...it was home to every living thing. [03:23.83] [03:26.38]This was mostly a gentle era of soft bodied creatures like jellyfish. [03:30.98] [03:35.62]Jellyfish are 95% water and have no skeleton at all. [03:39.89] [03:41.56]Unlike us, they don't need one - [03:43.86] [03:44.33]Because water gives them all the buoyancy and support they need. [03:47.79] [03:49.07]Water suspends all life in a three dimensional world. [03:52.83] [03:53.31]For a diver, it's a bit like floating in space. [03:56.64] [04:07.95]It enables these kelp fronds to tower 30 meters upwards from the ocean bed. [04:13.45] [04:21.97]In fact, just the like animals, for hundreds of millions of years, [04:25.73] [04:25.94]plants could only exist in the sea. [04:28.57] [04:44.32]On land, this watery support would all be gone, so what would happen then? [04:50.09] [04:53.93]The kelp would collapse. [04:55.37] [05:00.94]And the diver would suddenly aware of the weight of his equipment [05:04.60] [05:04.84]because gravity is now dragging him down. [05:07.51] [05:13.02]How could any animal designed for life in the sea ever make it on dry land? [05:18.55] [05:19.76]One group of sea creatures had just the right kit to get up and go. [05:23.99] [05:25.37]Arthropods with their hard jointed skeleton. [05:29.73] [05:30.10]This living suit of armor holds it up, so gravity can't pull it down [05:35.20] [06:06.04]Armor gave arthropods the staying power they needed to make it on land. [06:11.00] [06:13.31]And its made crabs expert land grabbers [06:16.18] [06:33.83]In Florida, crabs have even invaded peoples gardens. [06:37.50] [06:38.84]And the lawns are riddled with their burrows. [06:41.17] [06:49.65]These blue land crabs can live up to 5 kilometers from water [06:53.64] [07:05.16]Crabs are actually well adapted to life on land. [07:07.79] [07:08.27]Their jointed, outer skeletons may have evolved in the sea... [07:11.50] [07:11.80]...but it also supports their weight on land making it easier for them to get around, [07:16.37] [07:16.94]...and it helps stop them from drying out. [07:19.00] [07:22.72]They roam at will, and pop up in surprising places. [07:26.41] [07:29.46]And they're not fussy about what they eat. [07:31.32] [07:42.27]On the other side of the world... [07:43.60] [07:43.80]In the Indian Ocean, there's a land where crabs have really made themselves at home. [07:48.60] [07:54.18]Christmas Island, is completely overrun with red land crabs. [07:58.41] [08:02.59]120 million of them live here on this small island outnumbering the human [08:07.15] [08:07.36]residents by 300,000 to 1 [08:10.49] [08:12.43]Once a year, vast numbers hit the road. [08:15.06] [08:25.14]They're heading for the beach. [08:26.30] [08:26.88]Why? Because despite their armor they can't shake off their ties to the sea. [08:32.08] [08:42.26]There's a vital purpose to this mass maneuver. [08:44.46] [08:44.96]The females clasp thousands of soft eggs beneath them. [08:48.70] [09:00.15]Like all land crabs they have to return to the water to release their eggs. [09:04.61] [09:05.22]Their young still have to grow up in the sea. [09:08.15] [09:18.23]But the first creatures ever to venture on land lived long [09:21.06] [09:21.27]before crabs even came existed. [09:23.26] [09:27.24]We know because they left their footprints. [09:30.37] [09:34.45]Preserved in the rock in what is now Ontario Canada [09:37.58] [09:37.78]are the oldest footprints anywhere on earth... [09:40.55] [09:44.29]Just a few small steps for a bug - but one giant leap for life! [09:48.89] [09:51.33]These prints are 500 million years old, [09:54.63] [09:54.83]their maker is believed to have been an armored arthropod... [09:58.07] [09:58.60]...a bit like a giant wood louse more than 30 cm long... [10:02.73] [10:07.01]What made this ancient trailblazer drag itself ashore onto the barren land? [10:12.45] [10:17.16]500 million years ago, [10:18.85] [10:19.06]there were so many hardened hunters in the ocean that