0:01:49 > 0:01:52These mountains stand in the middle
0:01:52 > 0:01:54of the biggest desert on Earth, the Sahara.
0:01:54 > 0:01:57It stretches right across the width of Africa,
0:01:57 > 0:02:01three and a half million square miles of it.
0:02:01 > 0:02:06At night, it gets so cold that it can freeze.
0:02:06 > 0:02:09During the day, the sun strikes it so ferociously
0:02:09 > 0:02:14that the highest land temperatures ever recorded were measured here -
0:02:14 > 0:02:1858 degrees centigrade, 137 degrees Fahrenheit.
0:02:18 > 0:02:22And, in turn, those oven-like temperatures rob the land
0:02:22 > 0:02:26of all its moisture. All in all, there could hardly be
0:02:26 > 0:02:29a more hostile environment for life on Earth.
0:02:29 > 0:02:33But it wasn't always this way.
0:02:33 > 0:02:36And if you want evidence of that, here it is.
0:02:41 > 0:02:44A group of antelope, probably sable.
0:02:44 > 0:02:47Creatures that can't live anywhere in the Sahara today,
0:02:47 > 0:02:50because there's simply not enough vegetation for them.
0:02:50 > 0:02:52These aren't the only wild animals
0:02:52 > 0:02:54that have been painted on these rocks.
0:02:57 > 0:02:59A giraffe.
0:03:01 > 0:03:04A kind of wild goat, probably a moufflon.
0:03:04 > 0:03:05And antelope.
0:03:05 > 0:03:08Obviously, at the time these pictures were painted,
0:03:08 > 0:03:11there was good grazing here.
0:03:11 > 0:03:14Indeed, there was sufficient vegetation
0:03:14 > 0:03:17to sustain not only wild animals,
0:03:17 > 0:03:20but whole herds of cattle.
0:03:20 > 0:03:23We don't know exactly who drew these pictures.
0:03:23 > 0:03:26The artists may have been ancestors of the nomads
0:03:26 > 0:03:30who today follow their herds of long-horned piebald cattle just
0:03:30 > 0:03:31south of the Sahara.
0:03:31 > 0:03:35But we know what they looked like, because they left their portraits.
0:03:35 > 0:03:38They lived here, it seems, some 5,000 years ago.
0:03:38 > 0:03:42But eventually the rains began to fail, the pastures disappeared,
0:03:42 > 0:03:45and with it the cattle and their keepers.
0:03:45 > 0:03:49But there are one or two living survivors from that time.
0:03:51 > 0:03:55This ancient cypress, judging from the number of rings in the trunks of
0:03:55 > 0:04:00others like it, is probably between 2,000 and 3,000 years old.
0:04:00 > 0:04:04In fact, it was probably already growing here when the last of
0:04:04 > 0:04:09the paintings were being made. It still bears fertile seed,
0:04:09 > 0:04:12but there are no little seedlings growing here to replace it.
0:04:12 > 0:04:15The land now is far too dry for them.
0:04:15 > 0:04:18Indeed, the cypress itself only survives
0:04:18 > 0:04:22because it sends its huge roots deep into the ground to tap
0:04:22 > 0:04:23underground water.
0:04:24 > 0:04:28The drying out of the Sahara seems to be connected
0:04:28 > 0:04:32with the great changes in climate at the end of the last ice age.
0:04:32 > 0:04:36As the glaciers retreated northwards across Europe,
0:04:36 > 0:04:39so the belt of rains that fell regularly along their southern edge
0:04:39 > 0:04:42left Africa and moved up into Europe,
0:04:42 > 0:04:45and the Sahara was robbed of its rains.
0:04:45 > 0:04:47Indeed, it seems to be that most,
0:04:47 > 0:04:49if not all of the great deserts in the world,
0:04:49 > 0:04:53were formed around that time, and most, if not all of them,
0:04:53 > 0:04:57are therefore comparatively recent environments.
0:04:57 > 0:05:01To see why deserts lie where they do,
0:05:01 > 0:05:03we can look at the Sahara from the west.
0:05:03 > 0:05:07The equator runs away from us across the width of Africa.
0:05:07 > 0:05:12It's along this line that the sun's rays strike from directly overhead,
0:05:12 > 0:05:14and therefore with the greatest strength.
0:05:14 > 0:05:16The heated air rises along the equator
0:05:16 > 0:05:20and flows away, north and south, to cooler parts of the world.
0:05:20 > 0:05:24Because it's warm, it carries with it a lot of moisture.
0:05:24 > 0:05:26But as it rises and cools,
0:05:26 > 0:05:29the moisture condenses first to form clouds
0:05:29 > 0:05:31and then to fall as rain.
0:05:31 > 0:05:35By the time the air comes down again over the Sahara to the left
0:05:35 > 0:05:37and the Kalahari to the right,
0:05:37 > 0:05:41it's lost most of its moisture and creates very few clouds.
0:05:41 > 0:05:44So the Sahara, with few clouds to shield it from the sun,
0:05:44 > 0:05:48becomes roastingly hot during the day.
0:05:48 > 0:05:51And at night, with no blanket of clouds to keep in its warmth,
0:05:51 > 0:05:53it gets desperately cold.
0:05:55 > 0:05:58Deserts are not placed symmetrically around the world,
0:05:58 > 0:06:00because the continents themselves
0:06:00 > 0:06:02are distributed in a very irregular way.
