Conquest David Attenborough's First Life


Conquest

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I'm on a fantastic journey to look for the origins of life.

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I shall be travelling, not only around the world, but back in time,

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to try and build a picture

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of what life was like in that very early period.

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Last time I saw how, 600 million years ago,

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simple cells evolved into the first multi-cellular animals.

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In this programme, I investigate what happened next.

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I will look for evidence in both

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fossils and living creatures of what happened in that

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far, distant past,

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when the fundamental features of modern animals

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were being established for the first time.

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One group, the arthropods, were the great pioneers.

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They were the first big predators.

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They had eyes.

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Legs.

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And hard external skeletons,

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They were the first to crawl out of water

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to conquer the land and the air.

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600 million years ago, the world was very different

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from the planet we know today.

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The land was entirely without animals or plants.

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But the oceans were teeming with life.

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The first proto-animals were immobile organisms

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that lived on the sea floor and extracted their nourishment

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from the water flowing around them.

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But once animals developed mouths

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and the ability move, evolution took off.

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Canada's Rocky Mountains.

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Here we can find evidence of a sudden explosion of life

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when animals started to evolve with astonishing rapidity.

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It happened during a period called the Cambrian.

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And it began 542 million years ago.

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During the next 10-20 million years,

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animals increased in numbers, diversity and size as never before.

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And as they got bigger, so they became more complex.

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And they're preserved to an extraordinary degree of perfection

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in the rocks right below me.

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The Burgess Shales, where a rich seam of fossils

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documents this Cambrian explosion in astonishing detail.

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All this area was once the floor of a shallow sea, teeming with life.

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As sediment settled down onto the floor, so it became compressed

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and turned into mudstones and shales that you can see around me here.

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About a century ago,

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an American geologist from the Smithsonian Institution

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was making a survey of this part of the Rockies.

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And he came walking along this particular path.

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And when he got to precisely this spot,

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he noticed a tiny fossil of a kind he had never seen before.

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He bent down and picked it up and it looked like this.

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What sort of a creature could this be?

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It was only the first of the enigmatic creatures

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to come from the Burgess Shales.

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Since then over 65,000 different specimens of now extinct

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Cambrian animals have been from this one small quarry.

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Many species have never been found elsewhere.

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It seems that the Burgess Shales were deposited in a place

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where conditions for fossilisation were uniquely perfect.

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As a consequence, even bodies of animals that were soft

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and lacking any hard parts were, nonetheless, preserved.

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They survive as thin, almost imperceptible layers,

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that you only see if you get the light just right.

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It's these fossils that have transformed our understanding

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of how animals we know today have come to be the way they are.

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In some of these specimens we can glimpse shapes and forms

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that look faintly familiar.

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But many of these bizarre creatures seem like nothing we know of today.

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This is one of the more mysterious animals from the Shales.

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There are two clues as to how this creature might have lived.

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It has flaps along the side of its body,

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but no legs, and also a broad, flat tail.

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So it's reasonable to assume that they helped it swim

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and that it lived not crawling along the floor,

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but up higher in the water.

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But the really, truly mysterious thing about it is that here

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on its head it had five eyes,

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each of them like a kind of little mushroom.

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And beneath that it had a long proboscis

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with which it grabbed things.

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It's a truly primitive animal

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and one that, still, we don't fully understand.

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It's been named opabinia.

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And it seems to have been a kind of evolutionary experiment.

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It's almost as if an assortment of different body parts

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had been put together in something of a hurry.

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What other animal has five eyes?

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And opabinia wasn't the only oddball.

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Wiwaxia was once thought to be an ancestor of earthworms,

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but now is considered to be an early snail.

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Most of the Burgess Shale creatures

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are unlike anything ever discovered before.

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There were countless bizarre creatures

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living in the Cambrian Seas,

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This unprecedented surge of diversity was something

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that had never happened before and would never happen again.

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For many years, scientists excavated and scrutinised the Shales

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looking for the causes of the Cambrian explosion.

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Their first task was to try and reconstruct

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what these strange animals must have looked like when they were alive

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and that was not at all easy.

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This is one of the oddest of the fossils from Burgess Shales.

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It seems to have five legs along the bottom,

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and curious kind of lobes along the top,

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which presumably were some devices, which help it to feed.

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But what kind of animal is that with five walking legs

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and feeding lobes along the top of its back?

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It was such an extraordinary thought that the scientist

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who described it thought it was a kind of hallucination,

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and he called it "hallucigenia".

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But since then, more specimens have shown that in fact,

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this is probably the wrong way up and that it was really like that.

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The projections at the bottom are, in fact, legs.

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And those along the top are tipped with sharp spines

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that were presumably, defensive.

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Perhaps these animals evolved these strange shapes

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because they needed to protect themselves?

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But if so, from what?

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Where were the predators?

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No-one could find a likely candidate.

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And then the answer came from a couple of fossil species

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that they had known almost from the very beginning.

