0:01:26 > 0:01:31The volcanoes of today are mere feeble flickerings
0:01:31 > 0:01:36compared with those that dominated the world at the beginning of its history.
0:01:36 > 0:01:40Then, enormous sheets of lava welled out of the craters,
0:01:40 > 0:01:44titanic explosions blew whole mountains into fragments
0:01:44 > 0:01:49and scattered them as dust and ash over the surface of the land.
0:01:49 > 0:01:53That sort of activity continued for millions and millions of years,
0:01:53 > 0:01:58and I'm talking about a period that was 4,500 million years ago.
0:02:00 > 0:02:04The forces of erosion, frost and rain, snow and ice,
0:02:04 > 0:02:07shattered the volcanic rocks into fragments.
0:02:07 > 0:02:10Rivers carried them down piecemeal to the edges of the continents
0:02:10 > 0:02:14and deposited them as sands and gravels and muds.
0:02:17 > 0:02:20As the continents drifted over the globe and collided,
0:02:20 > 0:02:25new mountain ranges were built up and, in their turn, worn down.
0:02:31 > 0:02:36And throughout this immensity of time, the land remained sterile.
0:02:36 > 0:02:39Nowhere was there even the smallest of animals
0:02:39 > 0:02:42or the tiniest speck of green.
0:02:43 > 0:02:48If you condense the whole history of life,
0:02:48 > 0:02:53from its very beginnings until the present moment, into a year,
0:02:53 > 0:02:56then it wasn't until about the end of September
0:02:56 > 0:03:01that the first creatures of any size, jellyfish and so on,
0:03:01 > 0:03:02appeared in the sea.
0:03:02 > 0:03:08And it wasn't until the beginning of November that the first life,
0:03:08 > 0:03:12a few patches of green, appeared on land.
0:03:12 > 0:03:16Maybe at the edge of water, like this.
0:03:17 > 0:03:20These first plants were simple algae
0:03:20 > 0:03:23that had developed cell walls thick enough to enable them to survive
0:03:23 > 0:03:26on the moist boulders and gravels.
0:03:26 > 0:03:31Slowly, they spread over the lake beaches and sandspits,
0:03:31 > 0:03:33pioneers of the great revolution
0:03:33 > 0:03:36that was to lead to the greening of the earth.
0:03:48 > 0:03:52Moving out of water for the plants had presented a number of problems.
0:03:52 > 0:03:55One of the most serious was the question of support.
0:03:55 > 0:04:01In water, algae like this can grow into long strands,
0:04:01 > 0:04:03but robbed of the support of water,
0:04:03 > 0:04:08none has a sufficiently rigid stem to allow it to grow upright.
0:04:12 > 0:04:15So the first land plants had to remain lowly,
0:04:15 > 0:04:20forming flat skins like liverworts or cushions like mosses.
0:04:20 > 0:04:25All of them lived in wet, moist places and for a very good reason.
0:04:26 > 0:04:29Their ancestors, the algae, had reproduced in two ways,
0:04:29 > 0:04:31by budding and sexually,
0:04:31 > 0:04:35and the sexual method involved the sex cells actually swimming
0:04:35 > 0:04:40through water in order to find one another and fuse.
0:04:40 > 0:04:44Well, mosses retain very much the same sort of method.
0:04:44 > 0:04:48And it's this that keeps them tied to water.
0:04:48 > 0:04:51So they can only live in places where at the very least,
0:04:51 > 0:04:53it's wet during some time of the year,
0:04:53 > 0:04:56so that sexual reproduction can take place.
0:04:56 > 0:05:01Of course, in places like this, they are literally in their element.
0:05:17 > 0:05:23Mosses and liverworts like this both produce two kinds of sex cells.
0:05:25 > 0:05:29These outgrowths on the liverwort, only a few centimetres high,
0:05:29 > 0:05:34develop tiny mobile sperms which actively swim.
0:05:34 > 0:05:38These different growths contain larger static sex cells, the eggs.
0:05:39 > 0:05:43Under the microscope, you can see the eggs at the base of tiny tubules
0:05:43 > 0:05:46surrounded by a protective sheath of smaller cells.
0:05:50 > 0:05:54When the outgrowths are ripe and conditions sufficiently wet,
0:05:54 > 0:05:56fertilisation begins.
0:05:59 > 0:06:01The wriggling sperm are released
0:06:01 > 0:06:05and swim in the film of water that covers the plant.
0:06:18 > 0:06:21The sperm appears as a milky fluid.
0:06:36 > 0:06:37At the same time,
0:06:37 > 0:06:41the female part of the liverwort that bears the egg cells
0:06:41 > 0:06:45releases a special chemical that attracts the sperms.
0:06:47 > 0:06:50Eventually, they reach the female organs.
0:06:50 > 0:06:53Fertilisation occurs and the eggs develop,
0:06:53 > 0:06:57repeatedly dividing to produce a capsule full of microscopic grains -
0:06:57 > 0:06:58spores.
0:06:58 > 0:07:03When they are ripe and the weather is dry, the capsules burst.
0:07:10 > 0:07:14Each minute spore is capable of growing into a new liverwort plant.
0:07:19 > 0:07:22Mosses also reproduce by these two alternating methods.
0:07:22 > 0:07:26The sexual stage provides the variety of offspring necessary
0:07:26 > 0:07:28for continued evolution.