0:06:02 > 0:06:05They're ridged with great mountain ranges,
0:06:05 > 0:06:07which interfere with the even flow of air,
0:06:07 > 0:06:10and the spin of the planet itself creates vast eddies
0:06:10 > 0:06:13in the atmosphere, which further complicates things.
0:06:13 > 0:06:15But even so, deserts do lie
0:06:15 > 0:06:19in two broad zones on either side of the equator.
0:06:19 > 0:06:22The pattern in Africa, with the Sahara in the north
0:06:22 > 0:06:26and the Kalahari and the Namib in the south, has its equivalent
0:06:26 > 0:06:28in the Americas.
0:06:29 > 0:06:33South of the lush equatorial jungles of the Amazon,
0:06:33 > 0:06:38beyond the great range of the Andes, lies the Atacama desert.
0:06:38 > 0:06:41On the other side of the equator, beyond the drenched tropical
0:06:41 > 0:06:46rainforests of Panama, stretch the deserts of Mexico and Arizona.
0:06:47 > 0:06:51Across the Pacific, the greatest expanse of water on the globe,
0:06:51 > 0:06:55lies, south of the equator, Australia,
0:06:55 > 0:06:57most of which is covered by desert.
0:06:57 > 0:07:02Its northern tip gets close enough to the equator to collect some rain.
0:07:02 > 0:07:04Farther north still,
0:07:04 > 0:07:08beyond the jungle that blankets Indonesia and Malaysia, Thailand
0:07:08 > 0:07:12and Burma, across the great snow-covered range of the Himalayas,
0:07:12 > 0:07:15stretch the vast deserts of central Asia -
0:07:15 > 0:07:17Mongolia and the Gobi.
0:07:17 > 0:07:20And beyond them, as we complete the circuit of the globe,
0:07:20 > 0:07:25the huge desert of the Middle East that covers Iran, Iraq and Jordan,
0:07:25 > 0:07:30Syria and Israel, the vast, sandy emptiness of Arabia,
0:07:30 > 0:07:33and runs on to join the Sahara.
0:07:39 > 0:07:43This is the biggest expanse of waterless land on Earth.
0:07:43 > 0:07:48Here, as in deserts everywhere, almost nothing moves
0:07:48 > 0:07:49during the heat of the day.
0:07:51 > 0:07:53But animals are here.
0:08:04 > 0:08:06If you want to see what made these tracks,
0:08:06 > 0:08:11you have to wait until the sun sinks and the desert begins to cool.
0:08:19 > 0:08:21A striped hyena,
0:08:21 > 0:08:23one of the commonest of the bigger desert animals
0:08:23 > 0:08:25in this part of the world.
0:08:35 > 0:08:37A fennec fox.
0:08:45 > 0:08:50Fennecs usually live in small family groups, and clearly enjoy
0:08:50 > 0:08:51one another's company.
0:08:59 > 0:09:03But there's not much time for frolicking. Food must be found.
0:09:03 > 0:09:07Faint smells from the sand tell them who has moved where
0:09:07 > 0:09:09since they were last out.
0:09:38 > 0:09:41As the moon rises, many more creatures emerge.
0:09:41 > 0:09:44A gecko. Just what the fennec wants.
0:09:56 > 0:10:00A jerboa. It, too, is looking for food. Seeds.
0:10:08 > 0:10:12Another little seed-eating rodent, a gerbil.
0:10:15 > 0:10:19And a caracal, a kind of cat, which loves both gerbils and jerboas,
0:10:19 > 0:10:21if it can get them.
0:10:37 > 0:10:41A smaller hunter, but nonetheless a deadly one - a scorpion.
0:10:41 > 0:10:44It is searching for beetles or other small insects.
0:10:44 > 0:10:47But sometimes the hunter becomes the hunted.
0:10:47 > 0:10:49A black widow spider has set
0:10:49 > 0:10:53her snare of silk underneath a thorn bush.
0:11:31 > 0:11:36In the intense struggle, the black widow loses one of her legs.
0:11:47 > 0:11:51She manages to get more ropes of silk around the scorpion,
0:11:51 > 0:11:53hampering it still further.
0:11:57 > 0:12:02The scorpion hangs on to its trophy, but to no purpose.
0:12:02 > 0:12:04The battle is as good as lost.
0:12:15 > 0:12:18Methodically, the spider trusses up her victim
0:12:18 > 0:12:20and hangs it in her larder.
0:12:26 > 0:12:30Wolves, perhaps surprisingly, are quite common in these
0:12:30 > 0:12:34Middle Eastern deserts, but they're not the same as those farther north.
0:12:34 > 0:12:38They're smaller, lighter-coloured, and with only the thinnest fur,
0:12:38 > 0:12:41and they scavenge as much as they hunt.
0:12:45 > 0:12:49The cool night is coming to an end. Hunting is over.
0:12:49 > 0:12:52The animals must go back to their dens and hiding places
0:12:52 > 0:12:56to shelter from the heat that is to come.
0:13:16 > 0:13:18The sun returns, and very soon
0:13:18 > 0:13:22the desert will be heating up once again.
0:13:22 > 0:13:23The mammals that were active
0:13:23 > 0:13:26during the night have to go for shelter.
0:13:26 > 0:13:28And the day will belong not to them,
0:13:28 > 0:13:31but to those creatures that get their heat
0:13:31 > 0:13:33directly from the sun, the reptiles.