it had become a dangerous place. [10:24.46] [10:26.00]For the first time in the history of life [10:27.97] [10:28.23]there was good reason to leave the crowded seas. [10:30.96] [10:37.48]There's another ancient creature that re-traces those pioneering steps each year. [10:42.97] [10:50.49]The horseshoe crab has been around for hundreds of millions of years. [10:54.32] [11:00.13]And every year it still makes a dramatic pilgrimage to a beach [11:03.50] [11:03.70]in North America to breed. [11:08.50] [11:21.09]Horseshoes crabs lay their eggs on land... [11:23.39] [11:23.59]...to put them out of reach of marine predators. [11:26.06] [11:28.96]And its probably the same reason that brought those early trailblazers to shore. [11:33.06] [11:39.24]Job done, the horseshoe crabs turn round and crawl straight... back into the sea. [11:43.97] [11:45.58]Those first track makers were probably just visitors too. [11:49.11] [11:50.95]About 430 million years ago... [11:53.71] [11:53.92]...another group of arthropods abandoned the sea for good. [11:57.25] [12:01.09]The colonization of the land had truly begun. [12:04.59] [12:04.90]And it was creatures like these millipedes that lead the way. [12:08.66] [12:10.54]Others soon followed, spider and scorpion like creatures, [12:14.20] [12:14.44]almost a hundred million years before our ancestors left the sea. [12:18.60] [12:22.11]Today, more than 90% of land dwellers are arthropods. Most of them insects [12:27.92] [12:32.09]Amongst the most successful are the ants, [12:34.65] [12:34.96]which keep their eggs and young moist in nests underground. [12:38.62] [12:42.17]Driver ants have evolved sophisticated strategies to breed and feed on land. [12:47.23] [12:47.67]By massing in their millions around the nest, [12:49.83] [12:50.04]they raise its temperature and speed up the development of the young inside. [12:54.17] [12:58.82]Their sturdy skeletons carry them easily across the ground [13:01.75] [13:01.95]and stop them from drying out. [13:03.39] [13:14.10]These fearsome jaws have... made them deadly hunters. [13:17.19] [13:17.44]Thousands work together as one giant predator devouring everything in its path. [13:23.50] [13:25.41]And many jaws make light work of creatures far bigger than themselves. [13:29.44] [13:33.02]But the arthropod's jointed construction does have drawbacks. [13:36.25] [13:36.49]The bigger they get, the heavier the armor, and the harder it is to breathe. [13:40.26] [13:42.33]It would take another group of animals to overcome this hurdle. [13:45.66] [13:48.00]It all started in the sea about 550 million years ago [13:50.94] [13:51.14]With a tiny wormlike sea creature... [13:53.47] [13:55.31]...this is its closed living relative - a lancelet. [13:58.77] [14:01.25]It has a revolutionary feature vital to our evolution. [14:04.77] [14:05.12]A rod running through the body - called the notochord. [14:08.61] [14:12.19]It provided a strong yet flexible lever for the muscles to pull against. [14:16.49] [14:16.70]And led the evolution to whole new family of backboned animals, The vertebrates. [14:22.46] [14:26.01]Having your skeleton inside allows you to grow much much bigger. [14:30.20] [14:34.15]The backbone's evolution happened only once, [14:36.94] [14:37.38]but this lucky break was a crucial turning point in the journey of life. [14:41.18] [14:50.23]It led to all the vertebrates - that's fish, amphibians, reptiles, birds and mammals. [14:56.66] [14:58.97]Including you and me. [15:00.70] [15:05.95]But to walk you need legs. How did they evolve? [15:09.61] [15:11.38]Underwater mudskippers look much like other fish. [15:14.65] [15:15.09]But watch what they can do with their fins. [15:17.45] [15:25.43]A fish that walks, well almost. [15:28.63] [15:29.24]The mudskipper can shuffle over land using two pairs of modified fins. [15:33.83] [15:36.34]This is how we once thought backboned creatures left the water... [15:39.64] [15:40.05]Ancient fish propped themselves