0:00:51 > 0:00:54The creatures that live among the coral heads
0:00:54 > 0:00:55of the Great Barrier Reef in Australia
0:00:55 > 0:00:58must surely be among the most beautiful
0:00:58 > 0:01:00and the most bewildering organisms
0:01:00 > 0:01:03you can find anywhere in the world.
0:01:51 > 0:01:54Sorting out these creatures into their various groups
0:01:54 > 0:01:56is baffling work.
0:01:56 > 0:01:58Often things are not what they seem.
0:01:58 > 0:02:02These are the tentacles of a worm.
0:02:03 > 0:02:06This is the cousin of a starfish.
0:02:06 > 0:02:10This is a flatworm, and the creature advancing on it
0:02:10 > 0:02:13is a snail that has lost its shell.
0:02:13 > 0:02:15One thing is clear.
0:02:15 > 0:02:18They're all animals without backbones, invertebrates.
0:02:18 > 0:02:20But how are they related to one another?
0:02:20 > 0:02:22Which is descended from what?
0:02:22 > 0:02:26One way to find out is to trace the various groups, as fossils,
0:02:26 > 0:02:30back through the rocks to their origins.
0:02:36 > 0:02:41These limestones here in Morocco are so old,
0:02:41 > 0:02:45getting on for 600 million years old,
0:02:45 > 0:02:49that they date long before the time of any backboned animals.
0:02:49 > 0:02:52There are no fish fossils here, for example.
0:02:52 > 0:02:55But there are invertebrate fossils.
0:02:55 > 0:03:00Not as many or as varied, it's true, as the invertebrates that live today
0:03:00 > 0:03:03on the Barrier Reef, but invertebrates nonetheless.
0:03:03 > 0:03:06And they fall roughly into three groups.
0:03:06 > 0:03:10There are little shells, like this.
0:03:10 > 0:03:12And a creature that looks like a flower
0:03:12 > 0:03:15but was covered in stony plates.
0:03:17 > 0:03:20And this, which is rather like a shrimp, with a shell,
0:03:20 > 0:03:24and its body divided into segments.
0:03:24 > 0:03:29What are the relationships between these three very, very early groups?
0:03:29 > 0:03:31If we can understand that,
0:03:31 > 0:03:35we will be close to understanding the origin of animal life.
0:03:35 > 0:03:36The obvious place to look
0:03:36 > 0:03:40is a few feet farther down in these limestones.
0:03:40 > 0:03:43A million or so years earlier.
0:03:43 > 0:03:46But suddenly, we come to a mystery.
0:03:46 > 0:03:51Although these limestones look exactly the same as those above,
0:03:51 > 0:03:54and must have been laid down in similar seas,
0:03:54 > 0:03:57there are no fossil shells to be found here at all.
0:03:57 > 0:04:00What's more, there are no fossil shells to be found
0:04:00 > 0:04:04in any rocks in the world of an age of these.
0:04:04 > 0:04:07And these extend for thousands of feet more,
0:04:07 > 0:04:11representing hundreds of millions of years of deposit.
0:04:11 > 0:04:15And not a fossil shell among them.
0:04:17 > 0:04:20The explanation is precisely in that word "shell".
0:04:20 > 0:04:23Shells fossilise easily.
0:04:23 > 0:04:29Soft animal tissues rot, and hardly leave any trace behind.
0:04:29 > 0:04:32There was life in the seas in which these limestones were deposited,
0:04:32 > 0:04:34but without shells.
0:04:38 > 0:04:42But why did it take so long for animals to develop shells?
0:04:42 > 0:04:46After all, if you condense the whole history of life
0:04:46 > 0:04:49from its beginnings until today into a year,
0:04:49 > 0:04:54it wasn't until early November that the first shelled animals appeared.
0:04:54 > 0:04:57Well, there's been a lot of debate on that question,
0:04:57 > 0:04:59and a lot of suggestions.
0:04:59 > 0:05:03One is that the chemistry of the seas wasn't suitable.
0:05:03 > 0:05:06They were either too cold or too acid
0:05:06 > 0:05:10to allow for the deposition of lime as shells.
0:05:10 > 0:05:15Whatever the answer, the fact remains that for that immense period of time,
0:05:15 > 0:05:19we have no fossil shells to help us chart the progress
0:05:19 > 0:05:22of the very early stages of animal life.
0:05:24 > 0:05:29But that doesn't mean we can't make some informed speculations.
0:05:29 > 0:05:32For example, take this group of creatures,
0:05:32 > 0:05:34the one like little shells.
0:05:34 > 0:05:37What could their early ancestors have been like?
0:05:39 > 0:05:43These microscopic creatures are among the simplest animals in the sea.
0:05:43 > 0:05:47They're the larvae of corals and jellyfish.
0:05:47 > 0:05:50We know that they appeared very early indeed.