0:07:28 > 0:07:31The asexual spores can be carried on the wind
0:07:31 > 0:07:34to distribute the plant over great distances.
0:07:34 > 0:07:37The spore capsules of mosses are very varied in shape,
0:07:37 > 0:07:39and they have the most ingenious ways
0:07:39 > 0:07:42of making sure that they only release their contents
0:07:42 > 0:07:45when the weather is suitably warm and dry.
0:08:01 > 0:08:03Many species have detachable caps
0:08:03 > 0:08:06which are blown off before the spores can be released.
0:08:07 > 0:08:11And beneath, a perforated lid, like a pepper pot.
0:08:11 > 0:08:16And the wind will now carry the microscopic spores for miles.
0:08:29 > 0:08:31With such mechanisms as these,
0:08:31 > 0:08:35the first plants colonised the moist places of the world,
0:08:35 > 0:08:38and green carpets bordered the lakes and rivers.
0:08:43 > 0:08:48Into these miniature jungles came the first land animals.
0:08:53 > 0:08:56Millipedes, then as now, were vegetarians,
0:08:56 > 0:09:00and they must have found plenty to eat among the mosses and liverworts.
0:09:00 > 0:09:03The biggest of them today are only a few inches long,
0:09:03 > 0:09:08but many ancient forms that pioneered life on land grew very much larger.
0:09:08 > 0:09:11One, indeed, was as long as a cow.
0:09:21 > 0:09:24Millipedes were descended from sea-living creatures
0:09:24 > 0:09:27very distantly related to crustaceans such as shrimps.
0:09:27 > 0:09:32From them, they inherited segmented bodies and an external skeleton
0:09:32 > 0:09:34which gave them the necessary support
0:09:34 > 0:09:37so they could move just as well in air, on land,
0:09:37 > 0:09:39as their ancestors had done in the sea.
0:09:42 > 0:09:45But breathing was another matter.
0:09:45 > 0:09:48Their ancestors had extracted dissolved oxygen from water
0:09:48 > 0:09:51with feathery gills alongside each leg.
0:09:51 > 0:09:53But such things wouldn't work in air.
0:09:53 > 0:09:57Instead, the first millipedes developed a system of branching tubes
0:09:57 > 0:10:02within each segment, along which air diffuses to all parts of the body
0:10:02 > 0:10:05so that the tissues can absorb oxygen directly.
0:10:05 > 0:10:07These tubes open to the outside
0:10:07 > 0:10:11through a tiny pore on the side of each segment.
0:10:18 > 0:10:24But the amiable browsing millipedes didn't have the land to themselves for long.
0:10:24 > 0:10:26Very soon after they had colonised it,
0:10:26 > 0:10:29hunters came up from the sea to prey on them.
0:10:32 > 0:10:35These hunters are still today active, mostly at night.
0:10:36 > 0:10:38The scorpions.
0:10:48 > 0:10:52They had evolved from a different group of segmented sea creatures,
0:10:52 > 0:10:56but, again, they had an external skeleton which worked very effectively on land.
0:10:56 > 0:11:01With powerful nipping claws and poisoned stings on their tails,
0:11:01 > 0:11:04scorpions are well-armed and ferocious,
0:11:04 > 0:11:07actively seeking out their prey wherever it may be hiding.
0:11:15 > 0:11:18Another closely related group became mainly day hunters
0:11:18 > 0:11:20in the miniature forests.
0:11:20 > 0:11:22The spiders.
0:11:26 > 0:11:30Although their sea-living ancestors had many pairs of legs,
0:11:30 > 0:11:35spiders and scorpions have only four pairs. Better for speed.
0:11:35 > 0:11:38And spiders have lost most signs of division in their bodies,
0:11:38 > 0:11:42except for some very primitive ones that live in Southeast Asia.
0:11:42 > 0:11:47Their abdomens show the last relics of that ancestral segmentation.
0:11:49 > 0:11:52Early in their history, the spiders developed glands in the abdomen
0:11:52 > 0:11:54with which they produce silk.
0:11:59 > 0:12:03They use it in hunting, sometimes laying long trip lines,
0:12:03 > 0:12:05sometimes constructing dense sheets.
0:12:05 > 0:12:10And they manipulate the threads with modified limbs, the spinnerets.
0:13:04 > 0:13:08By the time it's finished, any small creature trying to make its way here
0:13:08 > 0:13:11will blunder into a silken trap.
0:13:11 > 0:13:15And while it's still entangled, the spider will pounce on it.
0:13:21 > 0:13:26Reproduction for all these land creatures presented new problems.
0:13:26 > 0:13:30Without water to transport sperm to egg, there was nothing for it -
0:13:30 > 0:13:32male and female had to get together.
0:13:34 > 0:13:37For the millipede, this presented no real danger.
0:13:37 > 0:13:39They are vegetarians,
0:13:39 > 0:13:43so when individuals meet, neither risks being eaten by the other.
0:13:43 > 0:13:47Their difficulties are entirely ones of manipulation.
0:13:54 > 0:13:57The sex glands of both male and female
0:13:57 > 0:14:00are at the base of the second pair of legs.
0:14:00 > 0:14:04The male has reached forward with his seventh pair of legs
0:14:04 > 0:14:08and collected from his second segment a little packet of sperm.