up on fins, and once ashore the fins turned into legs. [15:45.48] [15:59.30]We now know that something even more surprising happened. [16:02.32] [16:04.44]Legs actually evolved for use in the water. [16:07.27] [16:08.41]How? [16:09.37] [16:12.21]Deep in the ocean other bizarre sea creatures seem to walk across the sea bed. [16:16.94] [16:21.12]Batfish never leave the sea, and they can swim perfectly well. [16:25.22] [16:25.69]But they too use their fins like pairs of legs. [16:28.75] [16:39.10]By stalking prey on stilt-like feet they don't stir up water or sand. [16:44.34] [16:51.32]So this shrimp doesn't have a clue. [16:53.38] [17:05.03]The frogfish has also evolved two... pairs of modified fin feet [17:09.80] [17:10.00]to help navigate the nooks and crannies of the ocean floor. [17:13.23] [17:24.05]So fishy fin feet weren't a new idea. [17:26.85] [17:27.39]But evolution didn't really run with it until around 370 million years ago [17:32.48] [17:32.76]when the climate changed [17:34.69] [17:35.26]The earth warmed up and became covered in shallow weedy swamps... [17:38.96] [17:39.16]...where normal fins got in the way [17:41.03] [17:48.11]Over time the paired fins began to develop a better shape for pushing through the weeds. [17:53.51] [17:53.75]Until they ended up more like four feet. [17:56.08] [18:00.15]And those early four-footed creatures left a legacy [18:03.32] [18:03.52]which can be traced throughout the tree of life right up to the present day. [18:08.19] [18:16.14]They were the ancestors of all land-living, back-boned animals... [18:21.07] [18:26.31]The four-legged blueprint had been set and all vertebrates [18:29.77] [18:29.98]that have ever lived on land have followed the same basic pattern. [18:33.38] [18:34.85]But this was just a fluke of nature, [18:36.75] [18:37.32]if our ancestral fish had had more fins, [18:39.72] [18:39.93]we might have... ended up with more set of arms. [18:42.92] [18:43.33]And we now know that we might easily have had many more fingers and toes. [18:47.06] [18:56.51]Fossils show that some of those early four-legged creatures [18:59.94] [19:00.15]that crawled through the swamps had six... seven... or even eight fingers. [19:07.11] [19:07.85]But the one successful species that became the ancestor of [19:10.79] [19:10.99]all the later vertebrates just happened to have five fingers and toes. [19:15.55] [19:20.67]So having five fingers and four limbs is an ancient blueprint all land [19:24.93] [19:25.14]- living animals inherited. [19:26.70] [19:32.48]But horses don't have five toes [19:36.68] [19:38.08]Well they don't have today, but their ancestors used to. [19:41.35] [19:42.62]And you can find some of those lost toes at the beginning of every horse's life. [19:46.72] [19:47.69]In the unborn foal, there are two extra toes, [19:51.15] [19:51.46]only present in the first months of development. [19:53.33] [19:53.53]In a replay of the horses' evolutionary past [19:56.93] [19:57.14]they fold back into the central cannon bone before the foal is born. [20:01.27] [20:15.09]We now know how our underwater ancestors evolved to walk... [20:18.78] [20:19.22]but there were other challenges they had to face in leaving [20:21.56] [20:21.76]the water for the hostile land. [20:23.35] [20:26.97]To start with, how would they manage to breathe? [20:29.63] [20:32.34]Down here we need special breathing equipment and in the same way... [20:36.36] [20:36.58]...that we'd drown without this diving gear, most fish would suffocate on land. [20:41.27] [20:47.72]Remarkably, though, there are some fish that can breathe air just like us... [20:51.21] [20:51.42]as they have done for 400 million years. [20:53.95] [20:57.96]Lungfish - still alive and well in Africa, Australia and South America today. [21:03.30] [21:22.32]For those first sea creatures that came ashore it took the winning formula of lungs... [21:28.28] [21:30.33]and legs... to make the quantum leap from ocean to