0:05:50 > 0:05:53But suppose some of them didn't grow up either to float
0:05:53 > 0:05:57or to build skeletons, but took to a creeping life.
0:05:57 > 0:06:00They might easily have become something like this.
0:06:02 > 0:06:04This is a juvenile flatworm.
0:06:04 > 0:06:07It has a cluster of spots on top at one end,
0:06:07 > 0:06:10which are sensitive to light and to gravity,
0:06:10 > 0:06:14and it swims with the aid of cilia that cover its surface.
0:06:18 > 0:06:22When that settles on the sea bed, it becomes this.
0:06:22 > 0:06:24Not a drifter like a jellyfish,
0:06:24 > 0:06:26but an animal that moves in a purposeful way
0:06:26 > 0:06:30with a definite front end and back end.
0:06:38 > 0:06:41Flatworms are very flat,
0:06:41 > 0:06:45and with such a great body surface in relation to their small bulk,
0:06:45 > 0:06:47they absorb all the oxygen they need
0:06:47 > 0:06:49through their beautifully patterned skin.
0:06:55 > 0:06:57Many of them move by rippling their bodies
0:06:57 > 0:07:00instead of relying entirely on the cilia.
0:07:11 > 0:07:15And some are so good at it that they can swim.
0:07:26 > 0:07:30A flat shape, however, is not so suited to burrowing.
0:07:30 > 0:07:33And as mud and sand began to spread over the sea floor
0:07:33 > 0:07:35about 1,000 million years ago,
0:07:35 > 0:07:38burrowing became a desirable thing to do.
0:07:38 > 0:07:41There were bits of food to be sifted from the mud,
0:07:41 > 0:07:44and hidden beneath it, there was safety.
0:07:44 > 0:07:49So some worms changed from being flat to being round and long,
0:07:49 > 0:07:51and buried themselves in the mud.
0:07:55 > 0:07:59Others were less active and remained with their front ends sticking out,
0:07:59 > 0:08:01ringed by tentacles.
0:08:05 > 0:08:07The beating of the cilia created currents
0:08:07 > 0:08:10that enabled the tentacles to absorb oxygen,
0:08:10 > 0:08:14and also swept food particles down to the mouth at their centre.
0:08:15 > 0:08:18About 600 million years ago,
0:08:18 > 0:08:22some of these worms secreted a pair of shields on the top
0:08:22 > 0:08:24to protect the delicate tentacles
0:08:24 > 0:08:27and channel the feeding currents over them.
0:08:30 > 0:08:33This was such a success that variations appeared.
0:08:33 > 0:08:36Shells were strengthened with lime and grew bigger
0:08:36 > 0:08:39to allow more efficient breathing tentacles.
0:08:39 > 0:08:43So eventually the original worm-like shape was lost.
0:08:44 > 0:08:48We know from fossils that these creatures, the brachiopods,
0:08:48 > 0:08:51were enormously abundant in the ancient seas.
0:08:51 > 0:08:55They grew in many shapes and to a considerable size.
0:08:55 > 0:08:59Some developed delicate coils of lime inside their shells
0:08:59 > 0:09:01to support their feeding apparatus.
0:09:01 > 0:09:06But some 70 million years ago, their fortunes waned,
0:09:06 > 0:09:09and today only a few species survive.
0:09:10 > 0:09:14One lives in some numbers on the muddy shores of a bay in Japan,
0:09:14 > 0:09:17and at low tide, they are collected for food.
0:09:18 > 0:09:20They call them shamisen-gai
0:09:20 > 0:09:25because their shape is like that of the Japanese guitar, the shamisen.
0:09:40 > 0:09:42These are the simplest type of brachiopod,
0:09:42 > 0:09:46that have outlasted all the more ambitious kinds
0:09:46 > 0:09:47that once were so abundant.
0:09:47 > 0:09:52In fact, they're virtually identical to those earliest fossil shells.
0:09:55 > 0:09:59It's an astounding example of survival
0:09:59 > 0:10:02which occurs several times in the history of life.
0:10:02 > 0:10:05An early species finds itself in surroundings
0:10:05 > 0:10:07which suit it to perfection.
0:10:07 > 0:10:10No other animal comes along later
0:10:10 > 0:10:13which exploits the surroundings any better.
0:10:13 > 0:10:18Its cousins may move away to colonise different environments,
0:10:18 > 0:10:20or their environments might change,
0:10:20 > 0:10:22and so they develop into different creatures.
0:10:22 > 0:10:28But this creature, encountering no change, sees no cause for change.
0:10:28 > 0:10:32So it plods doggedly on, an ultra-conservative.
0:10:32 > 0:10:37This formula of a simple, worm-like body enclosed in a protective shell
0:10:37 > 0:10:40had obviously a lot of potential.
