0:00:14 > 0:00:19I'm on a fantastic journey to look for the origins of life.
0:00:19 > 0:00:24I shall be travelling, not only around the world, but back in time,
0:00:24 > 0:00:26to try and build a picture
0:00:26 > 0:00:30of what life was like in that very early period.
0:00:31 > 0:00:35Last time I saw how, 600 million years ago,
0:00:35 > 0:00:41simple cells evolved into the first multi-cellular animals.
0:00:46 > 0:00:51In this programme, I investigate what happened next.
0:00:54 > 0:00:56I will look for evidence in both
0:00:56 > 0:01:00fossils and living creatures of what happened in that
0:01:00 > 0:01:01far, distant past,
0:01:01 > 0:01:04when the fundamental features of modern animals
0:01:04 > 0:01:08were being established for the first time.
0:01:08 > 0:01:13One group, the arthropods, were the great pioneers.
0:01:13 > 0:01:15They were the first big predators.
0:01:17 > 0:01:20They had eyes.
0:01:20 > 0:01:22Legs.
0:01:22 > 0:01:24And hard external skeletons,
0:01:27 > 0:01:30They were the first to crawl out of water
0:01:30 > 0:01:33to conquer the land and the air.
0:01:50 > 0:01:54600 million years ago, the world was very different
0:01:54 > 0:01:56from the planet we know today.
0:01:57 > 0:02:02The land was entirely without animals or plants.
0:02:02 > 0:02:06But the oceans were teeming with life.
0:02:12 > 0:02:16The first proto-animals were immobile organisms
0:02:16 > 0:02:19that lived on the sea floor and extracted their nourishment
0:02:19 > 0:02:21from the water flowing around them.
0:02:23 > 0:02:26But once animals developed mouths
0:02:26 > 0:02:30and the ability move, evolution took off.
0:02:46 > 0:02:48Canada's Rocky Mountains.
0:02:51 > 0:02:55Here we can find evidence of a sudden explosion of life
0:02:55 > 0:02:59when animals started to evolve with astonishing rapidity.
0:03:01 > 0:03:05It happened during a period called the Cambrian.
0:03:08 > 0:03:13And it began 542 million years ago.
0:03:17 > 0:03:21During the next 10-20 million years,
0:03:21 > 0:03:27animals increased in numbers, diversity and size as never before.
0:03:27 > 0:03:31And as they got bigger, so they became more complex.
0:03:31 > 0:03:36And they're preserved to an extraordinary degree of perfection
0:03:36 > 0:03:39in the rocks right below me.
0:03:42 > 0:03:45The Burgess Shales, where a rich seam of fossils
0:03:45 > 0:03:49documents this Cambrian explosion in astonishing detail.
0:03:59 > 0:04:06All this area was once the floor of a shallow sea, teeming with life.
0:04:06 > 0:04:10As sediment settled down onto the floor, so it became compressed
0:04:10 > 0:04:15and turned into mudstones and shales that you can see around me here.
0:04:18 > 0:04:20About a century ago,
0:04:20 > 0:04:23an American geologist from the Smithsonian Institution
0:04:23 > 0:04:27was making a survey of this part of the Rockies.
0:04:27 > 0:04:31And he came walking along this particular path.
0:04:31 > 0:04:35And when he got to precisely this spot,
0:04:35 > 0:04:40he noticed a tiny fossil of a kind he had never seen before.
0:04:40 > 0:04:46He bent down and picked it up and it looked like this.
0:04:49 > 0:04:53What sort of a creature could this be?
0:04:53 > 0:04:56It was only the first of the enigmatic creatures
0:04:56 > 0:04:59to come from the Burgess Shales.
0:04:59 > 0:05:04Since then over 65,000 different specimens of now extinct
0:05:04 > 0:05:08Cambrian animals have been from this one small quarry.
0:05:08 > 0:05:12Many species have never been found elsewhere.
0:05:12 > 0:05:16It seems that the Burgess Shales were deposited in a place
0:05:16 > 0:05:20where conditions for fossilisation were uniquely perfect.
0:05:21 > 0:05:25As a consequence, even bodies of animals that were soft
0:05:25 > 0:05:30and lacking any hard parts were, nonetheless, preserved.
0:05:30 > 0:05:34They survive as thin, almost imperceptible layers,
0:05:34 > 0:05:37that you only see if you get the light just right.
0:05:43 > 0:05:48It's these fossils that have transformed our understanding
0:05:48 > 0:05:52of how animals we know today have come to be the way they are.
0:05:55 > 0:05:59In some of these specimens we can glimpse shapes and forms
0:05:59 > 0:06:01that look faintly familiar.
0:06:05 > 0:06:11But many of these bizarre creatures seem like nothing we know of today.
0:06:16 > 0:06:21This is one of the more mysterious animals from the Shales.
0:06:21 > 0:06:24There are two clues as to how this creature might have lived.
0:06:24 > 0:06:28It has flaps along the side of its body,
0:06:28 > 0:06:34but no legs, and also a broad, flat tail.
0:06:34 > 0:06:37So it's reasonable to assume that they helped it swim
0:06:37 > 0:06:41and that it lived not crawling along the floor,
0:06:41 > 0:06:43but up higher in the water.
0:06:43 > 0:06:48But the really, truly mysterious thing about it is that here
0:06:48 > 0:06:50on its head it had five eyes,
0:06:50 > 0:06:55each of them like a kind of little mushroom.
0:06:55 > 0:06:58And beneath that it had a long proboscis
0:06:58 > 0:07:00with which it grabbed things.
0:07:00 > 0:07:03It's a truly primitive animal
0:07:03 > 0:07:07and one that, still, we don't fully understand.
0:07:09 > 0:07:11It's been named opabinia.
0:07:11 > 0:07:15And it seems to have been a kind of evolutionary experiment.
0:07:18 > 0:07:21It's almost as if an assortment of different body parts
0:07:21 > 0:07:24had been put together in something of a hurry.
0:07:24 > 0:07:27What other animal has five eyes?
0:07:30 > 0:07:33And opabinia wasn't the only oddball.
0:07:35 > 0:07:40Wiwaxia was once thought to be an ancestor of earthworms,
0:07:40 > 0:07:44but now is considered to be an early snail.
0:07:45 > 0:07:48Most of the Burgess Shale creatures
0:07:48 > 0:07:51are unlike anything ever discovered before.
0:07:53 > 0:07:56There were countless bizarre creatures
0:07:56 > 0:07:59living in the Cambrian Seas,
0:07:59 > 0:08:03This unprecedented surge of diversity was something
0:08:03 > 0:08:08that had never happened before and would never happen again.
