Category
📺
TVTranscript
00:30An eye from another world.
00:47A smell detector investigating the path ahead.
00:53We don't often see a snail that way.
00:57And that's because we've only recently had the tiny lenses and electronic cameras that
01:03we need to explore this miniature world.
01:10But when we meet its inhabitants face-to-face, we suddenly realise that their behaviour can
01:16be just as meaningful to us as the behaviour of many animals more our own size.
01:23Look at this, for example.
01:27It's an earwig, yes, but it's also a female and a mother.
01:33And like so many mothers, she's guarding her young.
01:41These two ants are not quite sure whether they like one another.
01:45Stroking Antennae is the equivalent of a cautious chat over the garden fence.
01:53When big animals go courting, they show off, and so do damsel flies.
02:03Courtship signals for a male wolf spider are rather more frantic, because if his female
02:08doesn't understand why he's approaching her, she'll eat him.
02:15This ant is a farmer, and these aphids, the cows, which it milks for a drink of honeydew
02:22every day.
02:26Other ants are eternally on the march.
02:31Powerfully armed soldiers guard the flanks of their column as they travel, protecting
02:35the workers who are carrying their helpless young.
02:44When it comes to craftsmanship, few can beat this wasp, using mud to construct an elegant
02:50jar in which to store her eggs.
02:55Mud is also used by termites.
02:57They build tower blocks that, in proportion to their size, are taller than New York skyscrapers.
03:04These two worlds, ours and theirs, influence one another to an extraordinary degree.
03:10If we and the rest of the backbone animals were to disappear overnight, the rest of the
03:16world would get on pretty well.
03:19But if they were to disappear, the land's ecosystems would collapse.
03:26For the fact is, they were the pioneers, the first animals of any kind to colonize the
03:32lands of the earth.
03:35To tell their story, we must go back to a time when the world was a very different place.
04:00Some 400 million years ago, the lands of planet earth were totally without life.
04:07They were bare, naked rock, roasted by sun during the day, freezing cold at night, and
04:15swept by terrible storms.
04:18But in the waters of the world, conditions were much more stable.
04:24Life had begun there some 2,000 million years earlier still.
04:29For a long time, it remained microscopic.
04:32But eventually, larger animals appeared, jellyfish and corals, starfish and snails, and animals
04:39with segmented bodies.
04:44All needed food.
04:46Many would have eaten unguarded eggs, given the chance.
04:51And then, around 400 million years ago, some enterprising creatures found it safer to lay
04:57their eggs out of the sea, up on a beach.
05:02They still do.
05:16Every spring, on a few special nights along the Atlantic coast of North America, thousands
05:22of horseshoe crabs emerge from the sea.
05:32And here, in the wet sand, they spawn.
05:36They may only stay for a few minutes or hours, but animals like these may well have been
05:41the first of any kind to leave the sea and venture onto land.
06:03Although these creatures spend virtually all their lives at sea, they can survive surprisingly
06:09well on land.
06:11It's almost as if they were pre-adapted.
06:14They have shells, external skeletons, and that means that their legs are rigid and jointed.
06:23And at the back, they have a series of plates called book lungs, which extract oxygen from
06:31seawater, but can also do the same thing if they're kept reasonably moist from the air.
06:37So creatures like this can, in fact, spend about a week on land.
06:43And it only requires minimal modifications to enable them to live up there permanently.
06:54It was difficult to abandon the sea altogether until the land became green, but eventually
07:01it did.
07:02Natural plants, algae and then mosses and liverworts, began to advance over the mud
07:07and rock to clothe the earth.
07:10And into these first green tangles came animals looking for food.
07:28Some had armor, for that in the sea had protected them from their enemies.
07:34Now it would help them conserve moisture.
07:52They were the ancestors of today's millipedes.
08:00Small holes had developed along the underside of their bodies that led to internal tubes
08:06with which they could absorb oxygen from the air.
08:13Their rigid, jointed legs, however, were largely unchanged and worked very well on land, even
08:18without the support of water.
08:27Battering ram heads enabled them to bulldoze their way through the vegetation to collect
08:31the rotting plants on which they fed.
08:42They grew big, increasing the number of segments in their bodies.
08:46Some had over 300, each with two pairs of legs.
08:53Some that didn't curl up reinforced their armor with plates along their backs.
09:11Crustaceans like shrimps came too.
09:16They were the ancestors of woodlice.
09:33So today there is a huge and varied population of animals living on the land with bodies
09:39that are a little different from those of their ancestors who lived in the sea so long
09:43ago.
09:47And they are extraordinarily successful.