0:10:40 > 0:10:44Several groups of creatures in early periods were based on it,
0:10:44 > 0:10:47and one group in particular, the molluscs,
0:10:47 > 0:10:49exploited it very well indeed.
0:10:49 > 0:10:53Today, there are around 80,000 different species of them.
0:11:13 > 0:11:17The flatworm ancestors of the molluscs developed their shells
0:11:17 > 0:11:21not over one end around the mouth, but in the middle of the back,
0:11:21 > 0:11:25originally like a small tent under which the animal could hide,
0:11:25 > 0:11:27as the limpet does today.
0:11:29 > 0:11:32The shell is deposited by a part of the back, the mantle,
0:11:32 > 0:11:36and the animal enlarges it by adding to the margins.
0:11:38 > 0:11:42Some species, though, don't do so at an equal rate all round,
0:11:42 > 0:11:46and that produces twists and coils in the shell.
0:12:04 > 0:12:07They have a well-developed head, with eyes,
0:12:07 > 0:12:08and sensory tentacles
0:12:08 > 0:12:11for feeling the way and tasting the water.
0:12:11 > 0:12:16And underneath it, a very efficient feeding organ.
0:12:16 > 0:12:18It's a long, tongue-like ribbon.
0:12:18 > 0:12:21The muscles around it press it down and pull it forward,
0:12:21 > 0:12:25rasping it over the surface on which the animal is crawling.
0:12:25 > 0:12:27Many species use it for eating algae.
0:12:34 > 0:12:36Looked at under the electron microscope,
0:12:36 > 0:12:38the reason for its efficiency is clear.
0:12:38 > 0:12:41It carries rows and rows of minute teeth.
0:12:41 > 0:12:45Each species, for some reason, with a different pattern.
0:12:46 > 0:12:50Cowries secrete their shell in a way all their own.
0:12:50 > 0:12:53They extend their mantle right round the shell
0:12:53 > 0:12:55and deposit material on the top,
0:12:55 > 0:12:58giving it that beautifully polished surface.
0:13:09 > 0:13:13The spider shell has its ribbon tongue on a stalk
0:13:13 > 0:13:17so it can scrape surfaces its shell would prevent it from reaching.
0:13:18 > 0:13:24It also has a stalked eye to help it prospect for hidden pastures.
0:13:26 > 0:13:30Its foot has become very muscular to help it get around.
0:13:35 > 0:13:40Molluscs with paired shells, bivalves, don't often move far.
0:13:40 > 0:13:43Their foot is used to pull them down into the sand
0:13:43 > 0:13:48where they can sit and filter food safely and unobtrusively.
0:13:51 > 0:13:53Scallops are also filter feeders.
0:13:53 > 0:13:55They live on the surface,
0:13:55 > 0:13:59and not only have good eyes, but a surprising way of moving.
0:14:30 > 0:14:35Biggest of all is another filter feeder, the metre-long giant clam.
0:14:35 > 0:14:38So huge, it can't move.
0:14:38 > 0:14:41Its fleshy mantle joins its two shells,
0:14:41 > 0:14:44forming a chamber through which water is sucked.
0:14:44 > 0:14:48Every so often, it gives a convulsive shudder
0:14:48 > 0:14:51and gets rid of a little waste.
0:14:55 > 0:15:00A few molluscs have gone to the other extreme and become free-swimming
0:15:00 > 0:15:04by reducing their shells to scales concealed within their bodies,
0:15:04 > 0:15:07or doing without them altogether.
0:15:19 > 0:15:21Unprotected by a shell,
0:15:21 > 0:15:24these creatures defend themselves with a nasty-tasting slime.
0:15:24 > 0:15:28And their brilliant colours may serve to warn off anything
0:15:28 > 0:15:31that might contemplate eating them.
0:15:31 > 0:15:35If that's so, they must be among the loveliest warning notices
0:15:35 > 0:15:36in all nature.
0:16:20 > 0:16:24These creatures are more complex and usually larger than flatworms,
0:16:24 > 0:16:28and they need special breathing apparatus, the gills.
0:16:28 > 0:16:29In some species,
0:16:29 > 0:16:34they're exposed as a kind of trembling bouquet at the back.
0:17:06 > 0:17:10Several kinds have developed feathery outgrowths
0:17:10 > 0:17:12that enable them to float close to the sea's surface.
0:17:12 > 0:17:18There, extraordinary though it may sound, they hunt for jellyfish.
0:17:19 > 0:17:24This one is called glaucus, and it has found its prey.
0:17:29 > 0:17:32The stinging cells of the jellyfish are no defence.
0:17:32 > 0:17:35Indeed, some of these floating molluscs welcome them,
0:17:35 > 0:17:37swallowing the stinging cells
0:17:37 > 0:17:40and storing them in their own tentacles
0:17:40 > 0:17:42to use as second-hand weapons.
