0:00:50 > 0:00:55One of the most crucial steps in the story of Life On Earth
0:00:55 > 0:01:02happened in a freshwater swamp about 350 million years ago.
0:01:02 > 0:01:05The fish began to haul themselves out onto the land.
0:01:05 > 0:01:09The land at the time was covered with the first plants.
0:01:09 > 0:01:12Very different from these mangrove plants of today,
0:01:12 > 0:01:14but nonetheless plants.
0:01:14 > 0:01:18In order to get out among them, the fish had to solve two problems.
0:01:18 > 0:01:21First, they had the mechanical problem
0:01:21 > 0:01:23of hauling themselves onto land,
0:01:23 > 0:01:27and second, they had to be able to breathe once they got there.
0:01:27 > 0:01:29The way they solved the problem
0:01:29 > 0:01:31of hauling themselves up onto the land,
0:01:31 > 0:01:34we can see from a small fish
0:01:34 > 0:01:37which lives in these mangrove swamps today.
0:01:37 > 0:01:40It's in no way closely related to those early fish,
0:01:40 > 0:01:44but it does give us an idea of what that scene must have been like.
0:01:44 > 0:01:45The mudskipper.
0:01:51 > 0:01:53They come up out of the water
0:01:53 > 0:01:57to browse on small creatures swarming on the mud.
0:02:01 > 0:02:04Their front fins have jointed bones
0:02:04 > 0:02:09so the fish can use them as legs to lever itself along.
0:02:17 > 0:02:20The mudskipper is not the only fish
0:02:20 > 0:02:23to have developed muscular fins like these.
0:02:23 > 0:02:26Fossils of one of the first have been found in rocks
0:02:26 > 0:02:28laid down just before the time
0:02:28 > 0:02:30that backboned animals ventured onto land.
0:02:30 > 0:02:32The coelacanth.
0:02:34 > 0:02:38Did this extremely ancient fish also use its fins as legs?
0:02:38 > 0:02:42Unfortunately, no fossils of them younger than 70 million years
0:02:42 > 0:02:46have ever been found, and up to 40 years ago,
0:02:46 > 0:02:49scientists concluded that they wouldn't be able to answer
0:02:49 > 0:02:53that question for certain as the fish was obviously extinct.
0:02:53 > 0:02:57And then, in 1938, a living coelacanth was caught
0:02:57 > 0:03:00off the coast of South Africa.
0:03:00 > 0:03:02It was the scientific sensation of the century.
0:03:02 > 0:03:06Before scientists could get to examine its entrails
0:03:06 > 0:03:10and see how they confirmed or denied the deductions they'd made
0:03:10 > 0:03:12from the very ancient fossil coelacanths,
0:03:12 > 0:03:14the fish was already rotting.
0:03:14 > 0:03:18Its guts were thrown away unexamined.
0:03:18 > 0:03:22So a huge search was mounted to find another.
0:03:22 > 0:03:27Leaflets were printed with pictures of the fish, offering a reward,
0:03:27 > 0:03:31and were distributed among the countless fishing villages
0:03:31 > 0:03:32off the African coast.
0:03:32 > 0:03:36But nothing... until, 14 years later,
0:03:36 > 0:03:39a second coelacanth was caught.
0:03:39 > 0:03:41It came from a place over 1,000 miles away
0:03:41 > 0:03:43from where the first one was landed.
0:03:43 > 0:03:47Here in the tiny Comoro Islands,
0:03:47 > 0:03:49a small group lying midway between
0:03:49 > 0:03:52Madagascar and the coast of East Africa.
0:03:52 > 0:03:55The first one, it seems, was a stray.
0:03:55 > 0:03:58These waters are the true and only home
0:03:58 > 0:04:01of this extraordinary rare fish,
0:04:01 > 0:04:03and the people who live in that tiny village
0:04:03 > 0:04:09are the world's experts in catching coelacanths.
0:04:09 > 0:04:13A villager still had a dried coelacanth which he let me see.
0:04:16 > 0:04:19From what we know of the habits of the living coelacanth,
0:04:19 > 0:04:21which is not much,
0:04:21 > 0:04:25it seems that these rear fins are used for swimming
0:04:25 > 0:04:29but the front ones are used for manoeuvring
0:04:29 > 0:04:31and for helping the fish to clamber about
0:04:31 > 0:04:34along the rocky bottom where it lives.
0:04:34 > 0:04:38All the fins have fleshy bases to them.
0:04:38 > 0:04:42The fishermen catch them at night from depths of 300 metres or so.
0:04:42 > 0:04:45Once hooked, the fish fight valiantly,
0:04:45 > 0:04:48and it may take all night to haul one to the surface.
0:04:48 > 0:04:51So it's usually dead on arrival.
0:04:51 > 0:04:54Scientists have still not been able to observe one alive.
0:04:54 > 0:04:58Then, while we were in the Comoros, one was caught.
0:04:58 > 0:05:02Although it was weak, it was still alive when the cameras arrived.
0:05:05 > 0:05:10350 million years ago, fish with fins like these
0:05:10 > 0:05:13were cruising the seas of the world.
0:05:13 > 0:05:15Some living in shallow waters produced descendants
0:05:15 > 0:05:18which eventually clambered onto the land,
0:05:18 > 0:05:21while this creature's ancestors moved down to the unchanging depths,
0:05:21 > 0:05:25there to remain unchanged themselves.
0:05:29 > 0:05:33The Comorians catch one or two coelacanths a year.