09:50Some are the most numerous of all land-living species, but we seldom see them.
09:57This pin will give you an idea of why.
10:00They're tiny.
10:04This minute little creature is a springtail.
10:08It's less than half a millimeter long, the size of a full stop.
10:14In one square meter of soil, there may be over 10,000 of them.
10:21Drying out is a very real danger for them, and some waterproof themselves regularly with
10:27a droplet of special grooming fluid.
10:30You might even say that they have turned bathing into an art form.
10:42They even have two inflatable tubes that enable them to get to those hard-to-reach places.
10:52To help them get around through the leaf litter, these springtails, as their name suggests,
10:57have a rather novel way of jumping.
11:07They have a tiny two-pronged lever beneath their abdomen.
11:11One small flick from it can catapult them six inches, some 15 centimeters, into the
11:17air.
11:23That's the equivalent of a human being jumping over the Eiffel Tower.
11:31And if they happen to land upside down, well, they have a special way of righting themselves.
11:39They use their grooming fluid dispenser to stick onto the ground so that they can pull
11:44themselves back onto their feet.
11:53So the foundations were laid for the ecosystems that now flourish on Earth, and on which we
12:00ourselves depend.
12:06It has to be said, however, that sometimes some of us regard a few of these pioneers
12:12more as our enemies than our friends.
12:19Many of the mollusks in the sea develop shells to protect themselves from predators.
12:26But on land, those shells serve just as well to keep the occupant nice and moist.
12:32So without any major change to their anatomy, mollusks were able to creep up out of the
12:38water and graze in the forests of algae and mosses that were then spreading over the land.
12:45And given the right conditions, they still do.
13:00With rain and the coming of night, a secret army comes out of hiding.
13:13These are the conditions they like best, dark and, best of all, wet.
13:39Walking along a carpet of slime works just as well on land as it does underwater.
13:46And a rasping tongue scrapes algae off rocks wherever they are.
13:57In times of drought, snails may be unable to move around for months on end.
14:02So when conditions are right, they eagerly set off to find food.
14:11Their upper pair of tentacles carry those eyes with which they look around.
14:17The lower ones smell what's beneath.
14:30And they breathe by means of a small pouch on the right-hand side of their body just
14:35within the shell, which, because it's permanently moist, is able to absorb oxygen.
14:49This is what they're seeking, a succulent green leaf, no time to be lost.
15:05Dawn will bring a change in conditions.
15:10So they have to return to their shelters and clamp down their shells once more so that
15:14they retain their moisture.
15:23The ancient forests were colonised by all kinds of plant eaters long before there were
15:28any frogs or lizards, birds or insect-eating mammals.
15:33But there were, nonetheless, hunters prowling through the vegetation.
15:40This was one of the first.
15:44Snails very like it have been found in rocks that are 540 million years old.
15:49This is a velvet worm.
15:53It too has scarcely changed since it lived in the sea.
15:57And today it's only found in wet, humid forests.
16:02It usually hunts at night, but infrared cameras can reveal it in action.
16:09Its soft, slumpy legs enable it to move in total silence, and it finds its way with long,
16:15sensitive feelers.
16:18It's a master of stealth.
16:22This cricket has huge eyes, but it's difficult to see what's going on around it.
16:34Although the velvet worm has fangs, it will attack its prey when it finds it with a very
16:39special weapon.
17:05Anxiously, the cricket probes around in the darkness with its long antennae.
17:15The velvet worm will only know if it's found its prey when it touches it, so when it does,
17:21it has to react immediately.
17:27There!
17:32And the cricket is trapped.
17:36A slow-motion camera shows the remarkable way in which the velvet worm attacks.
17:42Two nozzles beneath its feelers squirt twin streams of glue.
17:55The more the cricket struggles, the more it becomes entangled.
18:03With the prey immobilized, the velvet worm reclaims its glue by eating it, and then it
18:09starts on the cricket.
18:21There were other hunters, too, in the ancient forests, relatives of the horseshoe crabs,
18:26and they were even more formidable.
18:35This is a whip spider.
18:38Like its ancestors, it has a hard external skeleton.
18:44Two of its limbs have been turned into highly mobile, sensitive feelers.
18:50It uses them to probe around delicately both in front and behind.
18:54Any prey within a foot of it will be immediately detected.
19:04It's extremely territorial, and it has no hesitation in attacking one of its own kind.
19:15The width of its claws is a good indicator of strength, and a smaller animal will quickly
19:20back down.
19:23But these two are equally matched, and they will fight.
19:46The loser retreats.