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One of the strangest fossils found here is this.

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It's also one of the commonest.

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But what is it?

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Well, it has what looks like legs, so you might think

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it was some kind of caterpillar, or shrimp maybe.

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But the most mysterious thing about it was that

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they never found one with a head.

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Then there was another mystery,

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not as common as the headless shrimp,

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but one that looked like a sort of jellyfish,

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with radiating lines out, and this strange hole in the middle.

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And about twenty years ago,

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it was discovered that actually, there is a link between

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this and this.

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This bit is not a separate shrimp, it's actually a claw.

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And this bit is not a jellyfish, it's a mouth.

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And in the mouth you can see something

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that looks very significant.

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Could these be teeth?

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And were these not legs but spikes, used to stab and grab prey?

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The two were, in fact, connected.

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But now we have a most perfect fossil,

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which really demonstrates that that is indeed the case.

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This, you might say, is the Mona Lisa of the Burgess Shales.

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This specimen, at last, gave scientists a picture

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of the complete animal.

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It had plates along its back, and a tail at the rear end.

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It was a swimmer. And between those two spiked claws

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at the front there was a mouth...

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with teeth.

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This was the hunter they had been looking for.

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The scientist who discovered the claws called them anomalocaris,

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meaning strange shrimp.

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That name is now used for the whole animal.

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With its large tail and flexible plates along its flanks,

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anomalocaris could propel itself through the water at speed.

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Other specimens show that it could grow to a length of nearly a metre,

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two feet or so.

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It was, as far as we know, the first big predator on Earth.

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We can get clues as to what it was like

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from an animal that is alive today.

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It's much smaller than anomalocaris, though remarkably similar.

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And it lives in Australia, here on the Great Barrier Reef.

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Professor Justin Marshall has been studying these ferocious

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and powerful hunters for over 20 years.

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You have to very cautious about the way you handle them.

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If you pick them up they can knock the ends off your fingers.

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Fishermen call them thumb splitters because

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as they handle them they get thumbs and fingers split open.

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The other, slightly more technical name for them is mantis shrimp.

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They have a very ancient ancestry.

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Fossils of almost identical creatures have been found

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that date back 400 million years.

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This animal is almost as ancient as anomalocaris itself.

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It lurks in burrows, waiting for its victims

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to swim within range of its claws.

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Looking at the fossils of anomalocaris

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and comparing them to mantis shrimps,

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one could imagine that these animals are similar.

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They both have big raptorial appendages

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that are shot out at the front to grasp prey.

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You could imagine them lurking behind a rock

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waiting for unwitting prey to come past.

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And bang! Suddenly that's dinner.

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The mantis shrimp illustrates the essential characteristics

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of this brand new predator class of animals.

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Superb vision, great speed and superior size.

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Like anomalocaris, it's considerably larger than its victims.

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It also has extremely acute vision,

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with 12 different types of colour receptors in its eyes.

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We have just three.

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And it's one of the fastest animals alive,

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some species striking with the speed of a pellet from a gun.

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It's unlikely anomalocaris was as fast, or that it saw its prey

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so clearly, but nonetheless, it was a formidable predator,

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just as the mantis shrimp is today.

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Even a glimpse of a finger through glass is enough

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to make this animal strike,

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and with alarming force.

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So why did the mantis shrimp evolve in this way?

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Well, obviously...

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in order that it could

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outfox and outmanoeuvre, and eventually catch its prey.

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It's become very fast, very powerful,

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and capable of great patience.

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And those are characteristics of predators everywhere.

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So the fossilised remains of anomalocaris

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are evidence that hunting had begun in the Cambrian.

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And as predators became bigger, faster and stronger,

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so their prey had to develop increasingly elaborate defences.

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Opabinia's five eyes helped it spot trouble.

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And Hallucigenia protected itself with those spines along its back.

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One of the world's leading experts on the Burgess Shales,

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Dr Jean-Bernard Caron, believes that it was the arrival

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of predators like anomalocaris that stimulated the great

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Cambrian explosion of diversity.

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It is during the Cambrian

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that we can start seeing animals with legs, eyes, swimming.

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This didn't exist before and this evolved very, very quickly

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at the beginning of the Cambrian.

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But once you have a big predator, presumably the rest of life,

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which it was feeding on,

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had to evolve quite fast to develop some sort of defences.

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Would that be true?

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Well, we think that this evolution occurred relatively quickly because,

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in a place like the Burgess Shale you find organisms

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that may have had some kind of defensive mechanism,

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which is thought to be a response to higher predatory levels.

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Arms race, if you want, between predators and prey.

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One result of this duel between predators and prey

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was the development of armour.

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Animals everywhere were absorbing calcium carbonate

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and other inorganic substances from the seawater

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and mineralising their bodies.

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Many of them, like wiwaxia, that early mollusc,

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and ancestors of the squid, ammonites,

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developed protective shells.