land. [21:34.29] [21:37.07]Enter the first four-legged air-breathers... our ancestors. [21:41.13] [21:45.91]But even these groundbreaking creatures couldn't conquer dry land just yet. [21:50.18] [21:53.65]They needed shade and moisture, so they had to wait until another group invaded first. [21:58.99] [21:59.42]The woody plants [22:00.76] [22:07.10]Early plants were tiny algae and like every other form of life, confined to water. [22:12.40] [22:14.07]Just like animals to make it on the land they had to combat gravity. [22:18.63] [22:18.94]They evolved a skeleton, made up of rings of lignin [22:22.07] [22:22.35]- the plants' equivalent of bone. [22:24.14] [22:25.98]Now they could leave water and stand alone... [22:28.58] [22:33.19]Lignin gave plants the strength to grow up and up reaching towards the sunlight [22:39.96] [22:55.75]Lignin eventually expanded to fill up the cell walls and form wood [23:00.31] [23:03.86]And wood allowed plants to really make it big... [23:07.02] [23:07.46]...they evolved into the incredible hulks of the land, the trees. [23:12.29] [23:29.98]So lignin created wood and wood created forests [23:34.15] [23:34.35]- without which we wouldn't exist. [23:36.58] [23:48.97]A new damp shady world opened up for those early four-legged animals to explore. [23:54.46] [24:13.83]Even today, some 350 million years later, [24:17.32] [24:17.53]amphibians still can't escape the pull of their aquatic roots. [24:21.40] [24:28.14]Every spring, just after the snow melts, [24:30.61] [24:30.88]there's an extraordinary mass migration through the forests of North East America. [24:35.25] [24:39.28]Thousands of spotted salamanders that have [24:41.41] [24:41.62]spent the past year hidden underground emerge with one mission in mind... [24:46.39] [24:50.10]To get to water for a single night of passion. [24:53.33] [25:16.19]The following night the females spawn... [25:18.66] [25:30.04]Amphibian eggs are soft and dry out quickly in the air [25:33.77] [25:34.14]- which is why spotted salamanders still have to return to the water to breed. [25:38.63] [25:42.11]Ever since their ancestors first crawled out of the swamps, [25:45.08] [25:45.28]amphibians have come up with all kinds of weird [25:47.65] [25:47.85]and wonderful ways to break free from the water. [25:50.65] [26:05.40]The mountains of Northern Spain... A rocky barren landscape... [26:09.46] [26:15.51]But the midwife toad is perfectly at home. [26:18.17] [26:18.64]It has evolved to live and breed on dry land... [26:22.27] [26:23.48]So what do midwife toads do that salamanders don't? [26:26.74] [26:29.25]A pair meet up and start to mate... [26:31.55] [26:41.30]The male's embrace squeezes the female until she releases her eggs. [26:46.17] [26:53.04]She catches them in her back legs where he fertilizes them. [26:57.21] [27:01.62]But how will the eggs survive without water? [27:04.45] [27:10.29]The male midwife toad is one of nature's most devoted dads. [27:13.96] [27:14.36]He hoists the eggs up his legs, as if wriggling into a pair of shorts. [27:18.77] [27:28.01]From now on the eggs are his responsibility. [27:30.81] [27:31.28]And for the first few weeks he holes up under a rock to keep them moist. [27:35.62] [27:42.03]After that however, he has to emerge to find a pond. [27:45.69] [27:46.30]And there the tadpoles wriggle free and grow up just like other toads. [27:50.13] [28:07.25]What other solutions has evolution come up with so that frogs [28:10.24] [28:10.45]and toads can live further away from water? [28:13.05] [28:15.29]This forest in Costa Rica is full of frogs calling to their mates. [28:19.32] [28:21.83]There don't seem to be many pools and streams around here. [28:24.46] [28:25.13]So how do they breed? [28:26.50] [28:31.31]The strawberry poison arrow frog carries its tadpole on its back. [28:35.84] [28:36.28]The tadpole hatched out on the ground, [28:38.25] [28:38.45]but now it gets a piggyback- as mum begins a mother of a climb! [28:42.78] [28:54.16]One