0:17:52 > 0:17:54This is another creature eaten by glaucus.
0:17:54 > 0:17:59One of the most deadly of all jellyfish, a Portuguese man-of-war.
0:17:59 > 0:18:03Beneath it trail its tentacles, loaded with stings.
0:18:03 > 0:18:06Another mollusc also preys on this creature,
0:18:06 > 0:18:08and this time, one with a shell.
0:18:08 > 0:18:12It has a most ingenious solution to the problem of keeping afloat.
0:18:14 > 0:18:18It produces bubbles by trapping air in mucus
0:18:18 > 0:18:21with special movements of its spoon-like foot,
0:18:21 > 0:18:25and builds them into a raft, from which it hangs.
0:18:39 > 0:18:44When it drifts into a Portuguese man-of-war, it attacks immediately.
0:19:08 > 0:19:11The stinging cells of the jellyfish,
0:19:11 > 0:19:15lethal to other creatures, have no effect on the snail.
0:19:15 > 0:19:19It munches them with the rest of the tentacles.
0:19:24 > 0:19:27A raft of bubbles solves this snail's weight problems,
0:19:27 > 0:19:31but that won't work for bigger creatures.
0:19:31 > 0:19:34500 million years ago, however,
0:19:34 > 0:19:37a group of molluscs evolved another method.
0:19:40 > 0:19:44This fossil shell may look perhaps quite an ordinary sort of shell,
0:19:44 > 0:19:46albeit rather large,
0:19:46 > 0:19:51but inside it has got quite a complicated structure.
0:19:51 > 0:19:55Here's one in a boulder where the outside has been worn away
0:19:55 > 0:19:58so that we can see what's inside.
0:19:58 > 0:20:01This part was where the animal lived,
0:20:01 > 0:20:05and at the back of it, there were these chambers
0:20:05 > 0:20:11which in life were filled with gas and acted as flotation chambers.
0:20:11 > 0:20:14How can we be so sure?
0:20:14 > 0:20:18Well, because this is another of those creatures
0:20:18 > 0:20:20that have survived virtually unchanged
0:20:20 > 0:20:23for hundreds of millions of years.
0:20:23 > 0:20:29This is a nautilus, and there are nautilus swimming in the seas today.
0:20:31 > 0:20:36They live in the South Pacific, but few people ever see them alive,
0:20:36 > 0:20:40for they spend most of their time in depths of up to 500 metres.
0:20:40 > 0:20:42They can swim at any depth,
0:20:42 > 0:20:45by pumping fluid in and out of their chambers,
0:20:45 > 0:20:47and so controlling their buoyancy.
0:20:57 > 0:21:00Being so mobile, they need good sense organs,
0:21:00 > 0:21:03and their eyes, although they have no lenses,
0:21:03 > 0:21:06are the best of any creature we've seen so far.
0:21:06 > 0:21:11Their bodies have become modified into dozens of tentacles.
0:21:11 > 0:21:16Some carry sense organs to detect food, some are used in reproduction
0:21:16 > 0:21:18and others to grapple with their prey,
0:21:18 > 0:21:21which is usually carrion, or lobsters or crabs.
0:21:34 > 0:21:39This proved to be an immensely successful design.
0:21:39 > 0:21:44And from it came another great group of molluscs, the ammonites.
0:21:44 > 0:21:46The ammonites were to dominate the seas of the world
0:21:46 > 0:21:48for the next 200 million years.
0:21:48 > 0:21:51They left behind in the rocks,
0:21:51 > 0:21:53particularly here in Lyme Regis in southern England,
0:21:53 > 0:21:58fossils that to my mind are some of the loveliest fossils of all.
0:22:04 > 0:22:06Like the nautilus,
0:22:06 > 0:22:08the ammonites added new flotation chambers as they grew,
0:22:08 > 0:22:12while their bodies occupied only the outer one.
0:22:14 > 0:22:19Because ammonites were so numerous and their shells fossilised so well,
0:22:19 > 0:22:21we know a great deal about the way they developed
0:22:21 > 0:22:24over a period of 200 million years.
0:22:24 > 0:22:27But their history is full of puzzles.
0:22:27 > 0:22:31Why, for example, did some groups develop uncoiled species
0:22:31 > 0:22:35and then, over generations, slowly coil up again?
0:22:41 > 0:22:45And why did the junctions between the flotation chambers,
0:22:45 > 0:22:47which originally had been simple curves,
0:22:47 > 0:22:54become increasingly elaborate and intricate, and eventually florid?
0:22:58 > 0:23:01Small ones may have lived in shallow water near the bottom,
0:23:01 > 0:23:04but others grew to an immense size
0:23:04 > 0:23:08and probably sailed the upper waters of the prehistoric seas
0:23:08 > 0:23:10like galleons.
0:23:10 > 0:23:12And there is one final mystery.