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But one group, the arthropods, which had jointed legs,

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encased their entire bodies with hard armour plating.

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And what began as defensive armour, necessary for survival,

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brought with it another great advantage.

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Hard parts can be used not only to give protection,

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but to provide support for a body.

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Ha-ha!

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This spider crab is a crustacean.

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And it secretes chitin from its body,

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which it then strengthens with calcium carbonate.

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And a whole range of creatures

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have skeletons like this, based on chitin.

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Arthropods today include shrimps, lobsters and crabs,

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as well as land-living creatures,

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such as millipedes, scorpions and insects.

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But the ancestors of all of them first appeared in the Cambrian Seas.

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Over 50% of fossils in the Burgess Shales

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are arthropods of one kind or another.

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But one family was particularly abundant and varied.

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Just across the valley from the quarry,

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near the summit of Mount Stephen,

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almost every rock you turn over contains their remains.

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Here, they are found all over the place.

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They're called trilobites.

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Trilobites because their bodies were in three sections.

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Here on this slab there are several of them.

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That's the head.

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There's the middle bit. And there's the tail.

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One, two, three trilobites.

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Trilobites, at this particular time,

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right at the beginning of the Cambrian,

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began to proliferate into all sorts of forms.

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These creatures, for the next 250 million years,

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were probably the most advanced forms of life on this planet.

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To see how advanced the trilobites eventually became,

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I'm going to North Africa.

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In Morocco, on the southern flanks of the Atlas Mountains,

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the hills contain an amazing variety of them.

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They were only discovered a few years ago,

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but now the demand for them is so great

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that digging them out has become a major industry.

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These rocks, which were laid down about 150 million years after

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the Burgess Shale, also contain trilobites.

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The trouble is, the rock is very hard

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and the trilobites are quite rare.

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But when these people find them,

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their specimens are absolutely extraordinary.

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Some species have features that are so delicate

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that it can take days, sometimes weeks,

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to fully prepare a specimen.

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Skilled technicians use dentists' drills

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to get down to the finest detail.

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Every particle of rock must be carefully removed,

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with enormous patience and absolute precision.

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The end results reveal that trilobites

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moulded their external skeletons

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into an almost unbelievable variety of shapes.

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And that enabled them to colonise a great variety of habitats,

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just as modern arthropods still do today.

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There were about 50,000 different trilobite species that we know of,

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and doubtless there are still many more to be discovered.

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Their hard exoskeletons

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not only ensured their abundance in the fossil record,

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they also tell us a lot about their owners' lives.

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Many of the trilobites that are found in these cliffs

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are curled up like this one.

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Sometimes even more tightly than this is,

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with their tail tucked underneath their heads.

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And it's clear that this was some kind of protective posture,

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just as it is for some kinds of woodlice

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that you find in the garden today.

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That protected them against their enemies.

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But there are so many that are curled in these deposits,

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together with others that have their backs arched upwards

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and others in other strange postures,

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that it seems that they are the victim of some kind of catastrophe.

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The sea floor, it seems, was quite steep.

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And every now and again,

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the mud that accumulated on the bottom slipped down

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in a submarine avalanche,

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carrying the animals that lived in it and on it,

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higgledy-piggeldy, and burying them alive.

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Moroccan trilobites are big business these days.

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Particularly rare species sell for thousands of pounds.

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The world's leading trilobite experts,

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such as Professor Richard Fortey,

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come here to study these extraordinary animals.

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He believes that their external skeleton

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was the key to their success.

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The trilobites did almost everything

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you possibly can do with an exoskeleton.

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I think that skeleton was what gave them an advantage.

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They were protected. They could do all kinds of interesting things.

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They could grow spines.

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They could get flat, like pancakes.

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They could protect themselves by getting thick exoskeleton

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with pobbles all over it.

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It was a great advantage to them, just as it is to crabs and lobsters

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living today, which of course weren't around

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at the time of the trilobites.

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So they utilised the virtues of having a tough exoskeleton,

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to radiate into all kinds of ecological niches.

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You can see one of the most comprehensive collections

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of trilobite fossils

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just a few miles from where they're quarried, at Erfoud Museum.

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The collection here reveals just how varied

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the trilobite skeleton could be.

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There is no question that an exoskeleton

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gave the trilobites protection.

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But it also gave them something else of great value.

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There must have been many reasons why trilobites were so successful.

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But one of them, unquestionably, was their power of sight.

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They had eyes.

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not just eyespots that could tell the difference

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between light and dark,

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but complex eyes that could form detailed pictures

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of their surroundings, for the first time in the history of life.

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Eyes like these.

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Most animals on Earth today have eyes of one kind of another.

0:27:230:27:29

Most are made of soft tissue, as ours our.

0:27:290:27:33

But trilobite eyes are unique.

0:27:330:27:36

Their lenses are derived from their mineralised external skeleton.

0:27:360:27:41

They're made of rock.