0:14:08 > 0:14:11Now, if only he can get it into exactly the right position
0:14:11 > 0:14:15alongside the female's pouch in HER second segment,
0:14:15 > 0:14:16all will be well.
0:14:32 > 0:14:34And there it goes.
0:14:45 > 0:14:49The scorpion's sexual problems are much more complicated
0:14:49 > 0:14:51and potentially dangerous.
0:14:51 > 0:14:55They are hunters and have to make sure one doesn't regard the other
0:14:55 > 0:14:57not as a mate but as a meal.
0:14:57 > 0:14:59Courtship is necessary,
0:14:59 > 0:15:02ritualised in a number of set movements.
0:15:03 > 0:15:07First, those dangerous pincers have to be neutralised.
0:15:41 > 0:15:46Now, with the pincers held out of action, more rituals follow.
0:15:58 > 0:16:01The heads of male and female come close
0:16:01 > 0:16:03and even touch.
0:16:11 > 0:16:14Now a strange heaving back and forth,
0:16:14 > 0:16:18which will eventually lead to the actual transfer of sperm.
0:16:18 > 0:16:21The male's sex gland is on the underside of his body,
0:16:21 > 0:16:24and from it, he has deposited a packet of sperm on the ground.
0:16:24 > 0:16:27Now he has to tug the female into a position
0:16:27 > 0:16:31where her sexual pouch is directly above it.
0:16:40 > 0:16:42If this ritual is not performed correctly,
0:16:42 > 0:16:46the scorpion's hunting instincts will not be pacified.
0:16:46 > 0:16:50It's a delicate balance, and here it seems to be going wrong,
0:16:50 > 0:16:52because this probing with the sting
0:16:52 > 0:16:55is probably more to do with aggression than with mating.
0:17:23 > 0:17:24And they break.
0:17:25 > 0:17:28Spiders have the same kind of problem.
0:17:28 > 0:17:31They, too, are hunters, and a male advancing on a female
0:17:31 > 0:17:36has to make quite sure she knows who he is and what his intentions are.
0:17:36 > 0:17:40The female jumping spider has sharp eyes, eight of them.
0:17:40 > 0:17:43He signals with his front legs as though his life depended on it,
0:17:43 > 0:17:45which indeed it does.
0:17:57 > 0:17:59She signals back...
0:18:10 > 0:18:12..and he is encouraged.
0:18:16 > 0:18:18At close range,
0:18:18 > 0:18:22the male begins to use tactile signals rather than visual ones.
0:18:22 > 0:18:25He must constantly convince the female of his good intentions,
0:18:25 > 0:18:30for he has to achieve a more intimate and direct contact with the female
0:18:30 > 0:18:32than the male scorpion did.
0:18:32 > 0:18:35He's prepared for this encounter by spinning a tiny web of silk
0:18:35 > 0:18:39on which he's dropped some sperm from a gland under his abdomen.
0:18:39 > 0:18:43And he's taken up the sperm in two special feelers, the palps.
0:18:43 > 0:18:47Now he must reach over the female to pump sperm from one palp
0:18:47 > 0:18:50into one of the female's sexual pouches.
0:18:50 > 0:18:53It's rather like liquid being squeezed out of an eye dropper.
0:18:54 > 0:18:56And there it goes.
0:18:58 > 0:19:02Now the spider changes position to pass sperm from the other palp
0:19:02 > 0:19:04into the female's other sexual opening.
0:19:11 > 0:19:16The wolf spider is a larger and particularly aggressive species.
0:19:16 > 0:19:18He too is courting a female.
0:19:18 > 0:19:21His problem is especially dangerous here,
0:19:21 > 0:19:23because the female lives in a burrow
0:19:23 > 0:19:26from which she emerges only on hunting forays.
0:19:26 > 0:19:30It's hardly surprising, therefore, that he approaches with the greatest caution.
0:19:43 > 0:19:46At first, he uses a kind of semaphore.
0:19:46 > 0:19:49If he doesn't keep this up, the female may mistake him for prey
0:19:49 > 0:19:51and rush out and pounce on him.
0:20:09 > 0:20:13Within the confines of the burrow, visual signals are difficult,
0:20:13 > 0:20:18and so the male changes to delicate and sensitive strokings with his front legs.
0:20:30 > 0:20:34At last, she receives him and he can take up his risky mating position,
0:20:34 > 0:20:37reaching right round to the female's abdomen.
0:20:41 > 0:20:44The early jungles, filled with such creatures,
0:20:44 > 0:20:46were still only a few inches high,
0:20:46 > 0:20:50no more than a thick, moist carpet draping the sandspits and boulders.
0:20:50 > 0:20:55For plants like mosses and liverworts were still the only ones on land.
0:20:57 > 0:21:03And this is just about as big as any moss in the world ever grows.
0:21:04 > 0:21:08A series of isolated stems.
0:21:08 > 0:21:13It has no real roots. It just absorbs what moisture it requires through its surface.
0:21:13 > 0:21:17And it doesn't have true leaves. They're just simple scales.
0:21:17 > 0:21:22And to see why it's so frail, one has to look inside the stem.
0:21:24 > 0:21:27Sliced and examined under the electron microscope,
0:21:27 > 0:21:29this is how it appears in section.