at a time, she carries her four or five tadpoles all the way up into the trees [29:00.00] [29:13.05]Each to its own cradle in the canopy. [29:15.45] [29:16.25]An egg cup sized pond in these hanging gardens up to seven meters above the forest floor. [29:21.82] [29:26.13]And after all this effort, mum's still got more work to do! [29:29.86] [29:33.00]A few days later she must make the same climb all over again... [29:36.34] [29:36.77]because although her tadpole has water in its treetop nursery it has no food, [29:41.97] [29:42.31]so it waits... like a chick in its nest. [29:45.21] [29:52.36]As she lowers herself in, the tadpole head-butts her, even gives her a nip. [29:57.66] [29:59.86]Until she lays an infertile egg for it to eat [30:03.39] [30:07.04]With these regular food parcels the tadpole has all [30:09.87] [30:10.07]it needs in its tiny world to grow into a frog [30:13.13] [30:16.51]Strawberry poison arrow frogs may be the hardest working... [30:19.31] [30:19.52]...frog mothers on earth but another frog is the most devoted frog father. [30:23.98] [30:30.16]Guess where the Darwin's frog keeps his tadpoles! [30:33.46] [30:38.20]The male's throat is the nursery where they grow... he gives birth, [30:42.83] [30:43.04]as it were - by spitting out he froglets fully formed! [30:46.63] [31:01.99]So the Darwin's frog has reduced its need for water by carrying its own pond around inside. [31:08.09] [31:10.43]But despite all these extraordinary solutions [31:13.03] [31:13.24]almost all amphibians are still basically tied to water [31:16.80] [31:17.01]- just as their early ancestors were. [31:19.10] [31:23.18]So how did the first land-lovers colonize the deserts and dry lands? [31:27.91] [31:33.02]The entire spawning process had to be transformed... [31:36.01] [31:40.16]Eggs evolved their own internal life support system... [31:43.56] [31:43.77]And at the same time shells developed that could hold water. [31:47.43] [31:57.21]The arrival of the hard-shelled egg was one giant step forward on the journey of life. [32:03.12] [32:04.42]Each egg provided the growing youngster with its own private pond. [32:07.98] [32:08.19]Eggs could now be laid on dry land [32:10.66] [32:11.06]and this led to a whole new group of animals... the reptiles. [32:15.55] [32:25.98]Reptile eggs contain more food reserves than those of the amphibians, [32:29.91] [32:30.11]giving their babies a head start in life. [32:32.48] [32:45.33]Australian bearded dragons hatch out fully formed [32:48.35] [32:48.56]and ready to take on the world - just as they've done for millions of years. [32:52.80] [33:10.12]The reptiles evolved into a multitude of families, [33:13.32] [33:13.56]including one of the most impressive dynasties ever to dominate the land. [33:18.58] [33:27.30]The smash hit that was the hard-shelled egg led to the Age of the Dinosaurs. [33:32.83] [34:04.24]Dinosaurs ruled the earth for about 180 million years. [34:07.80] [34:08.24]There were as many as 700 species, from the size of a rat, to that of a whale. [34:13.65] [34:19.36]And the egg wasn't the reptiles' only strength. [34:21.95] [34:22.26]They also developed tough scaly armor... [34:24.49] [34:24.69]...that could withstand the toughest... conditions on earth. [34:27.29] [34:32.97]And you don't get much drier than the sand dunes of Namibia. [34:36.70] [34:43.25]But even here reptiles have solved the problem of drying out... [34:46.91] [34:47.18]...and they can turn the power of the sun to their advantage. [34:50.31] [34:55.96]First thing in the morning the cold-blooded chameleon heats up [34:58.98] [34:59.20]along the side facing the sun... [35:01.32] [35:14.08]Once it's recharged its solar batteries it's ready to go hunting. [35:18.01] [35:24.25]Here, the armored arthropods have met their match... [35:27.05] [35:27.46]Despite arriving more than 100 million years later, [35:30.43] [35:30.63]reptiles now appear to have the upper claw! [35:33.62] [35:57.99]But even