0:05:33 > 0:05:37They used not to value them much, for their flesh isn't good to eat.
0:05:37 > 0:05:41Now, however, big rewards are offered by scientific institutions,
0:05:41 > 0:05:44so the old man who caught this one will soon be rich.
0:05:44 > 0:05:46Some researcher in a few weeks' time
0:05:46 > 0:05:51will be absorbed in examining the structure of this fin
0:05:51 > 0:05:54which scientists agree must resemble closely those limbs
0:05:54 > 0:05:57that first took backboned animals onto the land.
0:06:01 > 0:06:06But how about that second problem? The problem of breathing up on land.
0:06:06 > 0:06:10The gills, which had served them well while swimming in water,
0:06:10 > 0:06:14extracting dissolved oxygen, wouldn't work in the air.
0:06:14 > 0:06:17How did the fish solve that problem?
0:06:17 > 0:06:21Well, this is East Africa and it's the height of the dry season.
0:06:21 > 0:06:26There is not a drop of water to be found in this parched landscape.
0:06:26 > 0:06:28And yet, here, close by me,
0:06:28 > 0:06:33there are fish that are living and breathing in air.
0:06:33 > 0:06:34If only I can find them.
0:06:54 > 0:06:59Six months ago, this was a pond several feet deep in water.
0:06:59 > 0:07:04But as the dry season progressed, the water evaporated
0:07:04 > 0:07:09and the fish in it burrowed down into this,
0:07:09 > 0:07:14which was soft liquid mud and is now brick-hard.
0:07:14 > 0:07:19And there, somewhere, they cocoon.
0:07:29 > 0:07:31And that...
0:07:32 > 0:07:37That looks like...the nose of one.
0:07:37 > 0:07:40Poking out from the mud, there.
0:07:40 > 0:07:44Now, if I take this and drop it in a tank of water,
0:07:44 > 0:07:48it should seem as though the rains have come early,
0:07:48 > 0:07:50and the fish should come to life.
0:08:13 > 0:08:17As the water soaks in, the mud softens and falls away,
0:08:17 > 0:08:20exposing a papery cocoon of dried mucus.
0:08:29 > 0:08:30And there is the throat of this
0:08:30 > 0:08:34extraordinary creature that can breathe in air and water.
0:08:34 > 0:08:36It's a lungfish.
0:09:11 > 0:09:13While its water-breathing apparatus,
0:09:13 > 0:09:16the gills, are getting working again,
0:09:16 > 0:09:18it snatches another gulp of air.
0:09:30 > 0:09:34It's able to breathe air because it has, opening from its gut,
0:09:34 > 0:09:37a long pouch lined with blood vessels,
0:09:37 > 0:09:41and they can absorb gaseous oxygen through its moist lining.
0:09:43 > 0:09:47The coelacanth has no lung, but it has got a simple leg,
0:09:47 > 0:09:51that fin with a fleshy base to it, supported by bones.
0:09:53 > 0:09:56Neither it nor the lungfish, therefore,
0:09:56 > 0:10:00can be close to the ancestral creature that first moved to land.
0:10:01 > 0:10:05But if those two crucial elements were to occur in one animal,
0:10:05 > 0:10:09then such a creature would be a strong candidate.
0:10:09 > 0:10:11And indeed, they do.
0:10:11 > 0:10:16This fossil fish, from rocks 450 million years old, has them both.
0:10:16 > 0:10:18It's called Eusthenopteron.
0:10:18 > 0:10:22When the rock and scales around its fin are removed,
0:10:22 > 0:10:26you can see the bones - one close to the body,
0:10:26 > 0:10:29then two, then a group of small ones.
0:10:29 > 0:10:32Exactly the pattern found in the limb of all land vertebrates.
0:10:34 > 0:10:38And that adventurous ancestor may have been very like this.
0:10:38 > 0:10:42But why should it have climbed onto the land?
0:10:42 > 0:10:44Perhaps it was forced out by droughts.
0:10:44 > 0:10:49Maybe it was tempted by food, the creatures that swarmed on the mud.
0:10:49 > 0:10:50Whatever the reason,
0:10:50 > 0:10:55its descendants came to spend more of their time out of water.
0:10:55 > 0:10:57And over millions of years they evolved bodies
0:10:57 > 0:10:59that were more suited to life on land
0:10:59 > 0:11:02and became the first amphibians.
0:11:21 > 0:11:25The vegetation of the time was different from that of today.
0:11:25 > 0:11:27There were no flowering plants or conifers,
0:11:27 > 0:11:30and one of the commonest was a kind of horsetail,
0:11:30 > 0:11:33rather like these growing in the north of England,
0:11:33 > 0:11:36except the horsetails then,
0:11:36 > 0:11:40300 million years ago, grew to about 50 or 60 feet tall
0:11:40 > 0:11:43and formed dense forests growing in swamps.
0:11:43 > 0:11:46When they died, the horsetail trunks
0:11:46 > 0:11:49fell into the water and formed a kind of peat.
0:11:49 > 0:11:52Over the years, there were variations in the sea level
0:11:52 > 0:11:55which flooded these swamps
0:11:55 > 0:11:58and buried the peat beneath deposits of sand.
0:11:58 > 0:12:02Under the accumulating weight of these sediments,
0:12:02 > 0:12:05the peat then turned to coal.
0:12:18 > 0:12:24And in the mine, you can see the sand that's been turned to stone
0:12:24 > 0:12:28and beneath it, the compressed remains of the plants.