0:21:29 > 0:21:33The cells are thin-walled with no rigidity to them,
0:21:33 > 0:21:35unable to support a tall plant.
0:21:35 > 0:21:38But that structure was soon to be strengthened.
0:21:39 > 0:21:44In the course of time, some plants developed that WERE able to grow upright
0:21:44 > 0:21:45and several feet tall.
0:21:45 > 0:21:49And the fossilised remains of some of the earliest of them
0:21:49 > 0:21:53have been found in the rocks of these bleak Welsh hillsides.
0:21:53 > 0:21:55To find fossils,
0:21:55 > 0:21:59you sometimes have to use violent methods.
0:22:33 > 0:22:35And here are some.
0:22:35 > 0:22:38They're just thin branching filaments,
0:22:38 > 0:22:41but they'll show up even better
0:22:41 > 0:22:43if I wet this slab.
0:22:47 > 0:22:49They look like tiny moss filaments,
0:22:49 > 0:22:53but when these flattened, 400-million-year-old stems are sectioned,
0:22:53 > 0:22:57the electron microscope reveals quite different cells.
0:22:57 > 0:23:01These have much thicker walls, forming tubes in the stem.
0:23:01 > 0:23:04A plumbing system, up which the plant draws water.
0:23:04 > 0:23:07And these new cells give the stem strength
0:23:07 > 0:23:10and the ability to grow tall.
0:23:10 > 0:23:14These very similar cells come not from a fossil plant
0:23:14 > 0:23:17but from a living one, from this plant,
0:23:17 > 0:23:20which grows on another Welsh hillside.
0:23:20 > 0:23:22It may look superficially like a moss.
0:23:22 > 0:23:25In fact, its common name is clubmoss,
0:23:25 > 0:23:28but actually, it's fundamentally different.
0:23:28 > 0:23:32By virtue of those tough, thick cells in its stem,
0:23:32 > 0:23:34it's much more rigid than any moss.
0:23:34 > 0:23:38Today, it only grows to that sort of height.
0:23:38 > 0:23:44But in the past, it grew to the size of trees and formed great forests.
0:23:45 > 0:23:48There were soon many kinds of plant with the new cell walls,
0:23:48 > 0:23:52and some of them, the horsetails, are still common worldwide.
0:23:52 > 0:23:56The highest, in South America, reaches three or four metres,
0:23:56 > 0:24:01but 300 million years ago, they grew to 30 metres, 90-feet tall.
0:24:01 > 0:24:06Then, as now, they developed a hard outer skin to prevent desiccation.
0:24:06 > 0:24:09Under the microscope, you can see minute pores
0:24:09 > 0:24:11through which the plant breathes,
0:24:11 > 0:24:15taking in carbon dioxide and giving out oxygen.
0:24:15 > 0:24:19And there was a third kind of plant that grew with the giant horsetails
0:24:19 > 0:24:21and the clubmoss trees in those first forests -
0:24:21 > 0:24:23tree ferns.
0:24:30 > 0:24:35But height for the horsetail and the tree fern accentuated yet again
0:24:35 > 0:24:39the problem of achieving sexual union
0:24:39 > 0:24:42with a male cell that has to swim.
0:24:43 > 0:24:47How could a microscopic cell swim from the top of THAT tree fern
0:24:47 > 0:24:51to the top of that one? Impossible.
0:24:51 > 0:24:55The structures that ARE up there produce spores,
0:24:55 > 0:25:00reproductive cells that do not require fertilisation in order to develop,
0:25:00 > 0:25:03just like those in the little capsules developed by mosses.
0:25:05 > 0:25:09The ferns produce their spores from structures beneath the fronds.
0:25:10 > 0:25:14Their shape and arrangement varies with each fern species.
0:25:50 > 0:25:52Ferns, like mosses,
0:25:52 > 0:25:55release their spores when the weather is dry,
0:25:55 > 0:25:57and the wind can carry them far and wide.
0:25:57 > 0:26:01Some fern spores are produced in cups at the end of curled strips,
0:26:01 > 0:26:04one side of which is woody and the other thin-walled.
0:26:04 > 0:26:07As these cups dry, they shrivel, pulling back the strip
0:26:07 > 0:26:10until the tension is too much, the strip snaps back
0:26:10 > 0:26:13and the spores are catapulted free.
0:26:22 > 0:26:27The spores have tiny spines and ridges that help them catch the wind.
0:26:32 > 0:26:34A few will fall on moist ground
0:26:34 > 0:26:38and then germinate to produce a different kind of plant altogether.
0:26:38 > 0:26:42This is the stage in the fern's life-cycle that bears the sex cells.
0:26:42 > 0:26:45And this has had to remain small and close to the ground
0:26:45 > 0:26:50in order that its sperm can swim from plant to plant.
0:26:50 > 0:26:53When wet weather comes, the male organs release the sperm
0:26:53 > 0:26:56which swim by thrashing their thread-like tails.
0:27:00 > 0:27:04Hundreds of thousands are produced from the underside of the flat plant
0:27:04 > 0:27:07and are carried away by the rainwater.
0:27:15 > 0:27:19Eventually, some reach the female organs of the plant
0:27:19 > 0:27:22and swim up the tubes that lead to the egg cells.