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Each one of these little dots is a lens.

0:27:450:27:48

And each is made from calcite,

0:27:480:27:50

a crystalline form of chalk.

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Trilobites were the only organisms

0:27:530:27:56

ever really to use this stuff as their lens material.

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And in doing so they evolved very sophisticated vision indeed.

0:28:020:28:06

For example, these sorts of trilobites had very large lenses.

0:28:060:28:12

And each lens is readily visible with the naked eye

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and each one is biconvex.

0:28:160:28:18

And it's been proven that individual lenses have little bowls inside them

0:28:180:28:23

to help them focus more precisely.

0:28:230:28:26

These creatures were among the first,

0:28:260:28:28

certainly, to actually focus a picture, weren't they?

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It wasn't just a question of telling light from dark,

0:28:310:28:34

they could do better than that?

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On no, these, these had really sophisticated vision.

0:28:360:28:39

The kind of trilobites that have these eyes were probably hunters.

0:28:400:28:44

Some people have claimed that they could form stereoscopic images,

0:28:440:28:48

using both eyes, so they could really home in on the prey.

0:28:480:28:52

May predators today, including ourselves,

0:28:550:28:58

have 3D, or stereoscopic vision.

0:28:580:29:02

It makes it possible for a hunter to accurately judge the distance

0:29:020:29:06

between itself and its prey.

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But not all trilobites were predators.

0:29:170:29:21

Some were inoffensive creatures that lived by munching mud.

0:29:210:29:24

But sight must have been valuable

0:29:240:29:27

for them too, enabling them to spot enemies in time to escape.

0:29:270:29:31

There are trilobite eyes with more than 5,000 lenses.

0:29:310:29:33

-5,000?

-More than 5,000 lenses.

0:29:330:29:37

Now each of those, does it have an image?

0:29:370:29:39

Each doesn't have an image, but if they go for lots of tiny lenses,

0:29:390:29:42

they're particularly sensitive to movement,

0:29:420:29:44

i.e. something changing between one lens and the next.

0:29:440:29:48

This trilobite's eyes are so big they extend right round its head

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and meet in the middle.

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And that suggests that the animal swam high above the sea floor

0:29:570:30:01

and had a 360-degree view of the scene below.

0:30:010:30:05

With each lens capable of detecting movement,

0:30:060:30:10

its owner must have been able to see an enemy coming from any direction.

0:30:100:30:14

But the shape of a trilobite's eyes can reveal more than the

0:30:160:30:20

kind of image they produced.

0:30:200:30:22

Eyes can tell us a surprising amount about how and where an animal lived.

0:30:240:30:30

This one with its eyes on turrets probably lived in the sea where it

0:30:300:30:36

was gloomy, but nonetheless there was enough light for the animal to

0:30:360:30:41

be able to see on either side of it.

0:30:410:30:44

This one, on the other hand, has eyes also on turrets, but at the top

0:30:440:30:49

it has flanges, like sun shades.

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So it's, er, likely that it lived in the shallow, sunlit sea

0:30:520:30:56

and valued shades above its eyes so it didn't get dazzled.

0:30:560:31:01

This one, however, has very reduced eyes, and it may well be

0:31:010:31:06

that it skated along the mud along the bottom,

0:31:060:31:09

where it was gloomy anyway and there wasn't much to see,

0:31:090:31:12

so like an animal living in a cave, it slowly lost the use of its eyes.

0:31:120:31:16

And finally there's this creature,

0:31:160:31:20

and this is the one I think is particularly delightful.

0:31:200:31:23

This one has its eyes on stalks.

0:31:230:31:27

And probably lived under the mud,

0:31:270:31:30

gobbling up food there with its, just its eyes

0:31:300:31:33

peeking out of the top, to see whether there was danger around.

0:31:330:31:37

So trilobites were the first animals to see clearly.

0:31:390:31:44

But they had other senses as well, perhaps some

0:31:440:31:47

we don't even know about.

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Take this species with this bizarre trident structure on its nose.

0:31:500:31:55

What was it for? Some kind of motion sensor?

0:31:550:31:58

Prehistoric radar, perhaps?

0:31:580:32:02

Trilobites were, without question,

0:32:030:32:05

the most successful animals of their time.

0:32:050:32:09

They flourished in all parts of the ocean.

0:32:090:32:12

Indeed, they could be counted as one

0:32:120:32:14

of the most successful kinds of animals in the entire history of life.

0:32:140:32:18

Most trilobites are quite small, rather like beetles are today.

0:32:200:32:25

But the biggest living beetle is about that big, the goliath beetle.

0:32:250:32:30

Trilobites, on the other hand, grew very big indeed. Like this one.

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And this is by no means the biggest.

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The biggest known is nearly a metre, nearly three feet long.

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And it's thought that these really big ones grew to this size

0:32:410:32:45

because they lived in cold waters, and that's a tendency of animals

0:32:450:32:50

in cold, to grow large.