0:08:08 > 0:08:14For many years, scientists excavated and scrutinised the Shales
0:08:14 > 0:08:17looking for the causes of the Cambrian explosion.
0:08:19 > 0:08:22Their first task was to try and reconstruct
0:08:22 > 0:08:26what these strange animals must have looked like when they were alive
0:08:26 > 0:08:29and that was not at all easy.
0:08:34 > 0:08:38This is one of the oddest of the fossils from Burgess Shales.
0:08:40 > 0:08:43It seems to have five legs along the bottom,
0:08:43 > 0:08:47and curious kind of lobes along the top,
0:08:47 > 0:08:53which presumably were some devices, which help it to feed.
0:08:54 > 0:08:59But what kind of animal is that with five walking legs
0:08:59 > 0:09:02and feeding lobes along the top of its back?
0:09:02 > 0:09:05It was such an extraordinary thought that the scientist
0:09:05 > 0:09:08who described it thought it was a kind of hallucination,
0:09:08 > 0:09:11and he called it "hallucigenia".
0:09:12 > 0:09:17But since then, more specimens have shown that in fact,
0:09:17 > 0:09:22this is probably the wrong way up and that it was really like that.
0:09:24 > 0:09:28The projections at the bottom are, in fact, legs.
0:09:28 > 0:09:32And those along the top are tipped with sharp spines
0:09:32 > 0:09:34that were presumably, defensive.
0:09:37 > 0:09:40Perhaps these animals evolved these strange shapes
0:09:40 > 0:09:43because they needed to protect themselves?
0:09:48 > 0:09:50But if so, from what?
0:09:50 > 0:09:53Where were the predators?
0:09:53 > 0:09:56No-one could find a likely candidate.
0:09:58 > 0:10:02And then the answer came from a couple of fossil species
0:10:02 > 0:10:05that they had known almost from the very beginning.
0:10:07 > 0:10:10One of the strangest fossils found here is this.
0:10:12 > 0:10:14It's also one of the commonest.
0:10:14 > 0:10:17But what is it?
0:10:17 > 0:10:21Well, it has what looks like legs, so you might think
0:10:21 > 0:10:25it was some kind of caterpillar, or shrimp maybe.
0:10:25 > 0:10:28But the most mysterious thing about it was that
0:10:28 > 0:10:31they never found one with a head.
0:10:31 > 0:10:33Then there was another mystery,
0:10:33 > 0:10:38not as common as the headless shrimp,
0:10:38 > 0:10:41but one that looked like a sort of jellyfish,
0:10:41 > 0:10:45with radiating lines out, and this strange hole in the middle.
0:10:45 > 0:10:48And about twenty years ago,
0:10:48 > 0:10:53it was discovered that actually, there is a link between
0:10:53 > 0:10:56this and this.
0:10:56 > 0:11:02This bit is not a separate shrimp, it's actually a claw.
0:11:02 > 0:11:06And this bit is not a jellyfish, it's a mouth.
0:11:08 > 0:11:11And in the mouth you can see something
0:11:11 > 0:11:13that looks very significant.
0:11:15 > 0:11:16Could these be teeth?
0:11:19 > 0:11:24And were these not legs but spikes, used to stab and grab prey?
0:11:26 > 0:11:29The two were, in fact, connected.
0:11:29 > 0:11:33But now we have a most perfect fossil,
0:11:33 > 0:11:37which really demonstrates that that is indeed the case.
0:11:37 > 0:11:42This, you might say, is the Mona Lisa of the Burgess Shales.
0:11:44 > 0:11:48This specimen, at last, gave scientists a picture
0:11:48 > 0:11:51of the complete animal.
0:11:51 > 0:11:55It had plates along its back, and a tail at the rear end.
0:11:55 > 0:11:59It was a swimmer. And between those two spiked claws
0:11:59 > 0:12:01at the front there was a mouth...
0:12:01 > 0:12:02with teeth.
0:12:05 > 0:12:08This was the hunter they had been looking for.
0:12:12 > 0:12:17The scientist who discovered the claws called them anomalocaris,
0:12:17 > 0:12:18meaning strange shrimp.
0:12:20 > 0:12:24That name is now used for the whole animal.
0:12:24 > 0:12:28With its large tail and flexible plates along its flanks,
0:12:28 > 0:12:32anomalocaris could propel itself through the water at speed.
0:12:34 > 0:12:39Other specimens show that it could grow to a length of nearly a metre,
0:12:39 > 0:12:42two feet or so.
0:12:42 > 0:12:46It was, as far as we know, the first big predator on Earth.
0:12:51 > 0:12:55We can get clues as to what it was like
0:12:55 > 0:12:57from an animal that is alive today.
0:12:59 > 0:13:05It's much smaller than anomalocaris, though remarkably similar.
0:13:05 > 0:13:09And it lives in Australia, here on the Great Barrier Reef.
0:13:15 > 0:13:19Professor Justin Marshall has been studying these ferocious
0:13:19 > 0:13:23and powerful hunters for over 20 years.
0:13:24 > 0:13:27You have to very cautious about the way you handle them.
0:13:27 > 0:13:32If you pick them up they can knock the ends off your fingers.
0:13:32 > 0:13:34Fishermen call them thumb splitters because
0:13:34 > 0:13:36as they handle them they get thumbs and fingers split open.
0:13:40 > 0:13:45The other, slightly more technical name for them is mantis shrimp.
0:13:47 > 0:13:49They have a very ancient ancestry.
0:13:49 > 0:13:52Fossils of almost identical creatures have been found
0:13:52 > 0:13:56that date back 400 million years.
0:13:56 > 0:14:02This animal is almost as ancient as anomalocaris itself.
0:14:02 > 0:14:05It lurks in burrows, waiting for its victims
0:14:05 > 0:14:08to swim within range of its claws.
0:14:39 > 0:14:42Looking at the fossils of anomalocaris
0:14:42 > 0:14:44and comparing them to mantis shrimps,
0:14:44 > 0:14:46one could imagine that these animals are similar.
0:14:46 > 0:14:49They both have big raptorial appendages
0:14:49 > 0:14:52that are shot out at the front to grasp prey.
0:14:52 > 0:14:54You could imagine them lurking behind a rock
0:14:54 > 0:14:57waiting for unwitting prey to come past.
0:14:57 > 0:14:59And bang! Suddenly that's dinner.
0:15:11 > 0:15:14The mantis shrimp illustrates the essential characteristics
0:15:14 > 0:15:18of this brand new predator class of animals.
0:15:18 > 0:15:24Superb vision, great speed and superior size.
0:15:25 > 0:15:31Like anomalocaris, it's considerably larger than its victims.
0:15:31 > 0:15:34It also has extremely acute vision,
0:15:34 > 0:15:38with 12 different types of colour receptors in its eyes.