19:51But even whip spiders were not the most formidable hunters in these forests.
19:56There were others with an even more venomous weaponry.
20:05This centipede has powerful jaws, poison fangs, and is very, very fast.
20:13It's a very good hunter, but it's only half as long as my little finger.
20:20There are centipedes in the world, however, that are as big as my forearm.
20:27This is one of these alarming giants.
20:30It's over 13 inches, 35 centimeters long, and with the muscular strength of a small
20:36snake, and the poison in its black-tipped fangs is lethal.
20:44It hunts in the dark, bat-haunted caves of Venezuela.
20:50Like the whip spider and the velvet worm, it uses its antennae to feel for its victims.
21:02The beetles that swarm on the rocky floor of the cave are of no interest to it.
21:07It's after bigger prey.
21:11And it knows it can find that by climbing.
21:20Its many legs give it a secure hold on the vertical rocks.
21:24It's heading for the ceiling.
21:38Now in the darkness, it can sense bats flying past it.
21:46Holding on with its hind legs, it reaches out into their flight path, and almost immediately,
21:52it has one.
22:00An injection of venom from its fangs kills the bat almost instantaneously.
22:08It will take it an hour or so, but it will eat all the bat's flesh.
22:21So all these animals, having left the sea, solved the problems of moving around and breathing
22:26air in their own differing ways.
22:29But there was another difficulty, mating.
22:33In the sea, animals need only release their eggs and sperm, and the water mix the two
22:37together.
22:38On dry land, that couldn't happen, even for the most moisture-loving of creatures.
22:45An individual slug carries both male and female organs, but even then, that was of no help.
22:52Each had to both give and receive.
22:56Somehow or other, pairs of individuals had to get together, and the ways they have evolved
23:03in which to do so are quite extraordinary.
23:06Indeed, some of them are almost beyond imagining.
23:12The leopard slug, you might think, has the simplest of habits.
23:16Maybe, but not when it comes to mating.
23:21When an individual is looking for a partner, it gives its trail of slime a special taste
23:26that advertises the fact.
23:28Another, if it feels the same way, will detect the invitation and start to follow.
23:35The pursuer, to confirm that it's there and ready to mate, gives the pursued a nibble.
23:47The leader heads upwards.
23:49An overhang is what's needed.
24:05The underside of a branch will do very nicely.
24:09The two start to circle one another more and more closely until they entwine.
24:18For an hour or so, they continue to wind themselves around one another.
24:35Then suddenly, the pair release their hold on the branch and start to slide downwards
24:41on a rope of mucus.
24:47Now, in midair, they move to the next stage in their pairing.
25:05Each averts its male organ from just behind its head.
25:19These grow longer and longer.
25:25Then they, too, begin to entwine.
25:46They fan out to form a translucent, flower-like globe.
26:10And now, at last, sperm passes from one slug to the other.
26:17The transfer is complete.
26:21Each has been fertilized.
26:28Finally, their strange, poetic relationship comes to an end with a bump.
26:43A millipede, unlike a slug, is either a male or a female.
26:49In southern Africa, where there are many different species, both sexes spend the winter in hibernation,
26:55curled up in the leaf litter or beneath the bark.
26:59As the temperatures rise with the coming of spring, they all unwind themselves and set
27:04off to look for a mate.
27:06Finding one in the tangled undergrowth is not easy.
27:10But this male forest millipede knows that he can increase his chances if he heads upwards
27:20into the trees.
27:27Leaving the safety of the undergrowth may seem a risky thing to do, but these millipedes
27:31secrete a poison from pores in their armor, and their conspicuous red and black colors
27:36warn predators to leave them alone.
27:39They emerge in thousands.
27:44Surprisingly, perhaps, a male, when he does find a female, is not met with a friendly
27:51greeting.
27:52Quite the reverse.
27:53She coils up.
27:59This is her way of sorting out the men from the boys.
28:03Only the strongest and fittest male will have the strength to force her coils apart.
28:09To help him do so, he has white suction pads on the bottom of his feet, which give him
28:14a good grip.
28:18Eventually she relaxes and he lifts her up so he can extend two specially modified legs
28:24with which he inseminates her.
28:32Once inserted, these legs swell so that the partners become fastened together.
28:38And that's important, because it will take him a couple of hours to transfer his sperm.
28:45But there are lots of males around, and before long, another one turns up.
28:54The new arrival checks out the pair with his antennae.
28:58If they're not tightly bound together, he may have a chance of taking over.
29:06He pushes between them, levering them apart.
29:10Gradually, he manages to unzip their legs.
29:18The first male's white mating legs are dragged out.