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And at the time that these rocks were laid down, Africa,

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where we are now, and where these are found,

0:32:560:32:59

was down by the South Pole.

0:32:590:33:01

Spectacular though these are,

0:33:040:33:06

they were by no means the largest arthropods in the ocean at the time.

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The trilobites had remote cousins, also arthropods, that had grown

0:33:110:33:16

into monsters.

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Their remains are much rarer, and often fragmentary,

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but some of the most complete have been found in Scotland.

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ALARM SOUNDS

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One of the best is held in the vaults

0:33:390:33:41

of Edinburgh's National Museum.

0:33:410:33:43

Gosh!

0:34:030:34:05

Well, this is a magnificent example of just how big an animal can grow

0:34:050:34:13

if it has an external skeleton.

0:34:130:34:15

This is a creature called the Eurypterid, or a sea scorpion.

0:34:150:34:20

And it was a hunter.

0:34:200:34:22

It had a pair of powerful pincers at the top, just behind its head.

0:34:220:34:27

It was obviously a monster, a terror of the seas.

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And this is by no means the biggest of the eurypterids.

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Sea scorpions were the top predators of their day.

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As far as we know, they were the biggest arthropod

0:34:430:34:46

that has ever existed.

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The discovery of a large fossilised claw suggests

0:34:500:34:54

that they could grow up to two and a half metres, eight feet in length.

0:34:540:34:58

So arthropods of one kind or another

0:35:070:35:09

were certainly dominant 420 million years ago.

0:35:090:35:15

The seas were full of life.

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From huge complex animals like this sea scorpion

0:35:180:35:21

creeping along the bottom,

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to simple creatures, like jellyfish, floating on the surface waters.

0:35:230:35:27

But the land was barren and without animals of any kind.

0:35:270:35:33

But there was food up there, simple plants,

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and that tempted some animals to venture out of the water.

0:35:390:35:45

Surviving on land, however, was a problem for them.

0:35:450:35:49

Coming from the sea, they had to evolve ways

0:35:490:35:51

of preventing their bodies from drying out.

0:35:510:35:54

And even more difficult, they had to develop a method of breathing air.

0:35:540:36:01

The very first animals had simply absorbed

0:36:010:36:03

dissolved oxygen from the water

0:36:030:36:05

through the skins of their soft bodies.

0:36:050:36:09

As they began to move and grow bigger, they needed more energy,

0:36:090:36:12

more quickly.

0:36:120:36:15

And that meant

0:36:150:36:17

they had to improve their method of collecting dissolved oxygen.

0:36:170:36:20

Bigger, more complex animals,

0:36:250:36:29

like for example, this lobster,

0:36:290:36:32

have to have specialised devices, which are called gills.

0:36:320:36:35

Here in the lobster they are these flaps underneath its abdomen,

0:36:350:36:40

which is flaps forwards

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and backwards to increase the flow of oxygenated water over them.

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But the trouble with gills is that they only work when they're wet.

0:36:470:36:51

In the dry, they do not absorb oxygen.

0:36:510:36:55

So if animals are to live on land,

0:36:550:36:58

they had have to have a new way of breathing.

0:36:580:37:03

The Burgess Shales,

0:37:080:37:10

that astonishingly rich treasury of Cambrian fossils,

0:37:100:37:14

contain the remains of just one

0:37:140:37:16

particularly rare species that may well have been the very first animal

0:37:160:37:21

to make that move onto land.

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It was not, as you might think, an amphibian, it was not even

0:37:230:37:27

a true arthropod, but one of their far distant cousins.

0:37:270:37:31

This little creature,

0:37:360:37:38

from the Burgess Shale seas, is thought to be the ancestor

0:37:380:37:44

of the very first creature that went on to land. It's called Aysheaia.

0:37:440:37:50

And we don't have to imagine what it was like in life,

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because there's a creature, that seems to be almost identical,

0:37:540:37:59

that is alive today.

0:37:590:38:00

It lives in many parts of the tropics, including the rainforest,

0:38:030:38:08

here in Queensland, Australia.

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It's nocturnal and seldom seen.

0:38:170:38:20

It spends most of its time hidden away inside rotten logs.

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Ah, it's nice and wet!

0:38:320:38:35

Certainly, er, perfect for what we're looking for.

0:38:350:38:38

You need local expertise to find one.

0:38:380:38:42

I generally find that it's just from the outside

0:38:440:38:46

of the, er, core of the tree.

0:38:460:38:48

-All nice and...

-Oh! What is that?

-Ooh, look at that.

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And this enchanting little creature

0:38:540:38:59

is what we were looking for.

0:38:590:39:01

Sometimes called a velvet worm,

0:39:060:39:10

or to give it its scientific name, Peripatus.

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If there is such a thing as a living fossil,

0:39:150:39:19

this surely must be one of them.