0:15:38 > 0:15:41We have just three.
0:15:43 > 0:15:46And it's one of the fastest animals alive,
0:15:46 > 0:15:49some species striking with the speed of a pellet from a gun.
0:15:51 > 0:15:56It's unlikely anomalocaris was as fast, or that it saw its prey
0:15:56 > 0:15:59so clearly, but nonetheless, it was a formidable predator,
0:15:59 > 0:16:02just as the mantis shrimp is today.
0:16:04 > 0:16:07Even a glimpse of a finger through glass is enough
0:16:07 > 0:16:10to make this animal strike,
0:16:10 > 0:16:12and with alarming force.
0:16:21 > 0:16:25So why did the mantis shrimp evolve in this way?
0:16:25 > 0:16:28Well, obviously...
0:16:28 > 0:16:30in order that it could
0:16:30 > 0:16:35outfox and outmanoeuvre, and eventually catch its prey.
0:16:35 > 0:16:39It's become very fast, very powerful,
0:16:39 > 0:16:42and capable of great patience.
0:16:42 > 0:16:46And those are characteristics of predators everywhere.
0:16:48 > 0:16:51So the fossilised remains of anomalocaris
0:16:51 > 0:16:57are evidence that hunting had begun in the Cambrian.
0:16:57 > 0:17:00And as predators became bigger, faster and stronger,
0:17:00 > 0:17:05so their prey had to develop increasingly elaborate defences.
0:17:05 > 0:17:09Opabinia's five eyes helped it spot trouble.
0:17:16 > 0:17:21And Hallucigenia protected itself with those spines along its back.
0:17:30 > 0:17:33One of the world's leading experts on the Burgess Shales,
0:17:33 > 0:17:37Dr Jean-Bernard Caron, believes that it was the arrival
0:17:37 > 0:17:41of predators like anomalocaris that stimulated the great
0:17:41 > 0:17:44Cambrian explosion of diversity.
0:17:47 > 0:17:49It is during the Cambrian
0:17:49 > 0:17:53that we can start seeing animals with legs, eyes, swimming.
0:17:53 > 0:17:58This didn't exist before and this evolved very, very quickly
0:17:58 > 0:18:00at the beginning of the Cambrian.
0:18:00 > 0:18:04But once you have a big predator, presumably the rest of life,
0:18:04 > 0:18:06which it was feeding on,
0:18:06 > 0:18:09had to evolve quite fast to develop some sort of defences.
0:18:09 > 0:18:11Would that be true?
0:18:11 > 0:18:16Well, we think that this evolution occurred relatively quickly because,
0:18:16 > 0:18:20in a place like the Burgess Shale you find organisms
0:18:20 > 0:18:24that may have had some kind of defensive mechanism,
0:18:24 > 0:18:28which is thought to be a response to higher predatory levels.
0:18:28 > 0:18:31Arms race, if you want, between predators and prey.
0:18:33 > 0:18:36One result of this duel between predators and prey
0:18:36 > 0:18:39was the development of armour.
0:18:43 > 0:18:46Animals everywhere were absorbing calcium carbonate
0:18:46 > 0:18:50and other inorganic substances from the seawater
0:18:50 > 0:18:53and mineralising their bodies.
0:18:54 > 0:18:58Many of them, like wiwaxia, that early mollusc,
0:18:58 > 0:19:01and ancestors of the squid, ammonites,
0:19:01 > 0:19:02developed protective shells.
0:19:04 > 0:19:07But one group, the arthropods, which had jointed legs,
0:19:07 > 0:19:12encased their entire bodies with hard armour plating.
0:19:18 > 0:19:24And what began as defensive armour, necessary for survival,
0:19:24 > 0:19:26brought with it another great advantage.
0:19:28 > 0:19:31Hard parts can be used not only to give protection,
0:19:31 > 0:19:34but to provide support for a body.
0:19:36 > 0:19:38Ha-ha!
0:19:38 > 0:19:42This spider crab is a crustacean.
0:19:42 > 0:19:45And it secretes chitin from its body,
0:19:45 > 0:19:50which it then strengthens with calcium carbonate.
0:19:50 > 0:19:52And a whole range of creatures
0:19:52 > 0:19:55have skeletons like this, based on chitin.
0:19:58 > 0:20:02Arthropods today include shrimps, lobsters and crabs,
0:20:02 > 0:20:04as well as land-living creatures,
0:20:04 > 0:20:07such as millipedes, scorpions and insects.
0:20:07 > 0:20:12But the ancestors of all of them first appeared in the Cambrian Seas.
0:20:16 > 0:20:19Over 50% of fossils in the Burgess Shales
0:20:19 > 0:20:22are arthropods of one kind or another.
0:20:22 > 0:20:27But one family was particularly abundant and varied.
0:20:29 > 0:20:31Just across the valley from the quarry,
0:20:31 > 0:20:34near the summit of Mount Stephen,
0:20:34 > 0:20:38almost every rock you turn over contains their remains.
0:20:39 > 0:20:43Here, they are found all over the place.
0:20:43 > 0:20:45They're called trilobites.
0:20:45 > 0:20:49Trilobites because their bodies were in three sections.
0:20:49 > 0:20:52Here on this slab there are several of them.
0:20:52 > 0:20:54That's the head.
0:20:54 > 0:20:58There's the middle bit. And there's the tail.
0:20:58 > 0:21:02One, two, three trilobites.
0:21:02 > 0:21:04Trilobites, at this particular time,
0:21:04 > 0:21:07right at the beginning of the Cambrian,
0:21:07 > 0:21:11began to proliferate into all sorts of forms.
0:21:11 > 0:21:16These creatures, for the next 250 million years,
0:21:16 > 0:21:21were probably the most advanced forms of life on this planet.
0:21:23 > 0:21:27To see how advanced the trilobites eventually became,
0:21:27 > 0:21:29I'm going to North Africa.
0:21:29 > 0:21:34In Morocco, on the southern flanks of the Atlas Mountains,
0:21:34 > 0:21:37the hills contain an amazing variety of them.
0:21:42 > 0:21:44They were only discovered a few years ago,
0:21:44 > 0:21:47but now the demand for them is so great
0:21:47 > 0:21:50that digging them out has become a major industry.
0:21:57 > 0:22:02These rocks, which were laid down about 150 million years after
0:22:02 > 0:22:06the Burgess Shale, also contain trilobites.
0:22:06 > 0:22:08The trouble is, the rock is very hard
0:22:08 > 0:22:11and the trilobites are quite rare.
0:22:11 > 0:22:13But when these people find them,
0:22:13 > 0:22:17their specimens are absolutely extraordinary.