0:12:28 > 0:12:33And in this particular seam have been found the bones
0:12:33 > 0:12:38of some of the animals that crawled in those ancient swamps.
0:12:38 > 0:12:42This is one of the most dramatic of them.
0:12:42 > 0:12:47It's a skull. Here are its huge teeth,
0:12:47 > 0:12:52which are simple teeth, rather like the peg-like teeth of the fish then.
0:12:52 > 0:12:56We know that this creature had a paddle-shaped tail
0:12:56 > 0:13:00and also four very good limbs.
0:13:00 > 0:13:04So it really was a true amphibian.
0:13:04 > 0:13:06It must have been a very formidable creature, too.
0:13:06 > 0:13:09It grew to a length of about 12 feet.
0:13:34 > 0:13:36There were many kinds of them,
0:13:36 > 0:13:40and they dominated the land for 100 million years.
0:13:46 > 0:13:51The largest amphibian alive today, the giant salamander from Japan,
0:13:51 > 0:13:55grows to over 1.5 metres, four feet or so.
0:13:55 > 0:13:59Even that is only a quarter as big as its ancestors.
0:14:03 > 0:14:07Most of its living relations, the rest of the salamanders and newts,
0:14:07 > 0:14:11are very much smaller, a few centimetres only from nose to tail.
0:14:14 > 0:14:17Though newts spend much of their time out of water,
0:14:17 > 0:14:18they don't go far from it.
0:14:18 > 0:14:22In early spring, after hibernating, they must move back into it.
0:14:26 > 0:14:29Their skin is permeable. It doesn't retain liquid very well.
0:14:29 > 0:14:33If they dry out, they die.
0:14:33 > 0:14:35They need to keep their skin moist,
0:14:35 > 0:14:38for, like most amphibians, they breathe through it,
0:14:38 > 0:14:41supplementing oxygen from their lungs
0:14:41 > 0:14:43with more absorbed from the air.
0:14:43 > 0:14:47And one final shackle keeps them tied to water.
0:14:47 > 0:14:50They have to return to it to breed.
0:14:55 > 0:14:57Once in water,
0:14:57 > 0:15:02it sheds the thin outer skin used to protect it on land
0:15:02 > 0:15:06and takes up an existence that is much more like that of a fish.
0:15:07 > 0:15:11It often seems the newt is more at home here than on land,
0:15:11 > 0:15:13and indeed, it retains many
0:15:13 > 0:15:16characteristics of its fish ancestors.
0:15:20 > 0:15:23The males become brightly coloured
0:15:23 > 0:15:26and develop flamboyant crests along their backs.
0:15:26 > 0:15:29Their courtship is reminiscent of that of fish.
0:15:29 > 0:15:32They flex the frills along their backs
0:15:32 > 0:15:35just as so many fish flex their fins,
0:15:35 > 0:15:37and they beat the water with their tails,
0:15:37 > 0:15:41sending powerful currents towards the female,
0:15:41 > 0:15:43which she detects with a line of sensors
0:15:43 > 0:15:46that resemble the lateral line system of the fish.
0:15:56 > 0:15:59Two males are courting one female. She's in the middle,
0:15:59 > 0:16:01without a crest.
0:16:06 > 0:16:12The female lays several hundred eggs, each stuck to a leaf.
0:16:14 > 0:16:18Development is swift. The tiny white sphere elongates.
0:16:22 > 0:16:25Pigment appears.
0:16:25 > 0:16:28And soon the young emerge,
0:16:28 > 0:16:30and they're even more fish-like than their parents.
0:16:30 > 0:16:35They have no legs, and breathe not with lungs but with feathery gills.
0:16:35 > 0:16:39But slowly, their legs and lungs do develop,
0:16:39 > 0:16:43and the newt tadpole for a short period can breathe both ways.
0:16:48 > 0:16:53But there's one tadpole that remains like this all its life.
0:16:54 > 0:16:59Its external gills are large and feathery and permanent.
0:16:59 > 0:17:02It lives in one lake in Mexico and the Aztecs
0:17:02 > 0:17:05called it the water monster, axolotl.
0:17:46 > 0:17:51But the most surprising thing about this overgrown, eternal tadpole
0:17:51 > 0:17:53is that it breeds in this condition.
0:18:09 > 0:18:12The eggs start developing immediately.
0:18:12 > 0:18:14The black part is the beginning of a body
0:18:14 > 0:18:18which will grow round and enclose the cream-coloured yolk.
0:18:18 > 0:18:20Food supply for further development.
0:18:27 > 0:18:29Though the axolotl never changes
0:18:29 > 0:18:31into a land-living salamander in the wild,
0:18:31 > 0:18:35it has a close relative in Mexico which retains its options.
0:18:35 > 0:18:39Sometimes it breeds like the axolotl, but if its lake dries,
0:18:39 > 0:18:43it can turn into a normal land-living salamander.
0:18:43 > 0:18:46The tadpoles, still with their feathery gills,
0:18:46 > 0:18:48wriggle in the tepid, shallowing pools.
0:18:52 > 0:18:55But as time passes, the gills disappear.
0:18:55 > 0:18:58For now, the animal has developed lungs.
0:19:00 > 0:19:05And eventually the little creature hauls itself up onto the mud.
0:19:05 > 0:19:09But many salamanders aren't enthusiastic walkers
0:19:09 > 0:19:13and show signs of abandoning the habit.
0:19:13 > 0:19:15This one, from California,
0:19:15 > 0:19:20has tiny legs and spends its time burrowing under stones.