0:13:33 > 0:13:37This is the desert of the American west in Arizona,
0:13:37 > 0:13:39and we've come here to look at one
0:13:39 > 0:13:42very special desert reptile - this one.
0:13:44 > 0:13:47This is the Gila monster,
0:13:47 > 0:13:51one of only two poisonous lizards in the world.
0:13:56 > 0:14:00Actually, he very seldom uses his poison in defence,
0:14:00 > 0:14:03and in any case, it's still quite early in the morning
0:14:03 > 0:14:07and he is so cold that he isn't very active.
0:14:07 > 0:14:12But in only about an hour, the desert will get so hot that
0:14:12 > 0:14:15he won't be able to stand it, and he, too, will have to seek shade.
0:14:15 > 0:14:18So in this short period of the early morning
0:14:18 > 0:14:22and in the cool of the evening is the time when he hunts.
0:14:37 > 0:14:39A tortoise, but he's far too big
0:14:39 > 0:14:43and well armoured for a Gila monster to tackle.
0:14:53 > 0:14:57This great nest of sticks, however, looks much more promising.
0:15:03 > 0:15:05HISS
0:15:06 > 0:15:08SQUEAK
0:15:12 > 0:15:14The victim, a desert mouse.
0:15:28 > 0:15:33The tortoise is on the lookout for food too, but it is a vegetarian.
0:15:51 > 0:15:53The day is now several hours old.
0:15:53 > 0:15:57Cool dawn is changing to baking noon.
0:15:57 > 0:16:00It's time for even a reptile to get out of the sun.
0:16:07 > 0:16:09Movement generates heat,
0:16:09 > 0:16:13so now nothing moves unless it absolutely has to.
0:16:13 > 0:16:17And there are some creatures that remain motionless
0:16:17 > 0:16:20even when you get within a few inches of them.
0:16:20 > 0:16:24One of them is on the ground right in front of me now,
0:16:24 > 0:16:27though you may find it difficult to see
0:16:27 > 0:16:29because it's so well camouflaged.
0:16:29 > 0:16:32It's a poorwill, a kind of nightjar.
0:16:36 > 0:16:39Fluttering the throat evaporates moisture from the mouth
0:16:39 > 0:16:41and so cools the bird.
0:16:41 > 0:16:45It consumes much less energy than heaving the chest and panting,
0:16:45 > 0:16:48as many mammals would do in this situation.
0:16:48 > 0:16:51The sand grouse of Africa uses the same trick.
0:16:59 > 0:17:02The sand grouse chicks start doing it almost as soon as they emerge
0:17:02 > 0:17:04from the shell.
0:17:08 > 0:17:10They also immediately peck for seeds,
0:17:10 > 0:17:12but there's little moisture in a seed,
0:17:12 > 0:17:15and unless they drink, they will die.
0:17:15 > 0:17:19The responsibility for providing that rests entirely with the male.
0:17:20 > 0:17:25Every day he flies to water, maybe as much as 25 miles from the nest.
0:17:25 > 0:17:28First he fills his own stomach with water.
0:17:33 > 0:17:38But then, very deliberately, he soaks his belly feathers.
0:17:42 > 0:17:45These feathers have a special spongy structure
0:17:45 > 0:17:48so that they can absorb a great deal of water.
0:17:48 > 0:17:52Once he has a full load on board, he flies back to his family.
0:18:03 > 0:18:05At last the chicks get their drink.
0:18:05 > 0:18:09No other bird has such an ingenious water-carrying device.
0:18:19 > 0:18:21The roadrunner of the American deserts
0:18:21 > 0:18:24provides water for its chicks in a quite different way.
0:18:24 > 0:18:29This parent bird has collected a cicada for its family.
0:18:29 > 0:18:32Its nest is up in a cholla cactus.
0:18:32 > 0:18:36The parent doesn't give its chicks their food immediately.
0:18:42 > 0:18:44The chick is gulping.
0:18:44 > 0:18:47The parent bird is producing liquid from its stomach and letting
0:18:47 > 0:18:50it trickle down its beak.
0:18:55 > 0:18:58Each youngster gets its share.
0:19:13 > 0:19:17Another ration of solid food. This time, a lizard.
0:19:27 > 0:19:30Each time, before the meal is handed over, the chicks get a drink,
0:19:30 > 0:19:33whether they like it or not.
0:19:43 > 0:19:46During the day, the parents sit on the nest,
0:19:46 > 0:19:48not to keep the chicks warm,
0:19:48 > 0:19:52but, on the contrary, to keep them cool by shading them.
0:19:52 > 0:19:54And the sitting bird not only flutters its throat,
0:19:54 > 0:20:00but protects itself from the sun by using its tail as a parasol.
0:20:02 > 0:20:07The ground squirrel of the Namib desert does the same thing,
0:20:07 > 0:20:11and very effectively, too, carefully angling itself as far as possible
0:20:11 > 0:20:14to keep its body in the shade.
0:20:24 > 0:20:28Many animals keep their blood cool with radiators.
0:20:28 > 0:20:31The hedgehog that lives in the desert of the Middle East
0:20:31 > 0:20:32has unusually large ears.
0:20:32 > 0:20:36Blood circulates through capillaries close to the surface of the skin
0:20:36 > 0:20:39and is cooled by the breeze.
0:20:45 > 0:20:49The fennec fox's huge ears serve the same purpose.
0:20:52 > 0:20:54And so do those of the American jackrabbit,
0:20:54 > 0:20:59which perhaps has the biggest ears of all in proportion to its body.