0:22:25 > 0:22:29Some species have features that are so delicate
0:22:29 > 0:22:32that it can take days, sometimes weeks,
0:22:32 > 0:22:35to fully prepare a specimen.
0:22:35 > 0:22:38Skilled technicians use dentists' drills
0:22:38 > 0:22:40to get down to the finest detail.
0:22:44 > 0:22:47Every particle of rock must be carefully removed,
0:22:47 > 0:22:51with enormous patience and absolute precision.
0:22:56 > 0:22:59The end results reveal that trilobites
0:22:59 > 0:23:01moulded their external skeletons
0:23:01 > 0:23:05into an almost unbelievable variety of shapes.
0:23:21 > 0:23:27And that enabled them to colonise a great variety of habitats,
0:23:27 > 0:23:30just as modern arthropods still do today.
0:23:36 > 0:23:41There were about 50,000 different trilobite species that we know of,
0:23:41 > 0:23:44and doubtless there are still many more to be discovered.
0:23:51 > 0:23:53Their hard exoskeletons
0:23:53 > 0:23:56not only ensured their abundance in the fossil record,
0:23:56 > 0:24:00they also tell us a lot about their owners' lives.
0:24:03 > 0:24:07Many of the trilobites that are found in these cliffs
0:24:07 > 0:24:10are curled up like this one.
0:24:10 > 0:24:12Sometimes even more tightly than this is,
0:24:12 > 0:24:16with their tail tucked underneath their heads.
0:24:16 > 0:24:19And it's clear that this was some kind of protective posture,
0:24:19 > 0:24:22just as it is for some kinds of woodlice
0:24:22 > 0:24:24that you find in the garden today.
0:24:24 > 0:24:28That protected them against their enemies.
0:24:28 > 0:24:31But there are so many that are curled in these deposits,
0:24:31 > 0:24:35together with others that have their backs arched upwards
0:24:35 > 0:24:37and others in other strange postures,
0:24:37 > 0:24:42that it seems that they are the victim of some kind of catastrophe.
0:24:42 > 0:24:46The sea floor, it seems, was quite steep.
0:24:46 > 0:24:48And every now and again,
0:24:48 > 0:24:51the mud that accumulated on the bottom slipped down
0:24:51 > 0:24:53in a submarine avalanche,
0:24:53 > 0:24:56carrying the animals that lived in it and on it,
0:24:56 > 0:25:00higgledy-piggeldy, and burying them alive.
0:25:09 > 0:25:13Moroccan trilobites are big business these days.
0:25:13 > 0:25:18Particularly rare species sell for thousands of pounds.
0:25:21 > 0:25:24The world's leading trilobite experts,
0:25:24 > 0:25:26such as Professor Richard Fortey,
0:25:26 > 0:25:30come here to study these extraordinary animals.
0:25:34 > 0:25:36He believes that their external skeleton
0:25:36 > 0:25:39was the key to their success.
0:25:39 > 0:25:42The trilobites did almost everything
0:25:42 > 0:25:46you possibly can do with an exoskeleton.
0:25:46 > 0:25:50I think that skeleton was what gave them an advantage.
0:25:50 > 0:25:54They were protected. They could do all kinds of interesting things.
0:25:54 > 0:25:57They could grow spines.
0:25:57 > 0:26:00They could get flat, like pancakes.
0:26:00 > 0:26:02They could protect themselves by getting thick exoskeleton
0:26:02 > 0:26:04with pobbles all over it.
0:26:04 > 0:26:08It was a great advantage to them, just as it is to crabs and lobsters
0:26:08 > 0:26:10living today, which of course weren't around
0:26:10 > 0:26:12at the time of the trilobites.
0:26:12 > 0:26:17So they utilised the virtues of having a tough exoskeleton,
0:26:17 > 0:26:20to radiate into all kinds of ecological niches.
0:26:25 > 0:26:28You can see one of the most comprehensive collections
0:26:28 > 0:26:30of trilobite fossils
0:26:30 > 0:26:34just a few miles from where they're quarried, at Erfoud Museum.
0:26:38 > 0:26:41The collection here reveals just how varied
0:26:41 > 0:26:44the trilobite skeleton could be.
0:26:47 > 0:26:50There is no question that an exoskeleton
0:26:50 > 0:26:53gave the trilobites protection.
0:26:53 > 0:26:57But it also gave them something else of great value.
0:26:59 > 0:27:02There must have been many reasons why trilobites were so successful.
0:27:02 > 0:27:07But one of them, unquestionably, was their power of sight.
0:27:07 > 0:27:08They had eyes.
0:27:08 > 0:27:11not just eyespots that could tell the difference
0:27:11 > 0:27:13between light and dark,
0:27:13 > 0:27:16but complex eyes that could form detailed pictures
0:27:16 > 0:27:20of their surroundings, for the first time in the history of life.
0:27:20 > 0:27:23Eyes like these.
0:27:23 > 0:27:29Most animals on Earth today have eyes of one kind of another.
0:27:29 > 0:27:33Most are made of soft tissue, as ours our.
0:27:33 > 0:27:36But trilobite eyes are unique.
0:27:36 > 0:27:41Their lenses are derived from their mineralised external skeleton.
0:27:41 > 0:27:42They're made of rock.
0:27:45 > 0:27:48Each one of these little dots is a lens.
0:27:48 > 0:27:50And each is made from calcite,
0:27:50 > 0:27:53a crystalline form of chalk.
0:27:53 > 0:27:56Trilobites were the only organisms
0:27:56 > 0:28:02ever really to use this stuff as their lens material.
0:28:02 > 0:28:06And in doing so they evolved very sophisticated vision indeed.
0:28:06 > 0:28:12For example, these sorts of trilobites had very large lenses.
0:28:12 > 0:28:16And each lens is readily visible with the naked eye
0:28:16 > 0:28:18and each one is biconvex.
0:28:18 > 0:28:23And it's been proven that individual lenses have little bowls inside them
0:28:23 > 0:28:26to help them focus more precisely.
0:28:26 > 0:28:28These creatures were among the first,
0:28:28 > 0:28:31certainly, to actually focus a picture, weren't they?
0:28:31 > 0:28:34It wasn't just a question of telling light from dark,
0:28:34 > 0:28:36they could do better than that?
0:28:36 > 0:28:39On no, these, these had really sophisticated vision.
0:28:40 > 0:28:44The kind of trilobites that have these eyes were probably hunters.
0:28:44 > 0:28:48Some people have claimed that they could form stereoscopic images,
0:28:48 > 0:28:52using both eyes, so they could really home in on the prey.
0:28:55 > 0:28:58May predators today, including ourselves,
0:28:58 > 0:29:02have 3D, or stereoscopic vision.