0:23:12 > 0:23:16Why, 50 million years ago, did they all die out?
0:23:16 > 0:23:19There is not one surviving ammonite today.
0:23:24 > 0:23:27But these paper-thin shells look remarkably like them.
0:23:27 > 0:23:28On very rare occasions,
0:23:28 > 0:23:32they are washed up on lonely beaches in New Zealand.
0:23:32 > 0:23:36They belong neither to an ammonite nor a nautilus but a relative,
0:23:36 > 0:23:38a kind of octopus called the argonaut,
0:23:38 > 0:23:40which is sometimes stranded with them.
0:23:40 > 0:23:43The animal doesn't live in the shell.
0:23:43 > 0:23:47It secretes it from one of its arms and then lays its eggs in it.
0:23:47 > 0:23:50Few people have ever seen that happen.
0:23:50 > 0:23:53Just once in a while, a storm catches the breeding shoals
0:23:53 > 0:23:56and drives these delicate cradles ashore,
0:23:56 > 0:23:59some of them still holding their eggs.
0:23:59 > 0:24:03For most of its life, the argonaut, like all other octopus,
0:24:03 > 0:24:05is totally without a shell.
0:24:05 > 0:24:09Only on this one occasion does it demonstrate its relationship
0:24:09 > 0:24:11with the nautilus so vividly.
0:24:17 > 0:24:21It's difficult to remember at times that the octopus is a mollusc
0:24:21 > 0:24:24and that most of its relations are weighed down with shells
0:24:24 > 0:24:28and a very long way from being quick-moving or intelligent.
0:24:28 > 0:24:33Its molluscan tentacles have become heavily armoured with suckers.
0:24:33 > 0:24:36The siphon, used by the clams for filter feeding,
0:24:36 > 0:24:39serves as a nozzle for jet propulsion.
0:24:39 > 0:24:44Its eyesight is excellent, and it has a lively brain and quick reactions.
0:24:46 > 0:24:49The squid is very similar.
0:24:49 > 0:24:52It has two more arms than the octopus
0:24:52 > 0:24:55and is a very much more active swimmer.
0:24:55 > 0:24:57Squids still keep within their bodies
0:24:57 > 0:25:00a last relic of their ancestral shell.
0:25:00 > 0:25:04A horny, sword-shaped structure that helps to support their long body.
0:25:04 > 0:25:08As they swim, they hold their tentacles out horizontally.
0:25:08 > 0:25:10They use jet propulsion for speed,
0:25:10 > 0:25:14but they can also idle along in either direction
0:25:14 > 0:25:17by waving fin-like extensions of their mantle.
0:25:33 > 0:25:38The squids and octopuses are the most active and intelligent of molluscs,
0:25:38 > 0:25:41able to solve complicated problems.
0:25:41 > 0:25:44They're also the largest.
0:25:47 > 0:25:51This giant squid that ran aground in Norway was nine metres long,
0:25:51 > 0:25:54and there are reports of others twice the size.
0:25:54 > 0:25:57They all developed from ancestors like flatworms
0:25:57 > 0:26:00that lived in the seas of 600 million years ago.
0:26:03 > 0:26:07But what about the second group of creatures from these ancient rocks?
0:26:07 > 0:26:11The ones represented by this flower-like fossil
0:26:11 > 0:26:13with a radial symmetry.
0:26:13 > 0:26:15Well, within the next few million years,
0:26:15 > 0:26:18these developed into a multitude of most beautiful forms
0:26:18 > 0:26:21that we call sea lilies or crinoids.
0:26:23 > 0:26:28It's a reasonable guess that these too evolved from worm-like creatures
0:26:28 > 0:26:33that developed limey plates to strengthen and protect themselves.
0:26:33 > 0:26:37These are about 300 million years old,
0:26:37 > 0:26:41and a very few species like them still survive in the ocean depths.
0:26:42 > 0:26:43But on the Barrier Reef,
0:26:43 > 0:26:47some close relatives still flourish in great numbers:
0:26:47 > 0:26:49feather stars.
0:26:49 > 0:26:51These are just like crinoids,
0:26:51 > 0:26:53but without stems, except when they're very young.
0:26:55 > 0:26:58These adults swim freely around,
0:26:58 > 0:27:01mostly at night, in search of feeding places
0:27:01 > 0:27:03where they can cling to the rocks
0:27:03 > 0:27:06and collect floating particles with their arms.
0:27:13 > 0:27:15Their relatives, the starfish,
0:27:15 > 0:27:19show clearly another characteristic of this group.
0:27:19 > 0:27:22Their bodies have a five-fold symmetry.
0:27:22 > 0:27:25The mouth is underneath, at the centre.
0:27:25 > 0:27:29They move on tube feet, another unique feature.