0:21:04 > 0:21:08The Dorcas gazelle also has radiator ears
0:21:08 > 0:21:11and is one of the best-adapted of all desert mammals.
0:21:11 > 0:21:14It's one of the very few that can survive without a drink at all.
0:21:14 > 0:21:17It gets all the liquid it needs from the vegetation.
0:21:20 > 0:21:24It doesn't waste liquid as urine, but gets rid of its uric acid
0:21:24 > 0:21:26as small, dry pellets.
0:21:43 > 0:21:48It's now approaching noon, the hottest time of the day.
0:21:48 > 0:21:51It's summer, the hottest time of the year,
0:21:51 > 0:21:55and I'm in one of the hottest places on Earth -
0:21:55 > 0:21:58Death Valley in the western United States.
0:21:58 > 0:22:05A thermometer on the ground here has risen to 201 degrees Fahrenheit.
0:22:05 > 0:22:09That's about 94 degrees centigrade.
0:22:09 > 0:22:13It's so hot that no creature can survive permanently out here.
0:22:13 > 0:22:16Even at the edge of these sand flats,
0:22:16 > 0:22:19where the ground is more broken,
0:22:19 > 0:22:22there is no sign of animal life whatever.
0:22:22 > 0:22:26All animals now have sought the shade and shelter
0:22:26 > 0:22:29from this ferocious sun.
0:22:29 > 0:22:32But there are some organisms that can't get out of the sun.
0:22:33 > 0:22:36Plants, being fixed to the ground,
0:22:36 > 0:22:41have to stay out in the heat of the day and simply endure.
0:22:41 > 0:22:46But all of them have special devices to help them to do so.
0:22:46 > 0:22:51The desert holly. Its leaves grow at about 70 degrees to the vertical,
0:22:51 > 0:22:55so that in the morning when it's less hot and in the evening when
0:22:55 > 0:22:59the plant needs light, the face of its leaves face the light.
0:22:59 > 0:23:03During the middle of the day, it shows only the edges
0:23:03 > 0:23:05and so it doesn't heat up so much.
0:23:05 > 0:23:09Not only that, but the plant extracts salt
0:23:09 > 0:23:11from the salt-laden ground
0:23:11 > 0:23:15and excretes it as a white coating on the surface of the leaf,
0:23:15 > 0:23:19which, like the white costume of an athlete, reflects the heat
0:23:19 > 0:23:23and so keeps the plant that much cooler.
0:23:23 > 0:23:25And this, the creosote bush.
0:23:25 > 0:23:31This is one of the most widespread of plants in American deserts,
0:23:31 > 0:23:33and its roots are better at extracting
0:23:33 > 0:23:37the last molecule of water from these parched sands
0:23:37 > 0:23:39than those of any other American plant.
0:23:39 > 0:23:42This has led to an extraordinary state of affairs
0:23:42 > 0:23:44that's only just been discovered.
0:23:44 > 0:23:46It seems that the creosote bush
0:23:46 > 0:23:49was the first plant to establish itself
0:23:49 > 0:23:53in the arid Mojave desert when the desert first appeared.
0:23:53 > 0:23:57Once it had established its extensive root system,
0:23:57 > 0:24:00it extracted moisture from the sand so efficiently
0:24:00 > 0:24:03that it was extremely difficult for any other plant
0:24:03 > 0:24:05to grow alongside it.
0:24:05 > 0:24:09And that applied not only to any other kind of plant but also to
0:24:09 > 0:24:10its own seedlings.
0:24:12 > 0:24:15So an individual creosote bush tended to spread
0:24:15 > 0:24:19not by setting seeds and producing a new generation,
0:24:19 > 0:24:24but by sending out new stems around its base.
0:24:24 > 0:24:26And as these spread outwards,
0:24:26 > 0:24:29so the stems in the middle tended to die away,
0:24:29 > 0:24:33and the bush grew into a ring shape like this.
0:24:33 > 0:24:39So these are not separate individual creosote bushes, as it might appear,
0:24:39 > 0:24:45but this is just one big, ring-shaped, individual plant.
0:24:47 > 0:24:52Over the centuries, the rings widened and changed their shape
0:24:52 > 0:24:58until now some are over 25 yards across, like this one.
0:24:58 > 0:25:01Of course, the individual stems
0:25:01 > 0:25:05and leaves of this plant are not very ancient.
0:25:05 > 0:25:08The first ones to grow, which appeared in the middle,
0:25:08 > 0:25:11have decayed and disappeared a long time ago.
0:25:11 > 0:25:14But now it's estimated that this plant started growing
0:25:14 > 0:25:18between 10,000 and 12,000 years ago,
0:25:18 > 0:25:22in fact, when the Mojave desert first appeared,
0:25:22 > 0:25:27and that makes it the oldest known living organism in the world.
0:25:32 > 0:25:35In the Mojave, the plants may have to survive
0:25:35 > 0:25:37for as long as ten years without rain,
0:25:37 > 0:25:40but if rain falls a little more frequently,
0:25:40 > 0:25:42as it does nearby in Arizona,
0:25:42 > 0:25:45plants can have different survival strategies.
0:25:47 > 0:25:51To many of us, the very symbol of the desert is the cactus.
0:25:51 > 0:25:56But in fact, this family of fleshy-stemmed plants lives only
0:25:56 > 0:25:57in the Americas.