0:27:30 > 0:27:33After fertilisation, a new growth develops from the egg,
0:27:33 > 0:27:35sending up a tiny stalk.
0:27:57 > 0:28:02These green shoots eventually grow tall and complete the cycle,
0:28:02 > 0:28:06becoming, once more, a familiar spore-bearing fern.
0:28:09 > 0:28:12Then, about 400 million years ago,
0:28:12 > 0:28:16as the forests began to rise, new animals appeared.
0:28:16 > 0:28:19These were descendants of the ancestral millipedes,
0:28:19 > 0:28:22and several kinds still survive today.
0:28:22 > 0:28:26This is a bristletail and it lives in soil worldwide.
0:28:28 > 0:28:32And this, the silverfish, that now often lives in houses.
0:28:32 > 0:28:35Faster than millipedes, they have fewer body segments
0:28:35 > 0:28:39and even fewer legs - just three pairs.
0:28:39 > 0:28:41They all feed on vegetable matter.
0:28:41 > 0:28:46But as plants grew taller, so leaves and spores became more inaccessible.
0:28:46 > 0:28:49And these little creatures doubtless
0:28:49 > 0:28:52clambered up the stems and trunks after them.
0:28:53 > 0:28:56The journey up must have been fairly easy,
0:28:56 > 0:29:00but getting down again, sometimes over upward-pointing spikes,
0:29:00 > 0:29:01may have been more laborious.
0:29:03 > 0:29:07Maybe that was the reason for a dramatic development.
0:29:09 > 0:29:13Some little creatures developed wings for flying from plant to plant.
0:29:16 > 0:29:20Just how wings evolved we can't be certain,
0:29:20 > 0:29:24but they may have first developed as tiny lobes on the back.
0:29:24 > 0:29:28Dragonflies today develop their wings in just this way,
0:29:28 > 0:29:32repeating millions of years of evolution in just one night.
0:30:31 > 0:30:35The wings are stretched taut by blood pumping into the veins.
0:30:43 > 0:30:46Later, the blood is drawn back into the body
0:30:46 > 0:30:49and the gauzy wings slowly dry and harden.
0:31:15 > 0:31:18Flight is the great achievement of the insects.
0:31:18 > 0:31:21They were the first creatures to take to the air
0:31:21 > 0:31:26and they were to have it almost to themselves for 100 million years.
0:31:32 > 0:31:35Dragonflies were among the first flyers,
0:31:35 > 0:31:37and they are still superb aeronauts.
0:31:37 > 0:31:41They can reach speeds of 20 miles, 30 kilometres an hour.
0:31:41 > 0:31:43They hunt in the air,
0:31:43 > 0:31:46holding their legs crooked in front of them like a basket.
0:31:46 > 0:31:48They even mate on the wing.
0:31:56 > 0:31:58The females lay their eggs in water.
0:32:01 > 0:32:06Their young, wingless larvae will grow up on the bottom of the pond,
0:32:06 > 0:32:08breathing through feathery gills
0:32:08 > 0:32:11and feeding on other small water-living creatures
0:32:11 > 0:32:14until the time comes for them too to climb up a reed
0:32:14 > 0:32:16and spread their wings.
0:32:18 > 0:32:23The dragonflies' smaller relatives, damselflies, also haunt ponds.
0:32:23 > 0:32:26The wings of these insects beat so rapidly
0:32:26 > 0:32:30that only a slow-motion camera can show clearly how they fly.
0:32:30 > 0:32:34This is the action slowed down 120 times.
0:32:34 > 0:32:38The insect gets lift on the downbeat of the wing by twisting it,
0:32:38 > 0:32:41so that the leading edge is inclined downwards.
0:32:41 > 0:32:45But at the bottom of each stroke the wing is twisted back
0:32:45 > 0:32:48so that it is effective on the upstroke as well.
0:32:48 > 0:32:51It's an intricate set of mechanical movements
0:32:51 > 0:32:53which man has never matched in the air.
0:32:53 > 0:32:55Here, the insect is hovering.
0:32:55 > 0:32:59The wings sweep alternately backwards and forwards,
0:32:59 > 0:33:01again changing angle at the end of each sweep
0:33:01 > 0:33:04in order to obtain lift on both strokes.
0:33:04 > 0:33:07Man has achieved something similar with a helicopter,
0:33:07 > 0:33:09whose blades rotate.
0:33:09 > 0:33:12The insect can't rotate its wings,
0:33:12 > 0:33:16but it's evolved a set of movements which are even more complex.
0:33:27 > 0:33:31The principal navigational equipment of dragonflies and damselflies
0:33:31 > 0:33:33are their superb eyes.
0:33:33 > 0:33:35Because they're so dependent on them,
0:33:35 > 0:33:38dragonflies normally fly only during the day.
0:33:41 > 0:33:45Today's splendid species are among the biggest of insects,
0:33:45 > 0:33:49but when the insects first had the air to themselves,
0:33:49 > 0:33:51the dragonflies grew gigantic
0:33:51 > 0:33:56and one appeared that had a wingspan of 70cm, over two feet.
0:33:56 > 0:33:59The largest insect that has ever existed.
0:34:01 > 0:34:05While all this was happening, some 300 million years ago,
0:34:05 > 0:34:09the plants themselves were on the brink of an important advance.