0:29:02 > 0:29:06It makes it possible for a hunter to accurately judge the distance
0:29:06 > 0:29:08between itself and its prey.
0:29:17 > 0:29:21But not all trilobites were predators.
0:29:21 > 0:29:24Some were inoffensive creatures that lived by munching mud.
0:29:24 > 0:29:27But sight must have been valuable
0:29:27 > 0:29:31for them too, enabling them to spot enemies in time to escape.
0:29:31 > 0:29:33There are trilobite eyes with more than 5,000 lenses.
0:29:33 > 0:29:37- 5,000?- More than 5,000 lenses.
0:29:37 > 0:29:39Now each of those, does it have an image?
0:29:39 > 0:29:42Each doesn't have an image, but if they go for lots of tiny lenses,
0:29:42 > 0:29:44they're particularly sensitive to movement,
0:29:44 > 0:29:48i.e. something changing between one lens and the next.
0:29:50 > 0:29:54This trilobite's eyes are so big they extend right round its head
0:29:54 > 0:29:57and meet in the middle.
0:29:57 > 0:30:01And that suggests that the animal swam high above the sea floor
0:30:01 > 0:30:05and had a 360-degree view of the scene below.
0:30:06 > 0:30:10With each lens capable of detecting movement,
0:30:10 > 0:30:14its owner must have been able to see an enemy coming from any direction.
0:30:16 > 0:30:20But the shape of a trilobite's eyes can reveal more than the
0:30:20 > 0:30:22kind of image they produced.
0:30:24 > 0:30:30Eyes can tell us a surprising amount about how and where an animal lived.
0:30:30 > 0:30:36This one with its eyes on turrets probably lived in the sea where it
0:30:36 > 0:30:41was gloomy, but nonetheless there was enough light for the animal to
0:30:41 > 0:30:44be able to see on either side of it.
0:30:44 > 0:30:49This one, on the other hand, has eyes also on turrets, but at the top
0:30:49 > 0:30:52it has flanges, like sun shades.
0:30:52 > 0:30:56So it's, er, likely that it lived in the shallow, sunlit sea
0:30:56 > 0:31:01and valued shades above its eyes so it didn't get dazzled.
0:31:01 > 0:31:06This one, however, has very reduced eyes, and it may well be
0:31:06 > 0:31:09that it skated along the mud along the bottom,
0:31:09 > 0:31:12where it was gloomy anyway and there wasn't much to see,
0:31:12 > 0:31:16so like an animal living in a cave, it slowly lost the use of its eyes.
0:31:16 > 0:31:20And finally there's this creature,
0:31:20 > 0:31:23and this is the one I think is particularly delightful.
0:31:23 > 0:31:27This one has its eyes on stalks.
0:31:27 > 0:31:30And probably lived under the mud,
0:31:30 > 0:31:33gobbling up food there with its, just its eyes
0:31:33 > 0:31:37peeking out of the top, to see whether there was danger around.
0:31:39 > 0:31:44So trilobites were the first animals to see clearly.
0:31:44 > 0:31:47But they had other senses as well, perhaps some
0:31:47 > 0:31:50we don't even know about.
0:31:50 > 0:31:55Take this species with this bizarre trident structure on its nose.
0:31:55 > 0:31:58What was it for? Some kind of motion sensor?
0:31:58 > 0:32:02Prehistoric radar, perhaps?
0:32:03 > 0:32:05Trilobites were, without question,
0:32:05 > 0:32:09the most successful animals of their time.
0:32:09 > 0:32:12They flourished in all parts of the ocean.
0:32:12 > 0:32:14Indeed, they could be counted as one
0:32:14 > 0:32:18of the most successful kinds of animals in the entire history of life.
0:32:20 > 0:32:25Most trilobites are quite small, rather like beetles are today.
0:32:25 > 0:32:30But the biggest living beetle is about that big, the goliath beetle.
0:32:30 > 0:32:36Trilobites, on the other hand, grew very big indeed. Like this one.
0:32:36 > 0:32:37And this is by no means the biggest.
0:32:37 > 0:32:41The biggest known is nearly a metre, nearly three feet long.
0:32:41 > 0:32:45And it's thought that these really big ones grew to this size
0:32:45 > 0:32:50because they lived in cold waters, and that's a tendency of animals
0:32:50 > 0:32:52in cold, to grow large.
0:32:52 > 0:32:56And at the time that these rocks were laid down, Africa,
0:32:56 > 0:32:59where we are now, and where these are found,
0:32:59 > 0:33:01was down by the South Pole.
0:33:04 > 0:33:06Spectacular though these are,
0:33:06 > 0:33:11they were by no means the largest arthropods in the ocean at the time.
0:33:11 > 0:33:16The trilobites had remote cousins, also arthropods, that had grown
0:33:16 > 0:33:19into monsters.
0:33:19 > 0:33:22Their remains are much rarer, and often fragmentary,
0:33:22 > 0:33:26but some of the most complete have been found in Scotland.
0:33:33 > 0:33:35ALARM SOUNDS
0:33:39 > 0:33:41One of the best is held in the vaults
0:33:41 > 0:33:43of Edinburgh's National Museum.
0:34:03 > 0:34:05Gosh!
0:34:05 > 0:34:13Well, this is a magnificent example of just how big an animal can grow
0:34:13 > 0:34:15if it has an external skeleton.
0:34:15 > 0:34:20This is a creature called the Eurypterid, or a sea scorpion.
0:34:20 > 0:34:22And it was a hunter.
0:34:22 > 0:34:27It had a pair of powerful pincers at the top, just behind its head.
0:34:27 > 0:34:31It was obviously a monster, a terror of the seas.
0:34:31 > 0:34:36And this is by no means the biggest of the eurypterids.
0:34:39 > 0:34:43Sea scorpions were the top predators of their day.
0:34:43 > 0:34:46As far as we know, they were the biggest arthropod
0:34:46 > 0:34:48that has ever existed.
0:34:50 > 0:34:54The discovery of a large fossilised claw suggests
0:34:54 > 0:34:58that they could grow up to two and a half metres, eight feet in length.
0:35:07 > 0:35:09So arthropods of one kind or another
0:35:09 > 0:35:15were certainly dominant 420 million years ago.
0:35:15 > 0:35:18The seas were full of life.
0:35:18 > 0:35:21From huge complex animals like this sea scorpion
0:35:21 > 0:35:23creeping along the bottom,
0:35:23 > 0:35:27to simple creatures, like jellyfish, floating on the surface waters.
0:35:27 > 0:35:33But the land was barren and without animals of any kind.
0:35:34 > 0:35:39But there was food up there, simple plants,
0:35:39 > 0:35:45and that tempted some animals to venture out of the water.