0:25:57 > 0:26:01There are several hundred species of them, but among the biggest
0:26:01 > 0:26:03is the saguaro.
0:26:05 > 0:26:07The saguaro has solved the problems
0:26:07 > 0:26:12of surviving in great heat and drought very successfully indeed.
0:26:13 > 0:26:15Its stem is pleated like an accordion,
0:26:15 > 0:26:18so when rain does fall, the cactus can
0:26:18 > 0:26:20expand and quickly absorb as much water
0:26:20 > 0:26:22as possible before it disappears.
0:26:22 > 0:26:26After a single storm, a saguaro can take up
0:26:26 > 0:26:29as much as a ton in a few days.
0:26:30 > 0:26:31Its leaves have become thorns,
0:26:31 > 0:26:35so reducing the surface area from which the plant might lose
0:26:35 > 0:26:36water by evaporation.
0:26:36 > 0:26:38The stem itself is green
0:26:38 > 0:26:41and has taken over the job of photosynthesis.
0:26:41 > 0:26:45The thorns protect the young plant from browsers, but they also
0:26:45 > 0:26:50break up the wind currents, so that the cactus is wrapped in still air,
0:26:50 > 0:26:53and evaporation of moisture from the stem is kept very low.
0:26:55 > 0:27:00These huge saguaro cacti can live for over 200 years and stand nearly
0:27:00 > 0:27:0450 feet high. A big one like this may weigh
0:27:04 > 0:27:08as much as eight tons, and 90% of that is water.
0:27:09 > 0:27:12If I was dying of thirst in this desert,
0:27:12 > 0:27:15I'd be tempted to cut inside that saguaro
0:27:15 > 0:27:18and raid its reservoir of water.
0:27:18 > 0:27:20But that would probably be a mistake,
0:27:20 > 0:27:23because the water in the saguaro contains a poison.
0:27:23 > 0:27:26But there are lots of desert-living plants
0:27:26 > 0:27:29which do have drinkable water within them,
0:27:29 > 0:27:32and desert-living people all over the world
0:27:32 > 0:27:35have become expert botanists, able to recognise
0:27:35 > 0:27:38from just the tiniest little leaflet or straggling stem
0:27:38 > 0:27:40where they can get a good drink.
0:27:43 > 0:27:47None are more skilled than the Bushman people of the Kalahari.
0:28:00 > 0:28:02By the end of the dry season,
0:28:02 > 0:28:05all their water holes have usually dried up.
0:28:05 > 0:28:09For liquid, they must now rely almost entirely on plants
0:28:09 > 0:28:13and their ability to recognise the right ones.
0:28:20 > 0:28:24This tuber is a kind that provides good drinking water.
0:28:34 > 0:28:37This much larger one is also full of liquid,
0:28:37 > 0:28:40but, unfortunately, it's so bitter, it's undrinkable.
0:28:40 > 0:28:43But it's worth having nonetheless.
0:28:46 > 0:28:49To extract the water, the root must be grated and pulped.
0:29:21 > 0:29:24The bigger root is grated as well.
0:29:28 > 0:29:32Drier though it is, it still contains valuable fluid.
0:29:38 > 0:29:42Since it cannot be drunk, the people use it to moisten their skin,
0:29:42 > 0:29:46and as it evaporates, it brings a delicious, refreshing coolness.
0:30:06 > 0:30:09200 miles west of the Kalahari
0:30:09 > 0:30:13lies an even hotter, drier desert, the Namib.
0:30:13 > 0:30:17Very few plants indeed can survive in these parched sands.
0:30:17 > 0:30:20Patches of grass sprouted after a rare shower
0:30:20 > 0:30:24and lived for a few weeks, but that was over four years ago
0:30:24 > 0:30:27and now only the dusty, withered stems are left.
0:30:27 > 0:30:31There is one plant that grows here, and indeed nowhere else,
0:30:31 > 0:30:34and one that is very odd indeed.
0:30:37 > 0:30:41The scientist who first described this extraordinary plant
0:30:41 > 0:30:45was an Austrian called Dr Welwitsch, who came to this part of Africa
0:30:45 > 0:30:47in the middle of the last century.
0:30:47 > 0:30:52He discovered many plants in Africa, but this perhaps is his most famous
0:30:52 > 0:30:55and the one that bears his name - Welwitschia.
0:30:55 > 0:30:58There are male plants and female plants.
0:30:58 > 0:31:02This one is a female, and these are the female's structures.
0:31:02 > 0:31:05These are young ones, which sprouted this year,
0:31:05 > 0:31:08and these are fully developed ones from last year.
0:31:08 > 0:31:12In structure, they are very like the cones of a fir tree.
0:31:15 > 0:31:19The male plant has growths rather like stamens, which produce pollen,
0:31:19 > 0:31:23so Welwitschia seems to be a kind of link between coniferous trees
0:31:23 > 0:31:26and true flowering plants.
0:31:28 > 0:31:32But the oddest thing about it, perhaps, are its leaves.
0:31:32 > 0:31:35They grow from the top of its central trunk,
0:31:35 > 0:31:38and what's more, do so extremely slowly,
0:31:38 > 0:31:43so that this leaf would have taken about 70 years to be produced.
0:31:43 > 0:31:46But if it hadn't frayed at the edges,
0:31:46 > 0:31:48it would be about 400 yards long,
0:31:48 > 0:31:53because this individual plant is thought to be about 1,500 years old.