0:39:190:39:22

Because it seems to be almost identical

0:39:220:39:26

with that fossil, Aysheaia, which we saw in the Burgess Shales.

0:39:260:39:32

It looks at first sight like a worm.

0:39:320:39:38

But of course no worm has legs. In fact,

0:39:380:39:43

it seems to be halfway

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between a worm

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and an insect.

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Aysheaia, of course, lived in the sea.

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But this little creature lives on land.

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And it has one further attribute,

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which Aysheaia could not have had.

0:40:030:40:08

It has tiny little holes all along its flanks,

0:40:080:40:12

which enable it to breathe air.

0:40:120:40:15

So this is one of the first creatures

0:40:150:40:20

that moved on to land,

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540 million years ago.

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Velvet worms may have been the first animals to set foot on land,

0:40:440:40:48

but they have hardly changed during the following half-billion years.

0:40:480:40:53

Why?

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Well, unlike true arthropods, their bodies are covered,

0:40:570:40:59

not by an exoskeleton, but by soft, permeable skin.

0:40:590:41:06

That lack of an external skeleton means that their bodies,

0:41:060:41:11

unsupported by water, can't grow any bigger.

0:41:110:41:13

It also means that in order to prevent themselves from drying out,

0:41:130:41:18

they have to stay in damp environments.

0:41:180:41:23

True arthropods, like this scorpion,

0:41:230:41:25

a descendent of those giant sea scorpions, were not so restricted.

0:41:250:41:30

They had external skeletons.

0:41:300:41:32

That meant that not only were their bodies protected from drying out,

0:41:340:41:38

but they were strong and rigid enough to allow them to grow bigger

0:41:380:41:42

and get around without the support of water.

0:41:420:41:45

So how and when did true arthropods with exoskeletons

0:41:530:41:57

draw their first breath of air?

0:41:570:41:59

The answer can be found in this.

0:42:050:42:07

It is perhaps the smallest and most fragmentary fossil I've seen so far,

0:42:070:42:12

but don't be fooled by appearances.

0:42:120:42:16

It's almost certainly one of the most significant.

0:42:160:42:19

This specimen was collected in Cowie Harbour, here in Scotland, in 2004.

0:42:250:42:33

Even though it's so small, under the microscope you can see

0:42:330:42:38

extraordinary detail.

0:42:380:42:40

This is the main body of the animal with its segments.

0:42:400:42:46

And here are its legs.

0:42:460:42:50

But above each there is a tiny hole.

0:42:500:42:55

That is a spiracle, through which the animal was able to breathe air

0:42:570:43:02

just as insects do today.

0:43:020:43:05

And since it breathed air, if it had gone into the water

0:43:050:43:09

it would have drowned.

0:43:090:43:11

So this is a truly land-living animal and what is more,

0:43:110:43:16

it's the first and oldest that we know.

0:43:160:43:19

It's 428 million years old.

0:43:190:43:24

But what kind of creatures were these early land-dwelling arthropods?

0:43:270:43:32

Animals very like them are still quite common

0:43:380:43:42

in many parts of the world.

0:43:420:43:44

There are certainly plenty of them in those Australian rainforests.

0:43:440:43:48

One sort are millipedes,

0:43:510:43:53

which today grow as long as that and live on vegetation

0:43:530:43:59

and rotting wood, harmless vegetarians.

0:43:590:44:01

But there's also another multi-leg creature, which is a much more

0:44:010:44:07

difficult customer.

0:44:070:44:08

This is one of them.

0:44:110:44:13

A centipede. A very formidable hunter, with a powerful bite,

0:44:130:44:18

and some centipedes have bites that are lethal to human beings.

0:44:180:44:23

What kind of a bite this one has,

0:44:230:44:26

I don't know.

0:44:260:44:28

But when I let him out I shall do so

0:44:280:44:30

very carefully, because I don't propose to find out.

0:44:300:44:36

Come on.

0:44:360:44:38

So multi-legged arthropods invaded the land and became

0:44:450:44:50

more successful than ever.

0:44:500:44:52

Back in Scotland,

0:44:590:45:00

there is impressive evidence of just how successful they became.

0:45:000:45:06

This is a small fishing village

0:45:080:45:11

on the East Coast of Scotland called Crail.

0:45:110:45:14

Nothing particularly strange about it, you might think...

0:45:140:45:18

until, that is, you go down to the shore.

0:45:180:45:22

And then you can see something that is really extraordinary.

0:45:220:45:26

Standing here and there on the beach are fossils,

0:45:310:45:34

not of animals, but of plants.

0:45:340:45:37

This huge circular stump looks just like the base of a tree.

0:45:400:45:46

And indeed that is what it is, or rather,

0:45:460:45:49

what it was, 335 million years ago.

0:45:490:45:54

But it wasn't a tree like trees we know today.

0:45:540:45:57

It was related

0:45:570:45:58

to the small plants that are alive today called horsetails.