0:27:29 > 0:27:32Each foot has a tiny suction pad at the end,
0:27:32 > 0:27:35and the many thousands of them are worked by hydraulics,
0:27:35 > 0:27:38for they are all connected to water-filled vessels
0:27:38 > 0:27:40that run through the body.
0:27:44 > 0:27:46Their cousins, the brittle stars,
0:27:46 > 0:27:49are much the speediest creatures in the group.
0:27:51 > 0:27:54Sea urchins are more typical.
0:27:54 > 0:27:56They too have tube feet,
0:27:56 > 0:27:59but they move largely with the help of their spines.
0:28:03 > 0:28:06Some of the tube feet are specialised for jobs
0:28:06 > 0:28:10such as moving bits of debris from around the mouth,
0:28:10 > 0:28:14which, like that of the starfish, is on the underside of the animal.
0:28:16 > 0:28:20Urchins feed by grazing slowly on algae.
0:28:20 > 0:28:23The food is gnawed by hard jaws, taken into the gut
0:28:23 > 0:28:27and then, in most species, excreted from a pore at the top.
0:28:30 > 0:28:34The spines are attached to the plates of the urchin's shell
0:28:34 > 0:28:35by ball-and-socket joints,
0:28:35 > 0:28:38so they can move in any direction.
0:28:38 > 0:28:42Those on the top are for defence. If a shadow falls on the urchin,
0:28:42 > 0:28:47it swivels its spines quickly to point towards a possible attacker.
0:28:47 > 0:28:51These creatures may seem different from the original crinoids,
0:28:51 > 0:28:54but they all have a radial symmetry and tube feet.
0:28:54 > 0:28:58Although we can't be sure of evolutionary pathways,
0:28:58 > 0:29:01relationships can be plainly seen.
0:29:01 > 0:29:05If the head of a crinoid drops on its face, it becomes a starfish.
0:29:07 > 0:29:10This, thinned down, turns into a brittle star,
0:29:10 > 0:29:15but if it thickens, curls its arms back on itself and grows spines,
0:29:15 > 0:29:18it becomes a sea urchin.
0:29:18 > 0:29:22One group became elongated and lay down on its side to feed.
0:29:22 > 0:29:26It's obvious why it's called a sea cucumber.
0:29:28 > 0:29:32Most of these creatures work their way over the sea floor,
0:29:32 > 0:29:33feeding on detritus.
0:29:33 > 0:29:36A pretty nondescript animal, you might think.
0:29:36 > 0:29:40But their tube feet give the clue to their true relationship.
0:29:47 > 0:29:51This whole group of hydraulically driven creatures
0:29:51 > 0:29:55hasn't produced any swift-moving highly intelligent forms,
0:29:55 > 0:29:57but in their own terms, they've been successful.
0:29:57 > 0:30:00There are about 5,000 species of them,
0:30:00 > 0:30:03and wherever there's a suitable opportunity,
0:30:03 > 0:30:07they miraculously appear, often in great numbers.
0:30:07 > 0:30:10The crown of thorns starfish is normally uncommon.
0:30:10 > 0:30:16But periodically, thousands appear on a reef and start to eat the coral.
0:30:16 > 0:30:20The secret of the group's success lies in their larvae.
0:30:24 > 0:30:27Too small to be noticed by the naked eye,
0:30:27 > 0:30:31these larvae swim in millions in the sea.
0:30:41 > 0:30:44This will eventually become a starfish.
0:30:47 > 0:30:51And this, similar in many ways, turns into a sea cucumber.
0:30:53 > 0:30:55Nearly all marine invertebrates -
0:30:55 > 0:30:57molluscs, sea urchins, worms, corals, jellyfish -
0:30:57 > 0:31:01all reproduce by larval forms like these
0:31:01 > 0:31:05which are swept by the currents into every part of the oceans.
0:31:05 > 0:31:07The vast majority will be eaten by fish.
0:31:07 > 0:31:11Great numbers fail to find a suitable home,
0:31:11 > 0:31:13and simply die and dissolve into nothing.
0:31:13 > 0:31:15But their presence everywhere
0:31:15 > 0:31:18ensures that no suitable corner goes unoccupied.
0:31:21 > 0:31:26The larval sea snails have to support the weight of their developing shells
0:31:26 > 0:31:29with lobes covered by beating cilia.
0:31:43 > 0:31:46The similarities between larval forms
0:31:46 > 0:31:50are just as valid evidence of relationship as those between adults.
0:31:50 > 0:31:54And the fact that this mollusc larva looks like this,
0:31:54 > 0:31:56the larva of a segmented worm,
0:31:56 > 0:32:01is a strong indication the two groups are descended from a common ancestor.
0:32:04 > 0:32:09Eventually, this larva becomes a worm such as this,
0:32:09 > 0:32:13the simplest member of our third group of animals, the segmented ones.