0:19:20 > 0:19:22One entire group of amphibians
0:19:22 > 0:19:26has opted totally for this way of life
0:19:26 > 0:19:29and lost their legs altogether - the Sicilians.
0:19:29 > 0:19:32You might well confuse these with large earthworms.
0:19:33 > 0:19:36This one comes from South-East Asia.
0:19:36 > 0:19:39Its eyes are covered in skin, and to replace them,
0:19:39 > 0:19:42it has small white feelers below its eye.
0:19:45 > 0:19:47Their bodies have become elongated
0:19:47 > 0:19:50and they've lost all traces of limbs.
0:19:59 > 0:20:03Most of them don't come up to the surface until night.
0:20:03 > 0:20:05But then you really see that they're not earthworms
0:20:05 > 0:20:08solidly champing through soil.
0:20:08 > 0:20:10Blind though they are, they're hunters.
0:20:14 > 0:20:18This one comes from South America.
0:20:33 > 0:20:37Sicilians constitute the smallest amphibian group.
0:20:37 > 0:20:41160 species are known, compared with over twice that
0:20:41 > 0:20:43for salamanders and newts.
0:20:43 > 0:20:45But they're so unobtrusive
0:20:45 > 0:20:49and so easily mistaken for worms and therefore ignored,
0:20:49 > 0:20:52that there may well be many more kinds
0:20:52 > 0:20:56still to be discovered in the soft, damp soils of the Tropics.
0:20:56 > 0:20:59But most of the amphibians living in the world today
0:20:59 > 0:21:01belong to a third group.
0:21:01 > 0:21:04A group that doesn't live below ground like the Sicilians,
0:21:04 > 0:21:08but above it, and far from having lost their legs,
0:21:08 > 0:21:13they have developed their legs to a spectacular degree -
0:21:13 > 0:21:14the frogs and toads.
0:21:14 > 0:21:17And this is the king of them all,
0:21:17 > 0:21:23the largest frog in the world, the Goliath frog.
0:21:23 > 0:21:29It's a very rare animal that lives in a small part of West Africa.
0:21:29 > 0:21:34In captivity, it lives on small birds and rats or mice,
0:21:34 > 0:21:40and even fish. But in the wild, its diet is not quite so ambitious.
0:21:40 > 0:21:43It takes dragonflies and other insects
0:21:43 > 0:21:48as well as crabs from the bottom of the river.
0:21:48 > 0:21:50It's a very good swimmer,
0:21:50 > 0:21:54with very large webs at the bottom of its feet.
0:21:54 > 0:21:58But these huge legs...these huge legs
0:21:58 > 0:22:01also enable it to jump very well.
0:22:01 > 0:22:07This particular one can jump nine or ten feet, ten times its body length.
0:22:07 > 0:22:11But in the kingdom of frogs, that's not much.
0:22:11 > 0:22:14Some of the smaller frogs can do very much better than that
0:22:14 > 0:22:16and are dazzling athletes.
0:22:28 > 0:22:32When each foot is webbed to form a parachute,
0:22:32 > 0:22:35your leaps are spectacular indeed.
0:22:52 > 0:22:54This is the famous flying frog,
0:22:54 > 0:22:58though it'd be more accurate to call it a glider.
0:22:58 > 0:23:02Even so, in one leap and glide, it can cover 15 metres or so,
0:23:02 > 0:23:05say, 100 times its body length.
0:23:05 > 0:23:08Several species have developed this talent.
0:23:08 > 0:23:10There's one in Japan, another in Malaya,
0:23:10 > 0:23:12and this one lives in Costa Rica.
0:23:20 > 0:23:24Its feet are not only webbed but each toe ends in a sucker
0:23:24 > 0:23:29so it can also cling to vertical leaves, if it has a mind to.
0:23:29 > 0:23:32But its unique splendour is only revealed
0:23:32 > 0:23:37when it leaps and opens its four extraordinary parachutes.
0:24:19 > 0:24:23The flying frog seeks safety by launching itself into the air.
0:24:23 > 0:24:26This frog takes refuge underground.
0:24:26 > 0:24:29And its pointed nose gives it a very good start.
0:24:36 > 0:24:39And then its legs provide a pile-driving thrust.
0:24:44 > 0:24:48The holy cross toad of Australia also buries itself,
0:24:48 > 0:24:52but it goes rear end first, with a different kind of leg action.
0:25:02 > 0:25:05It's easy to understand why they hide.
0:25:05 > 0:25:08Frogs to a hungry hunter appear appetising
0:25:08 > 0:25:11and vulnerable with their soft bodies.
0:25:11 > 0:25:13And indeed, many of them are.
0:25:13 > 0:25:19But some have developed defences, and very surprising ones, too.
0:25:20 > 0:25:24This grass snake is about to tackle an ordinary European toad.
0:25:42 > 0:25:45The combination of standing on tiptoe
0:25:45 > 0:25:49and inflating its body makes it look much bigger than it really is.
0:25:55 > 0:25:59Whether this frightens the snake or baffles it, who can say?
0:25:59 > 0:26:01Whatever its effect, it works.
0:26:04 > 0:26:05The fire-bellied toad. Watch.
0:26:08 > 0:26:11This extraordinary posture deters predators
0:26:11 > 0:26:14by revealing the pattern on its stomach,
0:26:14 > 0:26:17a combination of colours that's widely recognised
0:26:17 > 0:26:21by animals as a warning. It's not all bluff, either.