0:45:580:46:02

But this tree grew to 90 feet.

0:46:020:46:06

It was immense.

0:46:060:46:08

When they were alive, during a period called the Carboniferous,

0:46:110:46:15

long after the Cambrian,

0:46:150:46:17

this whole area was very different

0:46:170:46:19

from the windswept coastline of today.

0:46:190:46:21

This was a time when the continents of the world were grouped together

0:46:240:46:28

and forests were widespread.

0:46:280:46:30

So much plant life was pumping out oxygen

0:46:340:46:37

that the composition of the atmosphere began to change.

0:46:370:46:41

This had a profound effect on animal life.

0:46:450:46:49

In the forest that was growing near Crail, the ancient trees

0:46:540:46:58

were rooted in a sandy swamp.

0:46:580:47:01

And on the expanses of sand that stretched between those huge trees,

0:47:010:47:05

sand that's now turned to this sandstone,

0:47:050:47:09

there are tracks.

0:47:090:47:10

Tracks that come in pairs, there's one pair that goes up there.

0:47:100:47:15

There's another pair that goes up here.

0:47:150:47:18

And when you look at them in detail, you can see,

0:47:180:47:21

particularly on this pair, that each track has a number of dimples in it.

0:47:210:47:26

And those are the imprints of individual feet.

0:47:290:47:34

So this animal had a lot of feet.

0:47:340:47:36

It's thought to have been a giant millipede.

0:47:360:47:41

It was about...

0:47:410:47:43

four and a half feet long, one and a half metres.

0:47:430:47:46

And it had 26 or 28 segments.

0:47:460:47:50

A magnificent beast.

0:47:500:47:52

Arthropleura.

0:48:090:48:11

A giant millipede,

0:48:130:48:15

probably the biggest terrestrial arthropod that has ever existed.

0:48:150:48:21

The largest specimen discovered so far was nearly as long as a car...

0:48:210:48:26

two and a half metres.

0:48:260:48:27

The Carboniferous was the golden age for the arthropods,

0:48:290:48:34

for the air was now particularly rich in oxygen.

0:48:340:48:38

Today the atmosphere contains around 21% oxygen.

0:48:380:48:43

Back in the Carboniferous,

0:48:430:48:44

it was around 35% and that enabled animals to grow very big indeed.

0:48:440:48:50

But growing large was not their only success.

0:48:530:48:57

Some other arthropods in these carboniferous rainforests

0:48:570:49:01

were evolving in a different way.

0:49:010:49:03

Instead of becoming huge and ponderous,

0:49:030:49:07

they became agile and speedy.

0:49:070:49:09

To do that it's better to be short rather than long, and some

0:49:090:49:13

reduced their segments and ran around on just three pairs of legs,

0:49:130:49:17

as silverfish and bristletails do today.

0:49:170:49:20

These early insects then made another dramatic move...

0:49:250:49:29

they developed wings and became the first animals of any kind to fly.

0:49:290:49:35

Truly the invertebrates had colonised

0:49:400:49:44

not only the land, but the air.

0:49:440:49:46

And in an atmosphere so rich in oxygen,

0:49:470:49:51

they did so in a truly dramatic way.

0:49:510:49:54

This giant dragonfly,

0:49:560:49:58

the biggest flying insect that has ever existed, is called Meganeura.

0:49:580:50:03

Its wings were nearly three feet across.

0:50:120:50:15

But the golden age of the giant arthropods was not to last.

0:50:210:50:27

The rainforest died back, and oxygen in the atmosphere dropped.

0:50:270:50:32

Giant insects are no longer alive today and that may be

0:50:350:50:39

because the proportion of oxygen in the atmosphere is very much lower.

0:50:390:50:44

But nonetheless, insects have managed to find a way

0:50:440:50:47

of overcoming the problems of size.

0:50:470:50:50

They've become colonial.

0:50:500:50:52

Just as in the far distant, remote past,

0:50:540:50:57

individual cells clubbed together to form a larger organism,

0:50:570:51:02

such as a sponge,

0:51:020:51:03

so hundreds of thousands of individual insects, termites,

0:51:030:51:08

have cooperated to build this nest.

0:51:080:51:11

And a colony like this can crop as much vegetation

0:51:110:51:14

from the surroundings as a bigger animal like an antelope.

0:51:140:51:19

So by living in vast colonies like this,

0:51:330:51:36

arthropods can still dominate their surroundings.

0:51:360:51:39

They've become super-organisms...

0:51:410:51:43

hundreds of thousands of individuals all descended from the same female,

0:51:430:51:48

working and behaving as one.

0:51:480:51:50

So arthropods remain

0:51:580:52:00

one of the most successful groups of animals on the planet.

0:52:000:52:03

They've spread to all its corners.

0:52:070:52:11

Insects alone make up at least 80% of all animal species.

0:52:150:52:21

But arthropods weren't the only ones to make this move on to land.