0:35:45 > 0:35:49Surviving on land, however, was a problem for them.
0:35:49 > 0:35:51Coming from the sea, they had to evolve ways
0:35:51 > 0:35:54of preventing their bodies from drying out.
0:35:54 > 0:36:01And even more difficult, they had to develop a method of breathing air.
0:36:01 > 0:36:03The very first animals had simply absorbed
0:36:03 > 0:36:05dissolved oxygen from the water
0:36:05 > 0:36:09through the skins of their soft bodies.
0:36:09 > 0:36:12As they began to move and grow bigger, they needed more energy,
0:36:12 > 0:36:15more quickly.
0:36:15 > 0:36:17And that meant
0:36:17 > 0:36:20they had to improve their method of collecting dissolved oxygen.
0:36:25 > 0:36:29Bigger, more complex animals,
0:36:29 > 0:36:32like for example, this lobster,
0:36:32 > 0:36:35have to have specialised devices, which are called gills.
0:36:35 > 0:36:40Here in the lobster they are these flaps underneath its abdomen,
0:36:40 > 0:36:42which is flaps forwards
0:36:42 > 0:36:47and backwards to increase the flow of oxygenated water over them.
0:36:47 > 0:36:51But the trouble with gills is that they only work when they're wet.
0:36:51 > 0:36:55In the dry, they do not absorb oxygen.
0:36:55 > 0:36:58So if animals are to live on land,
0:36:58 > 0:37:03they had have to have a new way of breathing.
0:37:08 > 0:37:10The Burgess Shales,
0:37:10 > 0:37:14that astonishingly rich treasury of Cambrian fossils,
0:37:14 > 0:37:16contain the remains of just one
0:37:16 > 0:37:21particularly rare species that may well have been the very first animal
0:37:21 > 0:37:23to make that move onto land.
0:37:23 > 0:37:27It was not, as you might think, an amphibian, it was not even
0:37:27 > 0:37:31a true arthropod, but one of their far distant cousins.
0:37:36 > 0:37:38This little creature,
0:37:38 > 0:37:44from the Burgess Shale seas, is thought to be the ancestor
0:37:44 > 0:37:50of the very first creature that went on to land. It's called Aysheaia.
0:37:50 > 0:37:54And we don't have to imagine what it was like in life,
0:37:54 > 0:37:59because there's a creature, that seems to be almost identical,
0:37:59 > 0:38:00that is alive today.
0:38:03 > 0:38:08It lives in many parts of the tropics, including the rainforest,
0:38:08 > 0:38:11here in Queensland, Australia.
0:38:17 > 0:38:20It's nocturnal and seldom seen.
0:38:25 > 0:38:30It spends most of its time hidden away inside rotten logs.
0:38:32 > 0:38:35Ah, it's nice and wet!
0:38:35 > 0:38:38Certainly, er, perfect for what we're looking for.
0:38:38 > 0:38:42You need local expertise to find one.
0:38:44 > 0:38:46I generally find that it's just from the outside
0:38:46 > 0:38:48of the, er, core of the tree.
0:38:50 > 0:38:54- All nice and...- Oh! What is that? - Ooh, look at that.
0:38:54 > 0:38:59And this enchanting little creature
0:38:59 > 0:39:01is what we were looking for.
0:39:06 > 0:39:10Sometimes called a velvet worm,
0:39:10 > 0:39:13or to give it its scientific name, Peripatus.
0:39:15 > 0:39:19If there is such a thing as a living fossil,
0:39:19 > 0:39:22this surely must be one of them.
0:39:22 > 0:39:26Because it seems to be almost identical
0:39:26 > 0:39:32with that fossil, Aysheaia, which we saw in the Burgess Shales.
0:39:32 > 0:39:38It looks at first sight like a worm.
0:39:38 > 0:39:43But of course no worm has legs. In fact,
0:39:43 > 0:39:46it seems to be halfway
0:39:46 > 0:39:49between a worm
0:39:49 > 0:39:51and an insect.
0:39:52 > 0:39:57Aysheaia, of course, lived in the sea.
0:39:57 > 0:40:00But this little creature lives on land.
0:40:00 > 0:40:03And it has one further attribute,
0:40:03 > 0:40:08which Aysheaia could not have had.
0:40:08 > 0:40:12It has tiny little holes all along its flanks,
0:40:12 > 0:40:15which enable it to breathe air.
0:40:15 > 0:40:20So this is one of the first creatures
0:40:20 > 0:40:22that moved on to land,
0:40:22 > 0:40:26540 million years ago.
0:40:44 > 0:40:48Velvet worms may have been the first animals to set foot on land,
0:40:48 > 0:40:53but they have hardly changed during the following half-billion years.
0:40:54 > 0:40:57Why?
0:40:57 > 0:40:59Well, unlike true arthropods, their bodies are covered,
0:40:59 > 0:41:06not by an exoskeleton, but by soft, permeable skin.
0:41:06 > 0:41:11That lack of an external skeleton means that their bodies,
0:41:11 > 0:41:13unsupported by water, can't grow any bigger.
0:41:13 > 0:41:18It also means that in order to prevent themselves from drying out,
0:41:18 > 0:41:23they have to stay in damp environments.
0:41:23 > 0:41:25True arthropods, like this scorpion,
0:41:25 > 0:41:30a descendent of those giant sea scorpions, were not so restricted.
0:41:30 > 0:41:32They had external skeletons.
0:41:34 > 0:41:38That meant that not only were their bodies protected from drying out,
0:41:38 > 0:41:42but they were strong and rigid enough to allow them to grow bigger
0:41:42 > 0:41:45and get around without the support of water.
0:41:53 > 0:41:57So how and when did true arthropods with exoskeletons
0:41:57 > 0:41:59draw their first breath of air?
0:42:05 > 0:42:07The answer can be found in this.
0:42:07 > 0:42:12It is perhaps the smallest and most fragmentary fossil I've seen so far,
0:42:12 > 0:42:16but don't be fooled by appearances.
0:42:16 > 0:42:19It's almost certainly one of the most significant.
0:42:25 > 0:42:33This specimen was collected in Cowie Harbour, here in Scotland, in 2004.
0:42:33 > 0:42:38Even though it's so small, under the microscope you can see
0:42:38 > 0:42:40extraordinary detail.
0:42:40 > 0:42:46This is the main body of the animal with its segments.
0:42:46 > 0:42:50And here are its legs.
0:42:50 > 0:42:55But above each there is a tiny hole.
0:42:57 > 0:43:02That is a spiracle, through which the animal was able to breathe air
0:43:02 > 0:43:05just as insects do today.
0:43:05 > 0:43:09And since it breathed air, if it had gone into the water
0:43:09 > 0:43:11it would have drowned.