0:26:21 > 0:26:24All amphibians have mucus glands in their skin
0:26:24 > 0:26:26which help keep them moist,
0:26:26 > 0:26:28and some of these glands in the
0:26:28 > 0:26:32fire-bellied toad produce a bitter-tasting poison.
0:26:32 > 0:26:37Skin has become versatile in the amphibians for breathing, defence,
0:26:37 > 0:26:41and it comes in all sizes, shapes and colours.
0:26:50 > 0:26:54In South America, some frogs have developed defence
0:26:54 > 0:26:56so far that they've become real killers.
0:26:56 > 0:26:59The poison in their skins is so powerful,
0:26:59 > 0:27:03it can paralyse a monkey or a bird immediately.
0:27:03 > 0:27:06There are at least 20 different kinds of poison frogs
0:27:06 > 0:27:08in Central and South America,
0:27:08 > 0:27:13and conspicuousness is an important part of their defence strategy.
0:27:13 > 0:27:15They don't want to be eaten by mistake,
0:27:15 > 0:27:18for it's of no value to them if their attacker dies
0:27:18 > 0:27:20soon after they've been eaten.
0:27:20 > 0:27:23So they're all dressed in spectacular colours.
0:27:23 > 0:27:26Colour is of no use at night when it can't be seen,
0:27:26 > 0:27:28so unusually for frogs,
0:27:28 > 0:27:31these little creatures are active in the daytime,
0:27:31 > 0:27:34moving boldly around the forest,
0:27:34 > 0:27:37confident and secure in their brilliant livery.
0:27:45 > 0:27:49This particular species has good reason to be confident.
0:27:49 > 0:27:52It has the most poisonous skin secretion of all.
0:27:52 > 0:27:55It's only recently been discovered by science.
0:27:55 > 0:27:59And just a tiny smear from its skin could kill a man.
0:27:59 > 0:28:04The local Indians in Colombia use its poison on blowgun darts
0:28:04 > 0:28:08by rubbing the tips on the backs of the living frogs.
0:28:12 > 0:28:14One frog in Argentina has developed
0:28:14 > 0:28:17a unique way of safeguarding against that,
0:28:17 > 0:28:20and, at the same time, keeping itself watertight
0:28:20 > 0:28:22when the weather's dry.
0:28:22 > 0:28:24It gives itself a varnish.
0:28:30 > 0:28:33There are many wax glands in its skin,
0:28:33 > 0:28:36and when it feels its body is drying out,
0:28:36 > 0:28:39it gives itself a good going-over to produce a thin,
0:28:39 > 0:28:41waterproof covering.
0:29:17 > 0:29:20But there is, of course, another opposite strategy.
0:29:20 > 0:29:23If you're without defences of any kind,
0:29:23 > 0:29:27then it may be much more effective to spend the day concealed,
0:29:27 > 0:29:28camouflaged as part of a leaf.
0:29:32 > 0:29:36Some conceal themselves not to escape, but to lurk in ambush.
0:29:36 > 0:29:41This big toad will pounce on mice and fledglings as well as worms.
0:29:45 > 0:29:48Cleaning earth and twigs from the worm is important,
0:29:48 > 0:29:50for the toad has no teeth and swallows whole
0:29:50 > 0:29:52whatever gets into its mouth.
0:29:52 > 0:29:56It doesn't want any hard or spiky, inedible bits.
0:29:59 > 0:30:04A tongue that can be stuck out is an amphibian invention.
0:30:04 > 0:30:07No fish ever had one, and very effective it is, too.
0:30:12 > 0:30:15The blink is an essential part of swallowing.
0:30:15 > 0:30:18Frogs and toads have no bony base to their eye sockets,
0:30:18 > 0:30:21so their eyeballs bulge down into their mouths.
0:30:21 > 0:30:24When they blink, the underside of the eyes
0:30:24 > 0:30:28helps to squeeze the food in its mouth back towards its throat.
0:30:28 > 0:30:31Their tongue is not attached to the back of the mouth but to the front,
0:30:31 > 0:30:35so they can stick it out much farther than we can,
0:30:35 > 0:30:39which is very useful for an ungainly hunter without a neck like a toad.
0:30:39 > 0:30:41Its end is sticky and muscular
0:30:41 > 0:30:44and it grabs the worm with the underside.
0:30:44 > 0:30:50And then the tongue has one final function. It lubricates the food
0:30:50 > 0:30:52so it can be swallowed easily
0:30:52 > 0:30:56without scratching the delicate membranes of the throat.
0:30:58 > 0:31:02They eyes of the amphibians are fundamentally the same
0:31:02 > 0:31:05as those of fish. There was no need to change them,
0:31:05 > 0:31:07for they work as well in air as water.
0:31:07 > 0:31:10But they have to be kept moist and clean,
0:31:10 > 0:31:13so the amphibians have developed an ability to blink
0:31:13 > 0:31:15and a membrane to wipe the surface.
0:31:27 > 0:31:31Protection from strong light by closing the iris.
0:31:32 > 0:31:36Or by using a membrane which still lets light in.
0:31:38 > 0:31:42In air, however, you do need a different hearing apparatus
0:31:42 > 0:31:47than in water. Eardrums. And with them came a voice.
0:31:47 > 0:31:51Some frogs call during the day, like these edible frogs.
0:31:51 > 0:31:53RASPING CALL
0:32:09 > 0:32:12But most sing at night. Before the amphibians had crawled
0:32:12 > 0:32:16out of the water 300 million years ago,
0:32:16 > 0:32:19the only animal sounds on earth had been chirps of insects.