29:23He's been defeated.
29:24It will be the second, stronger male, who fertilises her eggs.
29:32So mating on land isn't as random as it had been for so many in the sea.
29:37Now it's selective.
29:39But brute force isn't the only basis on which to select.
29:44A female's springtail is bigger than a male, and she prefers a partner who can give her
29:49a sustained head-to-head push.
29:54Other males are eager to try their luck, but butting her sides won't get them anywhere.
30:14She seems unimpressed by any of them, but one is determined to stay as her dance partner.
30:33She simply can't get rid of him.
30:42He confidently signals victory with a couple of fancy twirls.
30:50Then he deposits a droplet of sperm onto the leaf, and she graciously takes it on board.
31:06One group of colonists were of particular importance, for they changed the nature of
31:11the soil, and thus made it possible for new kinds of plants and animals to evolve.
31:17They outweigh all other animals in any given area of a forest.
31:21A single hectare may be home to eight million of them.
31:26They spend nearly all their time below ground.
31:31Worms.
31:33They eat their way through the earth, extracting edible vegetable material and making it suitable
31:39for plants.
31:42And at night, they come to the surface and collect dead leaves.
31:59They also take the opportunity to call on their neighbours, poking their heads into
32:04next-door burrows.
32:09They're looking for partners.
32:11Like slugs, they're hermaphrodite, each individual both male and female.
32:17They mate by lying alongside one another.
32:21Two narrow grooves form between their two bodies.
32:24These are the conduits that carry sperm from one partner to the other.
32:32Their bodies slowly pulse as sperm travels along the space between them.
32:37But the process is a long one, and it may be three hours or so before they separate,
32:42each carrying the other's sperm.
32:52Like so many of the inhabitants of the undergrowth, earthworms can only live in a moist environment,
32:57but they are found in soils of every continent except Antarctica.
33:03This small valley in southern Australia is home to one of the rarest and the most extraordinary
33:10of all earthworms, and I know they're around because I can hear them.
33:21Those gurgling noises, believe it or not, are being made by giant earthworms as they
33:26squelch along their water-filled burrows.
33:36The vibrations of my footsteps are enough to stir them into activity.
33:45They never come to the surface, but in places where there's been a small landslip, you can
33:49sometimes find their burrows.
33:59They are over an inch in diameter, and in them, if you're very lucky, you may occasionally
34:05find one of these.
34:08This is one of their egg cocoons, and it's enormous.
34:14If I hold it up against the light, I can see the young worm inside wriggling.
34:21It'll take a year for this to develop, and when the young one finally does break free,
34:28it's already 20 centimetres long.
34:34Huge.
34:36It will take a further five years to reach full size and become this remarkable creature.
34:43So the question is, how long is a giant earthworm?
34:47Well, it's not an easy question to answer.
34:50The fact of the matter is, they're rather delicate creatures, and they break.
34:55If I was so unfeeling as to try and stretch it, well, I guess it might stretch to a couple
35:03of metres, almost six feet long.
35:10How long they live?
35:11Well, some say up to 20 years, but we really don't know, and we certainly don't know how
35:17they manage to mate deep underground as they sculpt their way through their lonely tunnels.
35:28The land may have been a safe place for eggs when horseshoe crabs first laid theirs up
35:33on the beach, but as new kinds of animals appeared, so it became increasingly important
35:39for animals to protect their eggs.
35:41Most creatures just hid them, but a few now actively defend them.
35:48The builder of this circular mud wall in the Central American rainforest is one.
35:53During the day, he conceals himself, but when night comes, he emerges to inspect his collection
36:00of eggs.
36:04His body is smaller than a grain of wheat.
36:08He's a relative of the spiders, a harvestman.
36:13His eggs, up to a hundred of them, are half buried in the floor of his nest, and he regularly
36:19inspects each one of them.
36:27If it has a fungus on it, he carefully cleans it before putting it back into its moist bed.
36:37He also continually repairs and improves his nest, for females will only call on those
36:43who have well-built and well-kept ones.
36:49Some males, however, follow a different policy.
36:52They don't bother to build a nest for themselves.
36:55They try to take over an existing one.
37:00A nest holder has to leave sometimes to feed, and that gives an intruder his chance.
37:14The owner is back almost immediately, and they fight, trying to bite one another in
37:20the weakest point of their armor, the joints of the legs.
37:41The intruder retreats, and the nest owner checks his eggs.
37:48No damage done.
37:58And then another, more welcome visitor arrives.
38:07This is a female.