0:34:09 > 0:34:14This tiny sexual stage of the fern's life cycle
0:34:14 > 0:34:16is obviously very vulnerable.
0:34:16 > 0:34:19It can only live in moist conditions like these,
0:34:19 > 0:34:23and down on the ground it's easily cropped by plant-eating animals.
0:34:23 > 0:34:27It would obviously be much safer if this stage could take place
0:34:27 > 0:34:29up in the top of the tree.
0:34:29 > 0:34:34But that would require some way of transferring the sex cells
0:34:34 > 0:34:36from tree to tree.
0:34:36 > 0:34:39Well, they could be blown there by the wind.
0:34:39 > 0:34:40But there was then, as there is now,
0:34:40 > 0:34:44also a regular traffic in-between the tops of the trees.
0:34:44 > 0:34:49Insects that go up there to seek the spores as food
0:34:49 > 0:34:51and fly from one tree to another.
0:34:51 > 0:34:54They could take them. And that's what happened.
0:34:54 > 0:34:58New plants appeared in which the sexual generation remained fixed
0:34:58 > 0:35:01to the asexual tree stage.
0:35:01 > 0:35:05And one of the first of them was a plant like this,
0:35:05 > 0:35:07a cycad.
0:35:07 > 0:35:10Cycads bear two kinds of cones,
0:35:10 > 0:35:14each of which represent, in effect, part of the tiny sexual stage
0:35:14 > 0:35:17that once grew down on the ground.
0:35:17 > 0:35:19The male cones produce pollen,
0:35:19 > 0:35:22the grains of which germinate to produce the male cells,
0:35:22 > 0:35:26and the female cones contain the large egg cells.
0:35:28 > 0:35:32Insects help to transport the pollen from the male cone to the female,
0:35:32 > 0:35:36and there it produces a tube down which swims the sperm.
0:35:40 > 0:35:43At its tip, within the female cone, a drop of water appears,
0:35:43 > 0:35:46and in that the sperm swims,
0:35:46 > 0:35:49re-enacting the journeys made through the primordial seas
0:35:49 > 0:35:53by the sperm cells of their algal ancestors.
0:35:53 > 0:35:56Only after several days does it fuse with the egg.
0:35:58 > 0:36:02This cycad leaf is about 200 million years old.
0:36:02 > 0:36:06That's to say it was fossilised around the end of November
0:36:06 > 0:36:08in the Life On Earth year.
0:36:08 > 0:36:12And at that time a new and revolutionary plant had appeared
0:36:12 > 0:36:15that was growing alongside these cycads.
0:36:15 > 0:36:19It was the conifer, and this is one of its trunks.
0:36:19 > 0:36:23It's not wood, as you might think, but solid stone.
0:36:28 > 0:36:33I'm in the middle of one of the most spectacular deposits
0:36:33 > 0:36:35of plant fossils in the whole world.
0:36:35 > 0:36:38The Petrified Forest in Arizona.
0:36:38 > 0:36:43These conifers grew to over 200 feet tall
0:36:43 > 0:36:48and they stood in thick, dense, dark forests alongside the swamps
0:36:48 > 0:36:50where the cycads grew.
0:36:50 > 0:36:53And when the trunks fell, they often dropped into a river
0:36:53 > 0:36:55which swept them down here
0:36:55 > 0:36:59so that they formed great logjams around here.
0:36:59 > 0:37:03And then the river muds and sands and silts buried them.
0:37:03 > 0:37:08And the silts eventually formed mudstones like those over there.
0:37:08 > 0:37:12When the mudstones eroded away, as they have done here,
0:37:12 > 0:37:16they re-exposed these trunks that have been turned to stone.
0:38:29 > 0:38:33Conifers are built on very similar lines to the cycads,
0:38:33 > 0:38:39except that they have both the male and the female cone on the same tree.
0:38:41 > 0:38:43These are the male cones,
0:38:43 > 0:38:46and they use wind to transport their pollen.
0:38:46 > 0:38:51But to ensure that such a haphazard method of fertilisation is successful,
0:38:51 > 0:38:54they have to produce pollen in huge quantities.
0:38:54 > 0:38:57One cone may produce several million grains,
0:38:57 > 0:39:01and there are many thousands of cones on an average-sized tree.
0:39:07 > 0:39:12The female cones are fewer in number and grow on the same branches.
0:39:12 > 0:39:16They're small globes in conspicuous positions on the tips of shoots,
0:39:16 > 0:39:18where they have a good chance of receiving pollen.
0:39:26 > 0:39:29Pollen falling on the female cone
0:39:29 > 0:39:32is only the beginning of a very long process.
0:39:32 > 0:39:37It takes a whole year for the grains to grow down to the eggs,
0:39:37 > 0:39:40and at the end of that year the cone looks like that.
0:39:40 > 0:39:42But even that's not the end of things.
0:39:42 > 0:39:46During the next year, the cone grows still more,
0:39:46 > 0:39:50it develops wrappings around the fertilised eggs
0:39:50 > 0:39:53and then it dries out and opens up.
0:39:53 > 0:39:57Out drop small, neatly packaged brown objects.
0:39:57 > 0:39:58Seeds.
0:39:58 > 0:40:02They contain the first kind of plant eggs to have been fertilised
0:40:02 > 0:40:04without the help of water.