0:31:56 > 0:32:00It's these amazing leaves that enable the plant to collect water
0:32:00 > 0:32:02in this rainless country.
0:32:02 > 0:32:05The Namib lies close to the western coast of Africa.
0:32:05 > 0:32:09At dawn, fogs regularly roll in from the Atlantic.
0:32:09 > 0:32:12As they swirl around the Welwitschia, their moisture
0:32:12 > 0:32:15condenses on the plant's huge leaves.
0:32:15 > 0:32:19Some droplets are absorbed through cracks in the leaves' skin.
0:32:21 > 0:32:25The rest of the water is channelled down to the ground,
0:32:25 > 0:32:29where it's sucked up by roots just below the surface of the sand.
0:32:31 > 0:32:33The fog also provides life-saving drinks
0:32:33 > 0:32:37for some of the desert animals. These are darkling beetles.
0:32:37 > 0:32:41On foggy mornings, they climb to the top of the dunes
0:32:41 > 0:32:46and stand in lines, head down, abdomen up, slowly marking time.
0:33:08 > 0:33:12Droplets of water from the fog collect on legs and antennae,
0:33:12 > 0:33:14and then, as the beetle lifts its feet,
0:33:14 > 0:33:16trickle down towards its mouth.
0:33:21 > 0:33:26The Namib's fogs never penetrate very far inland.
0:33:26 > 0:33:28Those deserts that lie a long way from the sea, therefore,
0:33:28 > 0:33:32can never receive moisture in such a way.
0:33:32 > 0:33:35Their water must come from the clouds.
0:33:37 > 0:33:41Often, the clouds that do build up above a desert
0:33:41 > 0:33:46sail off elsewhere without bursting, and the land remains parched.
0:33:55 > 0:33:58But when eventually rain does come,
0:33:58 > 0:34:01it's the trigger for immediate and urgent action.
0:34:03 > 0:34:06One or two drops are all that's necessary
0:34:06 > 0:34:09to activate these dead stems.
0:34:19 > 0:34:22Within half a minute, they're upright.
0:34:25 > 0:34:29Other plants begin to open their seed-heads.
0:34:38 > 0:34:39None of these plants is alive.
0:34:39 > 0:34:42All their movements are simply the result
0:34:42 > 0:34:44of the dead tissues absorbing water.
0:34:56 > 0:35:00The dead seed-heads have held the seeds securely during the drought.
0:35:02 > 0:35:03Now, since there has been rain
0:35:03 > 0:35:05and there's a chance of them germinating,
0:35:05 > 0:35:07they can be distributed.
0:35:09 > 0:35:14For some plants, the heavy raindrops are enough to dislodge the seeds.
0:35:32 > 0:35:37Others utilise the physical effects of absorbing water
0:35:37 > 0:35:38to shoot the seeds away.
0:35:48 > 0:35:52Now the seeds themselves, lying on the ground, begin to move.
0:35:58 > 0:36:01As the hairs absorb water, they swell and stiffen,
0:36:01 > 0:36:04so raising the seed into the right position
0:36:04 > 0:36:08for its first rootlets to strike straight downwards into the ground.
0:36:11 > 0:36:16But sometimes in the Arizona desert, maybe once in several years,
0:36:16 > 0:36:19there are real cloudbursts, and the desert is transformed.
0:37:10 > 0:37:13In the aftermath of the flood, new faces appear.
0:37:24 > 0:37:28A spadefoot toad. The males are the first to emerge from the soil
0:37:28 > 0:37:31where they've been buried for the past year or more.
0:37:37 > 0:37:41Hastily, they make their way down to one of the temporary pools that have
0:37:41 > 0:37:44appeared in the desert, and there they begin calling,
0:37:44 > 0:37:47summoning the females.
0:37:47 > 0:37:49There is great urgency. If they don't mate on this night,
0:37:49 > 0:37:51they may have lost their chance.
0:38:09 > 0:38:13Within 24 hours, the eggs have been laid and fertilised
0:38:13 > 0:38:15and are beginning to hatch.
0:38:25 > 0:38:29A day later, and the pool is full of tadpoles.
0:38:32 > 0:38:36Other creatures have appeared as if from nowhere.
0:38:36 > 0:38:39Fairy shrimp have hatched from tiny eggs
0:38:39 > 0:38:41blown with the dust all over the desert.
0:38:45 > 0:38:48The tadpoles are growing fast.
0:38:48 > 0:38:51These with small mouths feed on algae and bacteria,
0:38:51 > 0:38:55a diet usually abundant in these desert pools.
0:39:03 > 0:39:07But other individuals from the same batch of eggs develop bigger heads
0:39:07 > 0:39:12and more powerfully muscled jaws. They have become meat-eaters.
0:39:15 > 0:39:19Not all pools will provide enough food for them,
0:39:19 > 0:39:21but here they are fortunate.
0:39:30 > 0:39:34They even eat their vegetarian brothers.
0:39:39 > 0:39:41With such a protein-rich diet,
0:39:41 > 0:39:44they grow even faster than the algal feeders.
0:39:44 > 0:39:48Here they are the favoured few, more likely to survive if the pool
0:39:48 > 0:39:51evaporates very quickly indeed.
0:39:51 > 0:39:53They're an insurance for the continuation of the species,
0:39:53 > 0:39:57for which the payments are their vegetarian brothers.
0:39:58 > 0:40:00But now the pool is shrinking fast.