0:32:19 > 0:32:23So the first animal chorus to break the silence of the land
0:32:23 > 0:32:27may well have been like this.
0:32:27 > 0:32:29WAILING CALL
0:32:29 > 0:32:33The frog's lungs which blow air through its tiny vocal cords
0:32:33 > 0:32:36are feeble. But resonating sacs bulging from the
0:32:36 > 0:32:40angle of the jaws or the throat amplify it many times,
0:32:40 > 0:32:43so that some calls can be heard for over a mile away.
0:32:43 > 0:32:46HIGH-PITCHED CALL
0:32:53 > 0:32:55SHORT CHIRPS
0:33:03 > 0:33:05TWANGING CALL
0:33:08 > 0:33:10HAMMERING CALL
0:33:13 > 0:33:15VARIOUS CALLS
0:34:10 > 0:34:12CHORUS OF CALLS
0:34:25 > 0:34:28The cue for these choruses is usually a change in the weather,
0:34:28 > 0:34:31for these songs are the prelude to mating.
0:34:31 > 0:34:33In the Tropics, the trigger is usually
0:34:33 > 0:34:35the onset of the rainy season.
0:34:37 > 0:34:38THUNDER CLAPS
0:35:05 > 0:35:07As the forest is drenched,
0:35:07 > 0:35:10so the moisture-loving amphibians can get out,
0:35:10 > 0:35:13seeking mates and laying eggs.
0:35:26 > 0:35:29In this very rare species from Costa Rica,
0:35:29 > 0:35:32the male is yellow, the female red and brown.
0:35:37 > 0:35:41They abandon their eggs after they've laid them to return to land.
0:35:41 > 0:35:44Streams and ponds and other such spawning sites
0:35:44 > 0:35:48often swarm with fish that will eat any eggs or young they can find.
0:35:48 > 0:35:52So hundreds must be laid if just one or two are to survive.
0:35:54 > 0:35:58In temperate areas, breeding begins when the weather warms in spring.
0:35:58 > 0:36:02European toads migrate from miles around to a single favoured pond
0:36:02 > 0:36:06and assemble there in great numbers, all within a few days.
0:36:06 > 0:36:09The breeding period may only last a week or so,
0:36:09 > 0:36:13and towards the end, females with eggs still to lay become rare.
0:36:13 > 0:36:16And the males, in their frenzy to couple with them,
0:36:16 > 0:36:20clasp anything in the neighbourhood that moves, male or female,
0:36:20 > 0:36:23and so form tangled, writhing groups.
0:37:14 > 0:37:17These toads rely for breeding success on numbers.
0:37:17 > 0:37:22A single female may deposit 6,000 eggs.
0:37:22 > 0:37:24Mass production, be it in temperate or tropical places,
0:37:24 > 0:37:27seems to be a very effective strategy.
0:37:33 > 0:37:37These tadpoles, for example, developed from eggs
0:37:37 > 0:37:39that were laid in enormous numbers
0:37:39 > 0:37:41in pools beside this South American river.
0:37:41 > 0:37:46But amphibians have another option.
0:37:46 > 0:37:48They, after all, can climb up onto land,
0:37:48 > 0:37:55so they can lay their eggs in places no fish could possibly reach.
0:37:55 > 0:37:58When they had the land to themselves,
0:37:58 > 0:38:01that must have been a particularly effective strategy.
0:38:01 > 0:38:03Even today, there are many frogs
0:38:03 > 0:38:06that go to quite extraordinary lengths
0:38:06 > 0:38:10in order to lay their eggs away from ponds and rivers.
0:38:10 > 0:38:15Of course, they have to keep their eggs moist, or they would dry out.
0:38:15 > 0:38:19But around this river, at any rate, that's not too difficult.
0:38:28 > 0:38:32These are the Kaieteur Falls on the Potaro River in Guyana.
0:38:32 > 0:38:35The forest round here must be paradise for frogs,
0:38:35 > 0:38:38for here, in effect, there is permanent rain.
0:38:38 > 0:38:40And quite warm rain, at that.
0:38:48 > 0:38:52The Potaro, above the falls, is 100 or so metres across,
0:38:52 > 0:38:56and its waters fall sheer for over 200 metres.
0:38:56 > 0:38:58Much of these great masses of falling water
0:38:58 > 0:39:01turn to spray and drenching mists.
0:39:09 > 0:39:15And as the mist comes swirling up, it condenses into drops
0:39:15 > 0:39:18which fill the centre of such plants as this
0:39:18 > 0:39:23and turn them into miniature ponds, idea for the frogs' purposes.
0:39:23 > 0:39:28Inside this particular one lives a tiny, beautiful, golden frog.
0:39:35 > 0:39:37It shares its minute pool with a few larvae,
0:39:37 > 0:39:40but nothing that does it or its eggs any harm.
0:39:51 > 0:39:54There are many plants with water-filled chalices in their
0:39:54 > 0:39:56centres in the South American rainforest,
0:39:56 > 0:40:00and many of them grow high up on branches,
0:40:00 > 0:40:03their roots dangling in the moist air.
0:40:03 > 0:40:06So they are, in effect, ponds up trees.
0:40:06 > 0:40:08They provide the frogs with little oases
0:40:08 > 0:40:13where they can live and spawn away from predators for generations
0:40:13 > 0:40:16without ever coming down to the ground.
0:40:20 > 0:40:23In the African savannahs, with much less rain,
0:40:23 > 0:40:26there are no such plants, but there is a frog
0:40:26 > 0:40:30that manages to breed in the trees.