0:43:11 > 0:43:16So this is a truly land-living animal and what is more,
0:43:16 > 0:43:19it's the first and oldest that we know.
0:43:19 > 0:43:24It's 428 million years old.
0:43:27 > 0:43:32But what kind of creatures were these early land-dwelling arthropods?
0:43:38 > 0:43:42Animals very like them are still quite common
0:43:42 > 0:43:44in many parts of the world.
0:43:44 > 0:43:48There are certainly plenty of them in those Australian rainforests.
0:43:51 > 0:43:53One sort are millipedes,
0:43:53 > 0:43:59which today grow as long as that and live on vegetation
0:43:59 > 0:44:01and rotting wood, harmless vegetarians.
0:44:01 > 0:44:07But there's also another multi-leg creature, which is a much more
0:44:07 > 0:44:08difficult customer.
0:44:11 > 0:44:13This is one of them.
0:44:13 > 0:44:18A centipede. A very formidable hunter, with a powerful bite,
0:44:18 > 0:44:23and some centipedes have bites that are lethal to human beings.
0:44:23 > 0:44:26What kind of a bite this one has,
0:44:26 > 0:44:28I don't know.
0:44:28 > 0:44:30But when I let him out I shall do so
0:44:30 > 0:44:36very carefully, because I don't propose to find out.
0:44:36 > 0:44:38Come on.
0:44:45 > 0:44:50So multi-legged arthropods invaded the land and became
0:44:50 > 0:44:52more successful than ever.
0:44:59 > 0:45:00Back in Scotland,
0:45:00 > 0:45:06there is impressive evidence of just how successful they became.
0:45:08 > 0:45:11This is a small fishing village
0:45:11 > 0:45:14on the East Coast of Scotland called Crail.
0:45:14 > 0:45:18Nothing particularly strange about it, you might think...
0:45:18 > 0:45:22until, that is, you go down to the shore.
0:45:22 > 0:45:26And then you can see something that is really extraordinary.
0:45:31 > 0:45:34Standing here and there on the beach are fossils,
0:45:34 > 0:45:37not of animals, but of plants.
0:45:40 > 0:45:46This huge circular stump looks just like the base of a tree.
0:45:46 > 0:45:49And indeed that is what it is, or rather,
0:45:49 > 0:45:54what it was, 335 million years ago.
0:45:54 > 0:45:57But it wasn't a tree like trees we know today.
0:45:57 > 0:45:58It was related
0:45:58 > 0:46:02to the small plants that are alive today called horsetails.
0:46:02 > 0:46:06But this tree grew to 90 feet.
0:46:06 > 0:46:08It was immense.
0:46:11 > 0:46:15When they were alive, during a period called the Carboniferous,
0:46:15 > 0:46:17long after the Cambrian,
0:46:17 > 0:46:19this whole area was very different
0:46:19 > 0:46:21from the windswept coastline of today.
0:46:24 > 0:46:28This was a time when the continents of the world were grouped together
0:46:28 > 0:46:30and forests were widespread.
0:46:34 > 0:46:37So much plant life was pumping out oxygen
0:46:37 > 0:46:41that the composition of the atmosphere began to change.
0:46:45 > 0:46:49This had a profound effect on animal life.
0:46:54 > 0:46:58In the forest that was growing near Crail, the ancient trees
0:46:58 > 0:47:01were rooted in a sandy swamp.
0:47:01 > 0:47:05And on the expanses of sand that stretched between those huge trees,
0:47:05 > 0:47:09sand that's now turned to this sandstone,
0:47:09 > 0:47:10there are tracks.
0:47:10 > 0:47:15Tracks that come in pairs, there's one pair that goes up there.
0:47:15 > 0:47:18There's another pair that goes up here.
0:47:18 > 0:47:21And when you look at them in detail, you can see,
0:47:21 > 0:47:26particularly on this pair, that each track has a number of dimples in it.
0:47:29 > 0:47:34And those are the imprints of individual feet.
0:47:34 > 0:47:36So this animal had a lot of feet.
0:47:36 > 0:47:41It's thought to have been a giant millipede.
0:47:41 > 0:47:43It was about...
0:47:43 > 0:47:46four and a half feet long, one and a half metres.
0:47:46 > 0:47:50And it had 26 or 28 segments.
0:47:50 > 0:47:52A magnificent beast.
0:48:09 > 0:48:11Arthropleura.
0:48:13 > 0:48:15A giant millipede,
0:48:15 > 0:48:21probably the biggest terrestrial arthropod that has ever existed.
0:48:21 > 0:48:26The largest specimen discovered so far was nearly as long as a car...
0:48:26 > 0:48:27two and a half metres.
0:48:29 > 0:48:34The Carboniferous was the golden age for the arthropods,
0:48:34 > 0:48:38for the air was now particularly rich in oxygen.
0:48:38 > 0:48:43Today the atmosphere contains around 21% oxygen.
0:48:43 > 0:48:44Back in the Carboniferous,
0:48:44 > 0:48:50it was around 35% and that enabled animals to grow very big indeed.
0:48:53 > 0:48:57But growing large was not their only success.
0:48:57 > 0:49:01Some other arthropods in these carboniferous rainforests
0:49:01 > 0:49:03were evolving in a different way.
0:49:03 > 0:49:07Instead of becoming huge and ponderous,
0:49:07 > 0:49:09they became agile and speedy.
0:49:09 > 0:49:13To do that it's better to be short rather than long, and some
0:49:13 > 0:49:17reduced their segments and ran around on just three pairs of legs,
0:49:17 > 0:49:20as silverfish and bristletails do today.
0:49:25 > 0:49:29These early insects then made another dramatic move...
0:49:29 > 0:49:35they developed wings and became the first animals of any kind to fly.
0:49:40 > 0:49:44Truly the invertebrates had colonised
0:49:44 > 0:49:46not only the land, but the air.
0:49:47 > 0:49:51And in an atmosphere so rich in oxygen,
0:49:51 > 0:49:54they did so in a truly dramatic way.
0:49:56 > 0:49:58This giant dragonfly,
0:49:58 > 0:50:03the biggest flying insect that has ever existed, is called Meganeura.
0:50:12 > 0:50:15Its wings were nearly three feet across.
0:50:21 > 0:50:27But the golden age of the giant arthropods was not to last.
0:50:27 > 0:50:32The rainforest died back, and oxygen in the atmosphere dropped.
0:50:35 > 0:50:39Giant insects are no longer alive today and that may be
0:50:39 > 0:50:44because the proportion of oxygen in the atmosphere is very much lower.
0:50:44 > 0:50:47But nonetheless, insects have managed to find a way
0:50:47 > 0:50:50of overcoming the problems of size.