0:40:00 > 0:40:04Another couple of days and it's almost gone.
0:40:04 > 0:40:08Unless there is another shower of rain, all the tadpoles will die.
0:40:14 > 0:40:17If they do die, their bodies will not be wasted.
0:40:17 > 0:40:20They will decompose and fertilise the sand,
0:40:20 > 0:40:21so that when the next rains come
0:40:21 > 0:40:25and another pool collects in this hollow,
0:40:25 > 0:40:27the algae will grow fast and well.
0:40:29 > 0:40:33Ants are quick to attack the stricken tadpoles.
0:40:37 > 0:40:40But at the last minute, there is a reprieve. A shower of rain.
0:40:40 > 0:40:45Some tadpoles, though they still have a tail, now have legs,
0:40:45 > 0:40:49and they're able to leave the puddle just two weeks after hatching.
0:40:56 > 0:40:59Even among this tiny proportion of survivors,
0:40:59 > 0:41:01the mortality will be huge.
0:41:01 > 0:41:04But with luck, a few will join the adults as the desert dries
0:41:04 > 0:41:07and bury themselves to wait
0:41:07 > 0:41:10for the next shower of rain, many months from now.
0:41:32 > 0:41:36For several weeks after the rains, the desert blooms.
0:41:36 > 0:41:39The seeds shed by the shrivelled plants
0:41:39 > 0:41:41have sprouted and burst into flower.
0:41:41 > 0:41:44And deserts after rain all over the world,
0:41:44 > 0:41:48in Arizona and Australia, the Namib and the Sahara,
0:41:48 > 0:41:50put on one of the most dazzling displays
0:41:50 > 0:41:52of colour that you can see anywhere.
0:42:37 > 0:42:41Deserts are shaped by the sun and the wind.
0:42:41 > 0:42:43The roasting of rocks during the day,
0:42:43 > 0:42:44their chilling during the cold nights,
0:42:44 > 0:42:47eventually makes their surface crumble.
0:42:47 > 0:42:50Some of their minerals splinter and fray into dust.
0:42:50 > 0:42:52But quartz, the commonest, is very hard,
0:42:52 > 0:42:55and that remains as grains of sand.
0:42:55 > 0:42:58The wind catches them, sweeps them away, and collects them
0:42:58 > 0:43:01together as sand dunes.
0:43:28 > 0:43:31Dunes may be hundreds of feet high.
0:43:31 > 0:43:34If the wind is more or less constant,
0:43:34 > 0:43:37it blows the grains up the gently sloping side
0:43:37 > 0:43:38and over the steep front,
0:43:38 > 0:43:41so that the dune marches slowly across the desert.
0:43:55 > 0:44:00Trudging up the face of a dune like this is extremely hard work.
0:44:00 > 0:44:03The sand is so dry
0:44:03 > 0:44:08and the grains are so polished by the wind rubbing them together
0:44:08 > 0:44:11that the surface is continuously on the move,
0:44:11 > 0:44:15and it's quite impossible to get any firm foothold.
0:44:15 > 0:44:18And, of course, that problem faces not just me,
0:44:18 > 0:44:22but all the animals that live among these dunes.
0:44:22 > 0:44:27And some of them have developed some extremely ingenious solutions
0:44:27 > 0:44:28to the difficulty.
0:44:30 > 0:44:34These extraordinary tracks have been made by one of the swiftest movers
0:44:34 > 0:44:36across the dunes.
0:44:39 > 0:44:42The sidewinder, a kind of rattlesnake.
0:44:42 > 0:44:43It skims across the surface
0:44:43 > 0:44:46by throwing its body into a series of loops,
0:44:46 > 0:44:49which only touch the sand at two points.
0:44:49 > 0:44:52This not only enables it to move very fast,
0:44:52 > 0:44:55but keeps most of its body off the hot surface.
0:45:00 > 0:45:05In middle of the day, the sand is so hot that it's painful to touch.
0:45:05 > 0:45:07The Namib fringe-toed lizard
0:45:07 > 0:45:10prevents its feet from scorching by gymnastics.
0:45:27 > 0:45:30But eventually it gets so hot,
0:45:30 > 0:45:33the only thing to do is to shelter beneath the surface where
0:45:33 > 0:45:35the sand is very cool.
0:45:36 > 0:45:39But burrowing through this kind of sand also has its problems.
0:45:39 > 0:45:44An animal can't construct a tunnel like a mouse hole or a rabbit burrow
0:45:44 > 0:45:47because the sand is so smooth, it simply falls in behind it.
0:45:47 > 0:45:50So instead it has to wriggle through the sand
0:45:50 > 0:45:52almost as though it's swimming.
0:45:52 > 0:45:56And that's precisely what this little creature does.
0:45:56 > 0:45:58It may look like a worm, but in fact
0:45:58 > 0:46:01it's a lizard that has lost its legs.
0:46:01 > 0:46:06And you can see that it's a lizard when you look closely at its face.
0:46:06 > 0:46:10For it has a mouth and two eyes covered by transparent scales
0:46:10 > 0:46:13that protect them in the sand. It's a blind skink.
0:46:13 > 0:46:17It lives by hunting for insects below the sand surface,
0:46:17 > 0:46:22and when I put it down, it'll wriggle away, just like an eel.
0:46:28 > 0:46:33The most extremely specialised of these hunters in the dunes
0:46:33 > 0:46:37is not perhaps a reptile, but a mammal.