0:32:13 > 0:32:17They probably developed segments in their bodies,
0:32:17 > 0:32:19each with its own pair of movable bristles,
0:32:19 > 0:32:22because it made sustained burrowing easier.
0:32:24 > 0:32:28Soft-bodied animals hardly ever fossilise,
0:32:28 > 0:32:34but in one site in south Australia, in rocks 650 million years old,
0:32:34 > 0:32:36older than those limestones in Morocco,
0:32:36 > 0:32:40have been found what appear to be segmented worms.
0:32:40 > 0:32:43This is one of the earliest records of a soft-bodied animal
0:32:43 > 0:32:45that has ever been found.
0:32:53 > 0:32:56There is one other highly exceptional fossil site
0:32:56 > 0:32:59where soft bodies have left their impressions in the rocks.
0:32:59 > 0:33:03It lies in the heart of the Rocky Mountains in British Columbia.
0:33:06 > 0:33:09In the rocks here, you can get a unique glimpse
0:33:09 > 0:33:12of the animals that crawled around the bottom of the seas
0:33:12 > 0:33:16100 million years after those early Australian ones.
0:33:16 > 0:33:20In fact, at about the same time as those in Morocco.
0:33:30 > 0:33:32These rocks are shales.
0:33:32 > 0:33:36Mudstones, and of the very finest texture.
0:33:36 > 0:33:38From a detailed examination of them,
0:33:38 > 0:33:40we can be pretty sure they were laid down
0:33:40 > 0:33:44at the bottom of the sea about 500 feet deep.
0:33:44 > 0:33:48But this particular patch was a very special one.
0:33:48 > 0:33:54There were virtually no currents, and in consequence no oxygen.
0:33:54 > 0:33:57That meant that no creatures could actually live
0:33:57 > 0:34:00in this little part of the sea bottom.
0:34:00 > 0:34:03There were no scavenging animals, for example,
0:34:03 > 0:34:09and equally, there was no oxygen to fuel the processes of decay.
0:34:09 > 0:34:12So that meant that if any dead creatures
0:34:12 > 0:34:16drifted down to settle on these muds,
0:34:16 > 0:34:21their bodies would remain intact for a very long time.
0:34:21 > 0:34:23And come they did.
0:34:25 > 0:34:30Fine mud settled on top of them and so they were entombed.
0:34:30 > 0:34:36Over millions of years, the mud consolidated to form these shales.
0:34:36 > 0:34:39And here as fossils they have remained,
0:34:39 > 0:34:43miraculously escaping the distortions and crushings
0:34:43 > 0:34:47that happened when these rocks were rucked up by earth movements
0:34:47 > 0:34:49to form the Rocky Mountains.
0:34:49 > 0:34:51And these freak conditions
0:34:51 > 0:34:55have preserved the most delicate of creatures.
0:34:55 > 0:34:59Here, for example, is a little worm.
0:35:06 > 0:35:09Several species of segmented worms have been found,
0:35:09 > 0:35:12and their preservation is so remarkable
0:35:12 > 0:35:15that you can almost count their bristles.
0:35:30 > 0:35:32There's also a group of creatures that,
0:35:32 > 0:35:36while they seem to be related to the segmented worms
0:35:36 > 0:35:39and are rather more complex than they are,
0:35:39 > 0:35:42are nonetheless quite unlike any creatures alive today
0:35:42 > 0:35:45or any other later fossils we know of.
0:35:45 > 0:35:48You might call them experiments in animal design,
0:35:48 > 0:35:50experiments that didn't quite come off.
0:35:50 > 0:35:52They weren't efficient enough
0:35:52 > 0:35:54to survive in the battle for living
0:35:54 > 0:35:56that was becoming increasingly intense.
0:35:56 > 0:35:59Look at this one, for example.
0:35:59 > 0:36:02It appears to have seven pairs of supports,
0:36:02 > 0:36:05and above each, a tentacle with its own mouth.
0:36:07 > 0:36:10Even compared with some of today's strange creatures,
0:36:10 > 0:36:12it seems grotesque and outlandish.
0:36:14 > 0:36:19This five-eyed creature has a long trunk, here bent back along its body.
0:36:19 > 0:36:23It was probably used for detecting and manipulating food.
0:36:25 > 0:36:27This one is of particular interest,
0:36:27 > 0:36:30for it has stumpy little legs down each side.
0:36:30 > 0:36:35In this case, there does seem to be a close living parallel.
0:36:35 > 0:36:38It's not a sea creature but one that lives in moist jungles.
0:36:38 > 0:36:41Peripatus.
0:36:52 > 0:36:56Clearly, segmentation was a great evolutionary success.
0:36:56 > 0:36:59The appendages on each segment becoming more and more specialised
0:36:59 > 0:37:03as legs and gills and mouth parts.
0:37:05 > 0:37:09Some of the commonest fossils here are trilobites,
0:37:09 > 0:37:11like the one we saw in Morocco.