0:52:240:52:29

The Burgess Shales -

0:52:330:52:35

the place where the beginnings of all this proliferation of life

0:52:350:52:39

in the Cambrian period are recorded in unparalleled detail.

0:52:390:52:43

Among the ancestors of all the insects,

0:52:470:52:51

spiders, the scorpions, the shellfish, the crustaceans,

0:52:510:52:56

the shrimps, the sponges,

0:52:560:52:59

there's just one tiny little creature, very insignificant,

0:52:590:53:04

which we human beings might think is perhaps the most important of all.

0:53:040:53:10

Because this...

0:53:100:53:11

is the first creature to have the sign of a backbone,

0:53:110:53:16

and thus, therefore, is probably the ancestor of us all.

0:53:160:53:21

It's a tiny, worm-like creature called Pikaia.

0:53:250:53:29

It was not a fearsome hunter.

0:53:320:53:34

It had no teeth for attack and no external skeleton for defence.

0:53:340:53:41

But Pikaia did have something new.

0:53:410:53:44

Instead of an external skeleton,

0:53:460:53:49

it had an internal one, a thin gristly rod...

0:53:490:53:53

the beginnings of a backbone.

0:53:530:53:55

It, or something very like it, was the ancestor of all vertebrates.

0:53:550:53:59

From such a creature as this, the first fish evolved.

0:54:010:54:05

Some of them, living in swamps, started to gulp air and wriggled up

0:54:050:54:10

onto the land. They gave rise to moist-skinned amphibians.

0:54:100:54:17

Some of them developed scaly, impermeable skins that enabled them

0:54:170:54:21

to colonise the driest places...

0:54:210:54:22

they were the reptiles.

0:54:220:54:24

And from them came the birds.

0:54:240:54:27

And the mammals.

0:54:300:54:32

Today mammals, like this rhinoceros,

0:54:350:54:38

are the biggest of all living animals.

0:54:380:54:41

Hello, old boy. How are you?

0:54:430:54:46

How are you?

0:54:460:54:48

'All mammals, including ourselves, extract oxygen from the air with

0:54:480:54:53

'the end of internal lungs, and distribute it through our bodies

0:54:530:54:56

'in our blood.'

0:54:560:54:57

There we are. There's a good lad.

0:54:570:55:00

'But we also owe our success, and our size,

0:55:000:55:05

'to the nature of our skeletons.'

0:55:050:55:07

Animals with an internal skeleton, like this rhinoceros,

0:55:080:55:14

have a huge advantage over animals whose skeleton is external.

0:55:140:55:20

A white rhinoceros, like this,

0:55:200:55:24

is one of the biggest land animals alive today.

0:55:240:55:28

Compare him

0:55:280:55:31

with him... a rhinoceros beetle.

0:55:310:55:33

Its skeleton is external.

0:55:350:55:38

It's very powerful.

0:55:380:55:41

It can carry 850 times its own weight.

0:55:410:55:44

But it can't grow much bigger. Because the only way it can grow is

0:55:440:55:49

by shedding its skeleton and growing a new one.

0:55:490:55:51

And while its skeleton is not there, its body is unsupported.

0:55:510:55:57

And after a certain size, the body will collapse under its own weight.

0:55:570:56:04

Here.

0:56:040:56:05

Here we are, come on boy. Come on boy.

0:56:080:56:11

Despite these differences, it's no coincidence that

0:56:110:56:15

backboned animals evolved many of the same features as the arthropods.

0:56:150:56:21

Teeth.

0:56:210:56:23

Legs.

0:56:230:56:26

Shells. Eyes.

0:56:260:56:30

And wings.

0:56:300:56:31

Any animal group needs such things if they are to colonise

0:56:310:56:34

all the Earth's varied habitats.

0:56:340:56:37

A journey that began for me near my boyhood home in Charnwood Forest

0:56:440:56:49

has taken me around the world and through 600 million years

0:56:490:56:54

of evolutionary history.

0:56:540:56:55

I've seen evidence of how single-celled life

0:56:560:57:00

dominated the planet for billions of years,

0:57:000:57:02

until a global ice age triggered the emergence of the first animals.

0:57:020:57:08

Many animal groups lasted millions of years.

0:57:110:57:14

But eventually their time ran out and they disappeared.

0:57:140:57:18

But others endured.

0:57:290:57:31

And between them they evolved

0:57:350:57:37

into the wondrous variety of life that inhabits this planet today.

0:57:370:57:42

Life originated in the oceans.

0:57:440:57:47

After an immense period of time, some creatures managed to crawl up

0:57:470:57:53

onto the land.

0:57:530:57:55

Those animals may seem to us to be very remote,

0:57:550:57:58

strange, even fantastic.

0:57:580:58:01

But all of us alive today owe our very existence to them.

0:58:010:58:07

Subtitles by Red Bee Media Ltd

0:58:290:58:31

E-mail [email protected]

0:58:310:58:33

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