0:46:37 > 0:46:38It's very rarely seen,
0:46:38 > 0:46:41and your best chance of finding it is at night.
0:46:44 > 0:46:49These are its tracks, and that depression a place
0:46:49 > 0:46:51where it caught something.
0:46:59 > 0:47:02This is where it has burrowed again, and where, with luck,
0:47:02 > 0:47:05and if I dig very fast, I might catch it.
0:47:14 > 0:47:16Here it is, a golden mole.
0:47:18 > 0:47:22This one is a baby, but like its parents, it's totally blind.
0:47:22 > 0:47:26Eyes are of no use beneath the sand. Nor are ears,
0:47:26 > 0:47:28and it hasn't got those either.
0:47:28 > 0:47:30Its head ends in a kind of leathery wedge
0:47:30 > 0:47:34with which it pushes its way through the sand, or alternatively,
0:47:34 > 0:47:36through my fingers.
0:47:50 > 0:47:52Golden moles will eat quite large creatures -
0:47:52 > 0:47:55a blind skink, if it encounters one,
0:47:55 > 0:47:58or other creatures that might be wandering unsuspectingly
0:47:58 > 0:48:00across the surface.
0:48:13 > 0:48:15A cricket would do nicely.
0:49:04 > 0:49:09The great sandy deserts of the world in Arabia, central Australia and
0:49:09 > 0:49:13the Sahara have repelled even the hardiest of human travellers.
0:49:13 > 0:49:17Few people have managed to survive in them for long totally unaided.
0:49:17 > 0:49:22But some manage to make regular journeys through these wildernesses.
0:49:27 > 0:49:29These are the Tuareg.
0:49:29 > 0:49:32They travel from one side of the Sahara to the other,
0:49:32 > 0:49:34carrying great cakes of salt,
0:49:34 > 0:49:37which they trade for cloth and grain and dates.
0:49:51 > 0:49:54But even the Tuareg can only make these journeys
0:49:54 > 0:49:59with the help of an animal desert specialist - the camel.
0:49:59 > 0:50:01They have to take all the food
0:50:01 > 0:50:04that they and their camels will need with them.
0:50:07 > 0:50:11Water is carried in skins slung beneath the camels' bellies
0:50:11 > 0:50:15to minimise evaporation and keep it as cool as possible.
0:50:16 > 0:50:20The camel is marvellously adapted to life in the desert.
0:50:20 > 0:50:24Its toes are reduced to two, but connected by skin,
0:50:24 > 0:50:27so that they splay out on the sand and don't sink deeply into it.
0:50:34 > 0:50:36Their nostrils are closable,
0:50:36 > 0:50:39so they can shut out sand grains during a sandstorm.
0:50:49 > 0:50:51The hair on their body is restricted to the top,
0:50:51 > 0:50:54where it shields against the sun.
0:50:54 > 0:50:57Elsewhere, for coolness, their skin is virtually naked.
0:50:57 > 0:50:59Their hump is full of fat,
0:50:59 > 0:51:02which in emergencies can be converted to water.
0:51:02 > 0:51:05But the process wastes a lot of the fat's calories
0:51:05 > 0:51:09and is only used when the camel hasn't drunk for a long time.
0:51:09 > 0:51:12It can in fact live without liquid water
0:51:12 > 0:51:16for four times as long as a donkey and ten times as long as a man.
0:51:16 > 0:51:20But eventually, even a camel has to drink.
0:51:28 > 0:51:31At one or two places in the Sahara, water can be reached by digging
0:51:31 > 0:51:33deep into the ground.
0:51:33 > 0:51:36Here, camels can at last refill their stomachs,
0:51:36 > 0:51:39and they take a lot of filling.
0:52:10 > 0:52:13But if the Tuareg can't cross the Sahara without the camel,
0:52:13 > 0:52:16the camel can't do so without the Tuareg,
0:52:16 > 0:52:19for only men can dig wells for the essential water.
0:52:23 > 0:52:26Spring water is the key which unlocks abundant fertility.
0:52:26 > 0:52:28At Saharan oases like this one,
0:52:28 > 0:52:33all kinds of crops can be produced from the sand if it's watered -
0:52:33 > 0:52:35dates and vegetables and fruit.
0:52:35 > 0:52:39Insects whizz and buzz over the gurgling irrigation channels
0:52:39 > 0:52:42and birds sing in the palm trees.
0:52:44 > 0:52:48But these small islands of life are under constant threat.
0:52:48 > 0:52:52If the wind veers and blows steadily from another direction,
0:52:52 > 0:52:53nothing can stop the sand.
0:53:04 > 0:53:09Eventually, the advancing dunes may well overwhelm this oasis,
0:53:09 > 0:53:13and then this small world that's been brought into existence
0:53:13 > 0:53:16in the desert by the presence of water
0:53:16 > 0:53:17will be extinguished.
0:53:17 > 0:53:21The force that drives the dune, of course, is the wind,
0:53:21 > 0:53:24and the wind, too, has its own world of living organisms.
0:53:24 > 0:53:27Many of the spiders and beetles
0:53:27 > 0:53:31and other insects that live in the oasis arrived by air.
0:53:31 > 0:53:35And many of the plants, too, coming as windblown seeds
0:53:35 > 0:53:37or carried on the feet of birds.
0:53:37 > 0:53:40And that world, the world of the wind and the sky,
0:53:40 > 0:53:42we'll be exploring next time.