0:50:50 > 0:50:52They've become colonial.
0:50:54 > 0:50:57Just as in the far distant, remote past,
0:50:57 > 0:51:02individual cells clubbed together to form a larger organism,
0:51:02 > 0:51:03such as a sponge,
0:51:03 > 0:51:08so hundreds of thousands of individual insects, termites,
0:51:08 > 0:51:11have cooperated to build this nest.
0:51:11 > 0:51:14And a colony like this can crop as much vegetation
0:51:14 > 0:51:19from the surroundings as a bigger animal like an antelope.
0:51:33 > 0:51:36So by living in vast colonies like this,
0:51:36 > 0:51:39arthropods can still dominate their surroundings.
0:51:41 > 0:51:43They've become super-organisms...
0:51:43 > 0:51:48hundreds of thousands of individuals all descended from the same female,
0:51:48 > 0:51:50working and behaving as one.
0:51:58 > 0:52:00So arthropods remain
0:52:00 > 0:52:03one of the most successful groups of animals on the planet.
0:52:07 > 0:52:11They've spread to all its corners.
0:52:15 > 0:52:21Insects alone make up at least 80% of all animal species.
0:52:24 > 0:52:29But arthropods weren't the only ones to make this move on to land.
0:52:33 > 0:52:35The Burgess Shales -
0:52:35 > 0:52:39the place where the beginnings of all this proliferation of life
0:52:39 > 0:52:43in the Cambrian period are recorded in unparalleled detail.
0:52:47 > 0:52:51Among the ancestors of all the insects,
0:52:51 > 0:52:56spiders, the scorpions, the shellfish, the crustaceans,
0:52:56 > 0:52:59the shrimps, the sponges,
0:52:59 > 0:53:04there's just one tiny little creature, very insignificant,
0:53:04 > 0:53:10which we human beings might think is perhaps the most important of all.
0:53:10 > 0:53:11Because this...
0:53:11 > 0:53:16is the first creature to have the sign of a backbone,
0:53:16 > 0:53:21and thus, therefore, is probably the ancestor of us all.
0:53:25 > 0:53:29It's a tiny, worm-like creature called Pikaia.
0:53:32 > 0:53:34It was not a fearsome hunter.
0:53:34 > 0:53:41It had no teeth for attack and no external skeleton for defence.
0:53:41 > 0:53:44But Pikaia did have something new.
0:53:46 > 0:53:49Instead of an external skeleton,
0:53:49 > 0:53:53it had an internal one, a thin gristly rod...
0:53:53 > 0:53:55the beginnings of a backbone.
0:53:55 > 0:53:59It, or something very like it, was the ancestor of all vertebrates.
0:54:01 > 0:54:05From such a creature as this, the first fish evolved.
0:54:05 > 0:54:10Some of them, living in swamps, started to gulp air and wriggled up
0:54:10 > 0:54:17onto the land. They gave rise to moist-skinned amphibians.
0:54:17 > 0:54:21Some of them developed scaly, impermeable skins that enabled them
0:54:21 > 0:54:22to colonise the driest places...
0:54:22 > 0:54:24they were the reptiles.
0:54:24 > 0:54:27And from them came the birds.
0:54:30 > 0:54:32And the mammals.
0:54:35 > 0:54:38Today mammals, like this rhinoceros,
0:54:38 > 0:54:41are the biggest of all living animals.
0:54:43 > 0:54:46Hello, old boy. How are you?
0:54:46 > 0:54:48How are you?
0:54:48 > 0:54:53'All mammals, including ourselves, extract oxygen from the air with
0:54:53 > 0:54:56'the end of internal lungs, and distribute it through our bodies
0:54:56 > 0:54:57'in our blood.'
0:54:57 > 0:55:00There we are. There's a good lad.
0:55:00 > 0:55:05'But we also owe our success, and our size,
0:55:05 > 0:55:07'to the nature of our skeletons.'
0:55:08 > 0:55:14Animals with an internal skeleton, like this rhinoceros,
0:55:14 > 0:55:20have a huge advantage over animals whose skeleton is external.
0:55:20 > 0:55:24A white rhinoceros, like this,
0:55:24 > 0:55:28is one of the biggest land animals alive today.
0:55:28 > 0:55:31Compare him
0:55:31 > 0:55:33with him... a rhinoceros beetle.
0:55:35 > 0:55:38Its skeleton is external.
0:55:38 > 0:55:41It's very powerful.
0:55:41 > 0:55:44It can carry 850 times its own weight.
0:55:44 > 0:55:49But it can't grow much bigger. Because the only way it can grow is
0:55:49 > 0:55:51by shedding its skeleton and growing a new one.
0:55:51 > 0:55:57And while its skeleton is not there, its body is unsupported.
0:55:57 > 0:56:04And after a certain size, the body will collapse under its own weight.
0:56:04 > 0:56:05Here.
0:56:08 > 0:56:11Here we are, come on boy. Come on boy.
0:56:11 > 0:56:15Despite these differences, it's no coincidence that
0:56:15 > 0:56:21backboned animals evolved many of the same features as the arthropods.
0:56:21 > 0:56:23Teeth.
0:56:23 > 0:56:26Legs.
0:56:26 > 0:56:30Shells. Eyes.
0:56:30 > 0:56:31And wings.
0:56:31 > 0:56:34Any animal group needs such things if they are to colonise
0:56:34 > 0:56:37all the Earth's varied habitats.
0:56:44 > 0:56:49A journey that began for me near my boyhood home in Charnwood Forest
0:56:49 > 0:56:54has taken me around the world and through 600 million years
0:56:54 > 0:56:55of evolutionary history.
0:56:56 > 0:57:00I've seen evidence of how single-celled life
0:57:00 > 0:57:02dominated the planet for billions of years,
0:57:02 > 0:57:08until a global ice age triggered the emergence of the first animals.
0:57:11 > 0:57:14Many animal groups lasted millions of years.
0:57:14 > 0:57:18But eventually their time ran out and they disappeared.
0:57:29 > 0:57:31But others endured.
0:57:35 > 0:57:37And between them they evolved
0:57:37 > 0:57:42into the wondrous variety of life that inhabits this planet today.
0:57:44 > 0:57:47Life originated in the oceans.
0:57:47 > 0:57:53After an immense period of time, some creatures managed to crawl up
0:57:53 > 0:57:55onto the land.
0:57:55 > 0:57:58Those animals may seem to us to be very remote,
0:57:58 > 0:58:01strange, even fantastic.
0:58:01 > 0:58:07But all of us alive today owe our very existence to them.
0:58:29 > 0:58:31Subtitles by Red Bee Media Ltd
0:58:31 > 0:58:33E-mail subtitling@bbc.co.uk