0:40:25 > 0:40:29Ancient though the conifers' technique of reproduction is,
0:40:29 > 0:40:31it has proved a huge success.
0:40:31 > 0:40:36Today, about a third of the forests in the world are formed by conifers.
0:40:36 > 0:40:41Firs, larches, cedars, pines. They're all members of this group.
0:40:53 > 0:40:56The biggest living organism of any kind is a conifer,
0:40:56 > 0:40:59the giant sequoia of California
0:40:59 > 0:41:04that grows to 112 metres - 367 feet high.
0:41:04 > 0:41:08Some have a diameter of 12 metres, 40 feet.
0:41:27 > 0:41:31Conifers have a special way of healing wounds to their trunks.
0:41:31 > 0:41:34They seal them with resin.
0:41:34 > 0:41:38When it first flows, it's runny, but it soon forms a sticky lump
0:41:38 > 0:41:42which not only covers the wound but incidentally acts as an insect trap.
0:41:46 > 0:41:50Lumps of resin from the ancient coniferous forests survive as amber,
0:41:50 > 0:41:53and in them are insects,
0:41:53 > 0:41:56as perfect now as the day when they blundered into the resin
0:41:56 > 0:41:58100 million years ago.
0:42:07 > 0:42:11From fossils like these, we know that the insects by that time
0:42:11 > 0:42:14had developed into an enormous variety of forms
0:42:14 > 0:42:16that swarmed through the trees and over the ground,
0:42:16 > 0:42:19feeding on every part of the plants.
0:42:19 > 0:42:22Pollen and fruit, leaves and wood, root and branch,
0:42:22 > 0:42:24just as they do today.
0:42:28 > 0:42:33Bugs stab stems with stiletto-like mouthparts to reach the sap.
0:42:39 > 0:42:42There are over 3,000 species of aphids alone,
0:42:42 > 0:42:46tapping this ready source of food in plants all over the world.
0:42:46 > 0:42:49All they have to do is to pierce the plant vessels.
0:42:49 > 0:42:54They don't even need to suck, such is the pressure of the sap within the stem.
0:42:55 > 0:42:59Locusts and grasshoppers chew the leaves.
0:43:04 > 0:43:09Beetles munch through cuticles and even manage to digest wood.
0:43:09 > 0:43:12Some insects not only eat plants,
0:43:12 > 0:43:14but in order to hide while doing so
0:43:14 > 0:43:18they've come to look like plants, like leaves and sticks.
0:43:29 > 0:43:33Hunters from the ground pursue the insects up into the trees.
0:43:34 > 0:43:35Spiders.
0:43:37 > 0:43:41But lying in ambush on trunks and on leaves has its limitations.
0:43:41 > 0:43:42Most insects fly.
0:43:45 > 0:43:47Spiders never developed wings,
0:43:47 > 0:43:51so they were unable to pursue their prey into the air.
0:43:51 > 0:43:53Instead, they set traps for them.
0:44:01 > 0:44:06The silk that they had spread in sheets and trip lines on the ground
0:44:06 > 0:44:08they now wove into nets,
0:44:08 > 0:44:11setting them across the insect flyways.
0:44:59 > 0:45:02With these elegant and varied constructions,
0:45:02 > 0:45:06spiders began to take a heavy toll of flying insects
0:45:06 > 0:45:10and today spiders are one of the most effective predators
0:45:10 > 0:45:12on the insect populations.
0:45:24 > 0:45:29The insects developed their flying skills in many different ways.
0:45:29 > 0:45:33The two pairs of wings used by the dragonflies and their relatives
0:45:33 > 0:45:36were also used by other insects. This is a lacewing.
0:45:43 > 0:45:46But this design was modified by other insects.
0:45:46 > 0:45:51The caddis-fly, not needing the speed of a dragonfly to catch prey,
0:45:51 > 0:45:55overlapped its two pairs of wings, producing a unified surface area.
0:45:57 > 0:46:01On the other hand, bees must have compact wings
0:46:01 > 0:46:05which can be neatly folded back when visiting flowers or in the hive.
0:46:05 > 0:46:09To get the right lift, their smaller wings must beat faster.
0:46:10 > 0:46:14They look as though they only have one pair of wings,
0:46:14 > 0:46:15but in fact they have two.
0:46:15 > 0:46:19They're hitched together to form what is virtually a single surface
0:46:19 > 0:46:23by a line of hooks along the front edge of the back wing.
0:46:26 > 0:46:30Other insects spend more time among dense foliage.
0:46:30 > 0:46:33The front wings of this bug have thickened bases to them
0:46:33 > 0:46:38which strengthen them and protect the rear ones when folded.
0:46:38 > 0:46:40Beetles have gone one stage further.
0:46:40 > 0:46:43Many burrow through litter and dense vegetation,
0:46:43 > 0:46:48and their front wings have become converted into protective covers.
0:46:48 > 0:46:51In order to lift the heavy body during flying,
0:46:51 > 0:46:54the operational wings have to be large.
0:46:54 > 0:46:58If they're to be protected when not in use, they have to be folded,
0:46:58 > 0:47:01and the trick is done with spring-loaded joints
0:47:01 > 0:47:03in the veins of the wings.
0:47:05 > 0:47:09Once in the air, the wing covers have to be held up out of the way.