reptiles have their limitations [36:00.08] [36:00.29]because they slow down as the temperature drops, [36:03.52] [36:03.89]and when the sun is gone it can get very cold. [36:06.69] [36:10.80]We mammals on the other hand, can remain active whether its hot or its cold [36:14.86] [36:17.04]That's because we heat ourselves from the inside by burning food as fuel. [36:22.50] [36:28.48]A human face shows white hot, while this iguana's is a cool red. [36:33.32] [36:33.62]Because while we stay warm while we generate heat inside, [36:37.32] [36:37.53]the iguana is cooling down as it looses the heat it absorbed from the sun during the day. [36:43.26] [36:51.21]But we need to keep the heat we generate. [36:53.40] [36:53.71]Otherwise we'd have to eat non-stop just to stay alive. [36:57.04] [36:57.61]What's required is effective insulation. [37:00.28] [37:05.92]So there would have to be another breakthrough to allow [37:08.55] [37:08.76]land animals to reach the coldest corners of the world. [37:12.19] [37:18.37]About 200 million years ago a reptile-like ancestor of mammals [37:22.43] [37:22.64]started growing fine barbs underneath its scales. [37:25.73] [37:29.95]Over many generations, they became finer and longer, [37:33.78] [37:34.05]until eventually they turned into hair and fur. [37:36.92] [37:42.22]The mammals had arrived... and now they were ready to take on the elements anywhere. [37:47.69] [37:51.33]Musk oxen have the ultimate fur coats. [37:53.93] [37:54.27]Their insulation is so good they can survive at temperatures of minus 30 degrees Celsius. [38:00.44] [38:16.29]Warmed by their central heating and stoked up by regular meals... [38:19.73] [38:19.93]musk oxen have managed to beat the elements [38:22.33] [38:22.53]and eat their way north to the fringes of the Arctic ocean. [38:25.80] [38:46.16]Central heating and fur coats... [38:48.09] [38:48.29]coats let mammals range even to the icy reaches of the poles... [38:51.89] [38:57.10]And this must be the ultimate test. [38:59.90] [39:09.11]Dense fur allows a baby seal to undergo the biggest temperature shock experienced [39:14.01] [39:14.25]by any animal on earth... from around 40 Celsius degrees inside [39:18.85] [39:19.05]its mother's body to as low as minus 30 when it's born and it hits the ice. [39:24.69] [39:30.00]The miracle of this baby's survival here depends not just on fur [39:33.73] [39:34.00]but on its mother's dedication. [39:35.80] [39:36.24]She provides milk, the fuel that keeps it warm inside. [39:40.14] [39:51.12]But the ultimate mammal mother lives in Africa. [39:53.92] [40:15.91]It takes nearly 22 months to make a baby elephant, longer than any other animal. [40:21.41] [40:23.09]During that time the embryo floats in its own centrally heated world... [40:26.68] [40:26.99]an echo of life's earliest existence in warm primeval seas. [40:30.98] [40:40.20]And it's safer than inside an egg [40:42.47] [40:42.84]- mum has total control over the baby's growth. [40:45.77] [40:49.14]After investing all that effort even before birth [40:51.87] [40:52.11]it's not surprising mammals take exceptional care of their young. [40:55.88] [41:11.00]For a mammal mother, birth is just the start of a long and demanding job. [41:15.73] [41:29.08]Mum is a mobile milk bar. [41:31.35] [41:31.99]And delivering enough of the white stuff to build up baby [41:34.68] [41:34.96]is an even greater drain on her than pregnancy. [41:37.55] [41:42.36]Milk is highly nutritious and at this stage it's the only food that baby needs. [41:47.30] [41:50.41]No wonder others try to steal it! [41:51.96] [41:55.68]Like all mammals, baby elephants learn through play... [41:58.81] [41:59.82]and it's a good excuse to mess around. [42:01.37] [42:34.28]The evolution of mammals has been an extraordinary journey... [42:37.88] [42:38.79]our ancestors evolved... a backbone, lungs, [42:41.98] [42:42.29]and four limbs... ...with five fingers and toes. [42:47.25] [42:49.87]And we grew fur and