0:40:30 > 0:40:36Instead of water, it uses foam. The trick is done at night.
0:40:42 > 0:40:44The female excretes a liquid
0:40:44 > 0:40:48which she beats into a lather with her hind legs.
0:40:48 > 0:40:50The male joins her and fertilises
0:40:50 > 0:40:53the 150 or so eggs which she deposits in the foam.
0:40:53 > 0:40:57The sun will bake the outside into a hard crust.
0:40:57 > 0:41:01But inside, it remains liquid, and there the eggs develop.
0:41:01 > 0:41:04The nests are always made above water,
0:41:04 > 0:41:08so in due time, when the crust cracks and the young ooze out,
0:41:08 > 0:41:10they drop straight down into a river or a swamp.
0:41:39 > 0:41:41This frog from South America also
0:41:41 > 0:41:45has a way of keeping its eggs away from the river.
0:41:45 > 0:41:47Here, however, where the air is more humid,
0:41:47 > 0:41:49it doesn't need foam,
0:41:49 > 0:41:52because the jelly surrounding its eggs doesn't dry out.
0:41:52 > 0:41:56The young tadpoles develop inside the jelly, like many other species,
0:41:56 > 0:41:58but these stay there while they go through
0:41:58 > 0:42:00most of their larval development.
0:42:11 > 0:42:15They even develop gills inside the egg.
0:42:21 > 0:42:24And their hearts begin to beat.
0:42:27 > 0:42:33Eventually, they too will emerge and drop down into water.
0:42:33 > 0:42:36The tadpoles of one Caribbean frog have managed, astonishingly,
0:42:36 > 0:42:40to dispense with water altogether for their development.
0:42:40 > 0:42:43The whistling frog lays a cluster of eggs on the ground.
0:42:43 > 0:42:46They're only small, but inside each there is liquid.
0:42:46 > 0:42:50And in it, the young develop not only to the tadpole stage,
0:42:50 > 0:42:52but beyond.
0:43:06 > 0:43:08Their tiny stomachs are full of yolk
0:43:08 > 0:43:11that must fuel their entire development.
0:43:13 > 0:43:15The front legs are formed.
0:43:23 > 0:43:24And so are the back legs.
0:43:27 > 0:43:29And at last, it becomes virtually a tiny,
0:43:29 > 0:43:32fully-developed replica of its parent.
0:43:46 > 0:43:49On its nose, it has a tiny spike,
0:43:49 > 0:43:52and with that, it punctures the egg membrane.
0:43:52 > 0:43:56And after about 18 days, it hatches, having eliminated altogether
0:43:56 > 0:44:00the tadpole's normal need for open water.
0:44:00 > 0:44:02Laying eggs away from water and its dangers
0:44:02 > 0:44:05is a successful breeding strategy.
0:44:05 > 0:44:08But yet other frogs have taken a different line.
0:44:08 > 0:44:12Instead of abandoning their eggs in a safe place,
0:44:12 > 0:44:15they stay with them and look after them.
0:44:15 > 0:44:19The midwife toad lives in Europe. Its name is not accurate
0:44:19 > 0:44:25because it's the male that carries the eggs entangled round his legs.
0:44:25 > 0:44:27There may be 60 or so of them,
0:44:27 > 0:44:30and he carries them for six or seven weeks.
0:44:30 > 0:44:35At hatching time, he takes them down to water and the tadpoles swim away.
0:44:38 > 0:44:41Some toads spend all their time in water.
0:44:41 > 0:44:43This pipa from Brazil,
0:44:43 > 0:44:47instead of laying and abandoning 6,000 eggs like the European toad,
0:44:47 > 0:44:49lays a mere hundred or so.
0:44:53 > 0:44:57But they look after them in the most extraordinary manner.
0:44:57 > 0:45:02The male, with these elegant movements of its hind feet,
0:45:02 > 0:45:04takes care that as many eggs as possible
0:45:04 > 0:45:07are gathered on the female's back.
0:45:11 > 0:45:13And they stick.
0:45:39 > 0:45:43Then the skin on the female's back begins to swell.
0:45:45 > 0:45:48The eggs rapidly become embedded in it.
0:45:52 > 0:45:56Soon, a membrane grows over them to enclose them completely.
0:46:00 > 0:46:04After only 30 hours, almost all the eggs have disappeared
0:46:04 > 0:46:07and the skin is complete again.
0:46:07 > 0:46:10After nearly three weeks, it's moving.
0:46:12 > 0:46:17And then, after another three weeks or so, the young begin to emerge.
0:46:45 > 0:46:48Now the parent leaves the young to fend for themselves.
0:46:48 > 0:46:51But at least they're now independent swimmers,
0:46:51 > 0:46:53able to find hiding places.
0:46:53 > 0:46:56So a higher proportion is likely to
0:46:56 > 0:46:59survive than if they'd been abandoned as eggs.
0:47:01 > 0:47:03This little South American frog
0:47:03 > 0:47:05also keeps her eggs and young on her back
0:47:05 > 0:47:09in a pouch with an opening just above the base of her spine.
0:47:17 > 0:47:20Her developing young remain inside it for three months or more,
0:47:20 > 0:47:24until at last she releases them into a pool as tadpoles.
0:47:32 > 0:47:36They're on their way towards their final change into adults,
0:47:36 > 0:47:38for they have back legs.