0:47:09 > 0:47:13But they may also help a little in flight, acting as stabilisers,
0:47:13 > 0:47:15preventing rolling and yawing.
0:47:15 > 0:47:18Like many insects, this beetle increases lift
0:47:18 > 0:47:21by clapping its wings together at the top of the upstroke,
0:47:21 > 0:47:24thereby improving airflow over the wings.
0:47:29 > 0:47:32The chafer is the heavyweight of the insect fliers.
0:47:32 > 0:47:36Its wings beat comparatively slowly, about 40 times a second.
0:47:36 > 0:47:39And it's the least agile of insects in the air,
0:47:39 > 0:47:43ponderous and unable easily to bank and swerve.
0:47:43 > 0:47:46It holds its wing covers out of the way along its back
0:47:46 > 0:47:49and balances itself with outstretched legs.
0:47:49 > 0:47:51Its wing structure is tremendously strong,
0:47:51 > 0:47:54in order to support a heavy insect,
0:47:54 > 0:47:57and yet flexible enough to change its angle on each stroke
0:47:57 > 0:48:01and even fold back on itself when the insect stops flying.
0:48:07 > 0:48:09Even that feat is overshadowed
0:48:09 > 0:48:12by the achievement of the most skilled aeronauts of all,
0:48:12 > 0:48:13the flies.
0:48:13 > 0:48:17This one, the hoverfly, is perhaps the champion.
0:48:17 > 0:48:20It uses only one pair of wings, the front ones,
0:48:20 > 0:48:24which it keeps in perfect condition with frequent cleaning.
0:48:24 > 0:48:27It can hang absolutely stationary in the air,
0:48:27 > 0:48:30and does so even when it mates.
0:48:30 > 0:48:35It can compensate for any sudden current of wind to hold its position.
0:48:35 > 0:48:39It can fly backwards and dart off at great speed in any direction.
0:48:39 > 0:48:40And to perform these manoeuvres
0:48:40 > 0:48:46it beats its wings at the astonishing speed of 175 beats a second.
0:48:46 > 0:48:50A normal slow-motion camera still shows the wings as a blur.
0:48:52 > 0:48:56They control flight with a device which can be seen clearly in another fly,
0:48:56 > 0:48:59the crane-fly, or daddy-long-legs.
0:48:59 > 0:49:02Those two objects, like drumsticks, swinging up and down,
0:49:02 > 0:49:07are their back pair of wings after millions of years of evolution.
0:49:07 > 0:49:10They're jointed to the body just as the rear wings are,
0:49:10 > 0:49:12and they act like gyroscopes.
0:49:12 > 0:49:17By beating very fast, and here they're slowed down 120 times,
0:49:17 > 0:49:20they give the fly stability in the air.
0:49:20 > 0:49:24For, like gyroscopes in the automatic controls of an aeroplane,
0:49:24 > 0:49:28they enable the fly to be aware of the attitude of its body in the air
0:49:28 > 0:49:31and to detect when there's been any change in the flight path.
0:49:35 > 0:49:38Houseflies also have these "drumsticks",
0:49:38 > 0:49:40though they're much smaller.
0:49:40 > 0:49:43It's these that enable flies to perform such extraordinary
0:49:43 > 0:49:46and tantalising aerobatics.
0:49:48 > 0:49:52And the same organs perform similar functions for the hoverfly,
0:49:52 > 0:49:55giving it that superb flight control.
0:50:10 > 0:50:15The design of the insect body is particularly suited not to great size
0:50:15 > 0:50:17but to miniaturisation.
0:50:17 > 0:50:21The hoverfly is one of the most intricately constructed insects of all.
0:50:21 > 0:50:25A marvel of microscopic machinery
0:50:25 > 0:50:27that's built up from an egg in a few days
0:50:27 > 0:50:30and is often crushed beneath a thumb.
0:50:34 > 0:50:36The main developments of the insects
0:50:36 > 0:50:40took place at a comparatively early stage in the history of life on earth.
0:50:40 > 0:50:44At the time when these petrified forest trees were alive,
0:50:44 > 0:50:46200 million years ago,
0:50:46 > 0:50:49every single main type of insect that we know today
0:50:49 > 0:50:51was already in existence.
0:50:51 > 0:50:55Here, for example, is a piece of petrified wood,
0:50:55 > 0:51:01and before it was turned to stone some beetle had bored holes into it,
0:51:01 > 0:51:04just as beetles bore into dead wood today.
0:51:04 > 0:51:07And now the stage was set for a revolution,
0:51:07 > 0:51:11and one in which the insects were to play a crucial part.
0:51:11 > 0:51:15Charles Darwin called its history "an abominable mystery".
0:51:15 > 0:51:19Even today, we've only got a sketchy idea of just what happened.
0:51:19 > 0:51:22But some of the plants developed flowers.
0:51:22 > 0:51:25The woodlands and the lakes bloomed
0:51:25 > 0:51:28and colour came to the earth.
0:52:57 > 0:52:59Flowers became beautiful,
0:52:59 > 0:53:03not to delight the eye of man, but to attract insects.
0:53:03 > 0:53:07This led to some of the most intimate of all the relationships
0:53:07 > 0:53:12that have evolved between plants and insects - pollination.
0:53:12 > 0:53:15Subtitles by Red Bee Media Ltd