installed central heating [42:52.73] [42:52.94]which could cope with the extremes of temperature on land. [42:55.53] [42:59.34]But mammals needed other changes to move from the water to the air [43:03.68] [43:06.05]For one thing, they needed a totally new way of picking up sound [43:09.75] [43:12.62]And when it comes to ears, few mammals are more advanced than the bat-eared fox. [43:17.25] [43:22.30]It lives in the South African bush where it scans the ground listening for a meal. [43:27.23] [43:32.11]It hears in the same way as all mammals do - it's just more sensitive. [43:36.64] [43:39.25]Broad outer ears capture... ...the sound waves in the air [43:42.08] [43:42.28]and funnel them through to the eardrum, making it vibrate. [43:46.12] [43:49.52]Tiny bones in the middle ear transmit and amplify these vibrations. [43:53.82] [43:59.27]The sound waves are then converted into nerve impulses and sent to the brain. [44:03.80] [44:11.25]Bat-eared foxes can hear grubs and termites moving nearly 30cm below the ground. [44:16.74] [44:30.63]So how did the chance workings of evolution create such a remarkable device? [44:35.80] [44:37.51]Believe it or not it all began in the solid jaws of prehistoric reptilians. [44:42.71] [44:44.15]Two of the jaw bones became detached, [44:46.61] [44:46.88]they shrank and moved upwards to become the key components of the mammal ear... [44:51.62] [44:51.82]Hijacked by evolution for a totally different use. [44:55.69] [44:56.56]It's amazing to think that these tiny delicate bones... [44:59.19] [44:59.39]...evolved from the solid biting jaws of our reptile-like ancestors [45:03.63] [45:06.94]Mammals have now... reached all corners... of the globe... [45:10.10] [45:10.37]and can live almost anywhere... [45:12.34] [45:18.61]But this could have been a very different story. [45:22.55] [45:26.25]More than 100 million years [45:27.72] [45:27.92]after the mammal arrived... ...the land was still ruled by the dinosaurs. [45:32.49] [45:45.11]Most of... the early mammals were shrew-like creatures [45:48.24] [45:48.44]that ventured out mainly at night. [45:50.74] [45:51.58]The dramas of their lives took place entirely in the... shadow of the giants. [45:56.24] [46:39.19]But one cataclysmic event 65 million years ago played right into the mammals' hands... [46:45.16] [46:50.37]A massive meteorite collided with the earth... [46:54.33] [47:10.96]...and 85% of all land-dwelling animals were snuffed out. [47:15.26] [47:15.70]Dinosaurs became extinct over just a few thousand years. [47:19.57] [47:22.34]But among the animals that did survive were mammals... [47:25.77] [47:30.31]Perhaps that was because they fed on the other major group of survivors [47:33.84] [47:34.12]- the armored arthropods. [47:35.51] [47:44.93]With the dinosaurs gone the mammals seized their chance diversifying [47:49.56] [47:49.76]and expanding to fill the world. [47:52.03] [47:58.11]The mammal line exploded... Into the wonderful variety we... see today. [48:02.24] [48:08.88]Ultimately, that led to us. [48:11.22] [48:11.89]If that meteorite had missed the Earth, we probably wouldn't be here. [48:15.82] [48:19.13]We've come a long way since our ancestors first crawled out of the sea [48:23.29] [48:23.60]and coped with everything that this hostile new world can throw at us. [48:28.00] [48:32.14]But can we really call ourselves the true inheritors of the earth? [48:35.94] [48:38.28]No - that title belongs to the armored arthropods... [48:42.41] [48:42.78]...they colonized the land a hundred million years... before we vertebrates. [48:46.95] [48:47.16]And today they outnumber all other animals combined... [48:50.42] [48:55.10]On their journey of life... ...the land grabbing arthropods... [48:58.33] [48:58.53]...developed an... apparently bombproof design. [49:01.37] [49:04.31]No doubt they'll still be here long after we have gone. [49:09.40] [49:11.40]The End [49:57.40]

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