38:09She's bigger than he is, and she's touring all the nests in the neighborhood to choose
38:14the one where her eggs will be best cared for.
38:22She seems to approve of the standard of his housekeeping.
38:27So now, face to face through the tangle of legs, she mates with him.
38:35He has a rod with which he injects his sperm.
38:45He withdraws, and she's been fertilized.
38:56Half an hour later, she lowers her white tubular ovipositor, feeling for a suitable place for
39:01her egg.
39:06She thrusts the egg into the floor of the nest, and then covers it with a thin blanket
39:10of mud.
39:30She leaves.
39:31He will now tend and guard the egg with the rest of his collection for the month that
39:36it will take to hatch.
39:41His nest is clearly one of the best in the neighborhood, for throughout the night a succession
39:46of females call on him.
39:49But not all have come to lay.
39:53Life is not that simple.
39:56This one starts, as usual, with a routine inspection, and then, without more ado, she
40:02mates.
40:07He waits for her to produce a new egg, but nothing appears.
40:20He constantly checks the nest floor with his feelers, but there are no signs of a new
40:25egg.
40:32And then, she grabs one from his collection.
40:35She wants to eat one.
40:39She grabs again.
40:41He bites her leg joints and tries to pull her away.
40:51She's had enough, and he has rescued his egg.
40:58He checks it over, cleans it with great care, and then takes it away to rebury it.
41:09A month after the eggs were laid, his young begin to emerge.
41:28The skins from which they hatched provide them with their first meal.
41:34He will now guard his young for a couple of days until they leave the nest.
41:42Excellently adapted though harvestmen are to a life on land, they cannot survive for
41:47very long away from this damp undergrowth.
41:53In fact, most of the direct descendants of those early colonists that came from the sea
41:58are still trapped in a world of moisture.
42:10Those with no external skeletons are always in imminent danger of death by drought.
42:21Even those with exoskeletons are not safe, for most have armour that is not totally watertight
42:27and will eventually dry out and die if they leave the dank shelter of the undergrowth.
42:42But beyond the reach of the forests, in the centre of continents where little or no rain
42:47falls there is a very different territory, empty and hostile.
42:59Here there is little shelter from the scorching sun.
43:04Temperatures rise above 70 degrees centigrade and there may be no rain whatsoever for years
43:10on end.
43:20Deserts like this one in the south west of the United States represented the ultimate
43:26challenge for those ancient creatures whose ancestors first left the sea.
43:32Here there is virtually no water at all and yet those early creatures, the very first
43:40to walk on land, reached even here and they're still around.
43:47In order to survive the ferocious heat of the day they take refuge in little burrows
43:54like this which go quite a long way down into the ground.
43:59And I can use this special optical probe to see whether anyone's at home.
44:21And there it is, it's a scorpion.
44:25They won't come out for the rest of the day but at night when it gets cool scorpions
44:32all over the desert will be emerging.
44:36And then we have a very special way of finding them.
44:42In ultraviolet light scorpions are magically transformed, they glow with fluorescence.
44:50So with an ultraviolet torch you can get a better idea of just how abundant scorpions
44:55actually are even in this arid wilderness.
45:03That's because they have managed to develop external skeletons that are virtually watertight.
45:11They also have powerful stings and pincers so getting together to mate could be dangerous.
45:18A male looking for a female must be careful.
45:26She is powerful enough to kill and eat him.
45:34So he begins to dance.
45:45Is she impressed?
45:57Apparently so.
45:58And his solo becomes a pas de deux.
46:17But stings are still held high, ready to strike.
46:43He tries to sting him.
46:49His response is to give her a dose of her own medicine with a quick jab.
46:59But it's so slight it merely makes her a little drowsy.
47:05At last she seems more amenable.
47:08He pulls her to a part of the dance ground that is smooth and level.
47:13He has extruded a small packet of sperm on a stalk glued to the ground.
47:18He manoeuvres her so that as she dances she goes over the stalk and takes the sperm packet
47:24up into her body.
47:29The nuptial dance is over.
47:36Her fertilised eggs stay within a special chamber in her body for more than a year while
47:41they slowly develop.
47:44And then in her burrow deep underground she gives birth.
47:52She has produced up to 50 young ones.
47:55They cling tightly to her back for a few weeks after birth, each sustained by a small blob
48:00of yolk in its stomach.
48:05And then at last they're all ready to venture into the open desert for themselves.
48:17By colonising this, the most hostile of environments, the first animals to walk on land finally
48:24broke their link with open water.
48:27And they did that about 300 million years ago, at a time when the animals with backbones,
48:34including our own ancestors, were still swimming in the seas.