0:47:43 > 0:47:47These many differing ways of carrying their developing young
0:47:47 > 0:47:51may seem extraordinary enough, but other frogs in South America
0:47:51 > 0:47:55actually retain their tadpoles inside their bodies,
0:47:55 > 0:47:58in the most unlikely parts.
0:47:58 > 0:48:02There is one which calls in the beech forests of southern Chile.
0:48:02 > 0:48:03It's Darwin's frog.
0:48:22 > 0:48:25These creatures, only a few centimetres long,
0:48:25 > 0:48:28are all males, even though they vary in colour.
0:48:28 > 0:48:30They're still calling but the breeding is over.
0:48:30 > 0:48:35The females have laid their eggs in groups of 20 or 30 on the forest floor.
0:48:35 > 0:48:41As soon as the males see a movement in the eggs, they will, apparently, eat them.
0:48:41 > 0:48:44Each male may take a dozen or so but he doesn't swallow them.
0:48:44 > 0:48:48Instead, they go into his vocal sac down the front of his throat
0:48:48 > 0:48:52and there they develop...and wriggle.
0:48:52 > 0:48:56The males sit about, struck dumb by their own offspring.
0:48:56 > 0:48:58After some weeks, their extraordinary
0:48:58 > 0:49:01vocal pregnancy comes to an end.
0:49:21 > 0:49:25And here is that amazing birth once again, in slow motion.
0:49:35 > 0:49:39The male's vocal sac is now ready again for singing,
0:49:39 > 0:49:43before it's turned next season once more into a nursery.
0:49:45 > 0:49:49The prize for the care of the young must surely go to this frog,
0:49:49 > 0:49:52that lives only on a remote mountain in West Africa.
0:49:55 > 0:49:57The female, only about a centimetre long,
0:49:57 > 0:50:00keeps the eggs inside her distended oviducts
0:50:00 > 0:50:04and holds them there throughout the nine months of the dry season.
0:50:04 > 0:50:09As they grow, she secretes internally some white flakes.
0:50:09 > 0:50:13The tadpoles, moving around freely inside the oviduct,
0:50:13 > 0:50:15eat the flakes and digest them in
0:50:15 > 0:50:19their gut just as they would do in the pond.
0:50:19 > 0:50:22When the rain comes, she gives birth.
0:50:22 > 0:50:27Her stomach and oviduct don't have muscles which can expel the young,
0:50:27 > 0:50:31so she does it by bracing her body against the ground with her forelegs
0:50:31 > 0:50:35and inflating her lungs so they bulge into her abdomen
0:50:35 > 0:50:37and squeeze the young out.
0:50:37 > 0:50:41And they're born fully-formed froglets,
0:50:41 > 0:50:44a triumph of parental care.
0:50:44 > 0:50:47By providing their young with moisture of some kind
0:50:47 > 0:50:51and using all these varying and astonishing techniques,
0:50:51 > 0:50:54frogs and toads have managed to colonise almost all the world.
0:50:54 > 0:50:58Even so, you would think that with their thin, permeable skins,
0:50:58 > 0:51:02they would never be able to survive in the Australian desert.
0:51:02 > 0:51:05But one or two species manage to live even here.
0:51:05 > 0:51:10They spend nearly all their lives in the ground, away from the sun.
0:51:10 > 0:51:12They may lie here for years,
0:51:12 > 0:51:16waiting, but eventually the rains do come.
0:51:29 > 0:51:31When the frogs burrowed down here during the last rains,
0:51:31 > 0:51:33they were bloated with water,
0:51:33 > 0:51:35and they've conserved it in their chambers
0:51:35 > 0:51:40by sealing themselves inside a membrane secreted from their skins.
0:51:40 > 0:51:45But when the rain arrives again, they must get rid of their packaging
0:51:45 > 0:51:48to be ready for breeding.
0:52:04 > 0:52:07In the brief period when the desert is wet,
0:52:07 > 0:52:10these frogs will dig themselves out and mate,
0:52:10 > 0:52:12and their tadpoles will develop
0:52:12 > 0:52:15in the few days there's water in the pools.
0:52:15 > 0:52:20Then, as the desert dries out, the young frogs will bury themselves
0:52:20 > 0:52:24and remain underground for perhaps five years or more.
0:52:31 > 0:52:34In times of drought, the desert aborigines
0:52:34 > 0:52:39search eagerly for frogs like this, and this is why.
0:52:39 > 0:52:43If you squeeze one, you can get a reasonable drink of water from it.
0:53:01 > 0:53:04It's tasteless and really quite drinkable.
0:53:04 > 0:53:08And now I'm going to have to find a pond for this little creature,
0:53:08 > 0:53:11where it will survive until the next rains come.
0:53:11 > 0:53:13For the fact of the matter is
0:53:13 > 0:53:16that its success as a desert liver is limited.
0:53:16 > 0:53:18It can only be active and breed
0:53:18 > 0:53:21during that short period when there's rain.
0:53:21 > 0:53:24In order to survive in a desert and breed there,
0:53:24 > 0:53:27if necessary with no rain at all,
0:53:27 > 0:53:32you need a device that no frog or amphibian has got.
0:53:34 > 0:53:35This.
0:53:35 > 0:53:39An egg with a waterproof shell.
0:53:39 > 0:53:43That was the next great evolutionary breakthrough.
0:53:43 > 0:53:46And it was achieved by the reptiles.
0:53:46 > 0:53:49The astonishment is that without it,
0:53:49 > 0:53:53amphibians managed to colonise so much of the world.
0:54:15 > 0:54:18Subtitles by Red Bee Media Ltd