0:37:11 > 0:37:15These had hard shells, part calcium carbonate and part chitin,
0:37:15 > 0:37:18and they fossilised well all over the world,
0:37:18 > 0:37:23for they swarmed everywhere in the seas of 400 to 500 million years ago,
0:37:23 > 0:37:25during November in our "life on Earth" year.
0:37:32 > 0:37:34Because their body armour was not expandable,
0:37:34 > 0:37:39the trilobites had to shed their shells regularly in order to grow.
0:37:39 > 0:37:44Indeed, many trilobite fossils are of these discarded shells.
0:37:44 > 0:37:47Sometimes they occur in great drifts.
0:37:47 > 0:37:50Here, almost entirely the tail ends,
0:37:50 > 0:37:54presumably sorted out by the sea currents as shells are today.
0:37:57 > 0:38:00When the complete animal has been fossilised,
0:38:00 > 0:38:02we can see from various positions
0:38:02 > 0:38:07that some trilobites could roll up for protection, like woodlice today.
0:38:08 > 0:38:13More information can be discovered by X-raying some perfect fossils.
0:38:13 > 0:38:16They even reveal details of the gut
0:38:16 > 0:38:19and muscle fibres inside the animal's body.
0:38:21 > 0:38:25But perhaps the most astounding thing about trilobite fossils
0:38:25 > 0:38:27is the preservation of their eyes.
0:38:27 > 0:38:31Although our knowledge of the internal structure is limited,
0:38:31 > 0:38:36the hard part, the outer lens system, is often fossilised in superb detail.
0:38:36 > 0:38:40Even the earliest trilobites had compound eyes,
0:38:40 > 0:38:44each element providing a part of a mosaic picture,
0:38:44 > 0:38:50which in this species gave the animal an almost spherical field of view.
0:38:50 > 0:38:52If the fossil eye is sliced,
0:38:52 > 0:38:55we can discover how each lens was constructed.
0:38:56 > 0:38:59It was a single crystal of calcite,
0:38:59 > 0:39:02lined up in such a way as to give the clearest image.
0:39:02 > 0:39:05There could be several thousand in each eye.
0:39:05 > 0:39:08Later in their history,
0:39:08 > 0:39:11some trilobites evolved even more sophisticated eyes.
0:39:11 > 0:39:14Here, the lenses are less numerous but larger,
0:39:14 > 0:39:20and it's thought that each provided a separate image instead of a mosaic.
0:39:20 > 0:39:23By slicing one of these fossilised lenses,
0:39:23 > 0:39:25a remarkable discovery has been made.
0:39:25 > 0:39:30The lens is really a doublet. It has an upper and a lower element.
0:39:30 > 0:39:33This is the line of their contact.
0:39:33 > 0:39:36It's almost identical with the design recommended
0:39:36 > 0:39:39by mathematicians in the 17th century
0:39:39 > 0:39:42for correcting spherical aberration in thick lenses.
0:39:42 > 0:39:45Evolution solved the problem for the trilobites
0:39:45 > 0:39:48400 million years before man.
0:39:48 > 0:39:52The doughnut shapes of the lower lens elements
0:39:52 > 0:39:55have been preserved alone in these fossil eyes.
0:39:55 > 0:39:59In most cases, it's the upper lenses that can be seen.
0:40:00 > 0:40:02Although trilobites possessed
0:40:02 > 0:40:05the first sophisticated optical system on earth,
0:40:05 > 0:40:07some species were blind.
0:40:07 > 0:40:10They must have inhabited dark, muddy waters
0:40:10 > 0:40:13where there was no light and no need for eyes.
0:40:15 > 0:40:18The great variety of shape and size in trilobites
0:40:18 > 0:40:22suggests that they had a wide range of habits.
0:40:22 > 0:40:26It's probable that some scavenged their living on the muddy bottom,
0:40:26 > 0:40:29whilst others were quite active swimmer-hunters.
0:40:29 > 0:40:35Finally, some 250 million years ago, their great dynasty came to an end.
0:40:35 > 0:40:39Though one relative managed somehow to hang on.
0:40:39 > 0:40:42This is it. The horseshoe crab.
0:40:50 > 0:40:53It's sufficiently different from a trilobite,
0:40:53 > 0:40:55with this very big head shield,
0:40:55 > 0:40:58for us to put it in a group on its own.
0:40:58 > 0:41:01It's also sufficiently similar for us to be pretty sure
0:41:01 > 0:41:04that the two groups are closely related.
0:41:04 > 0:41:08It's got a pair of these eyes on the front,
0:41:08 > 0:41:11which are mosaic eyes, very like those of a trilobite,
0:41:11 > 0:41:16and underneath it's got a segmented body
0:41:16 > 0:41:19with a pair of legs on each segment.
0:41:21 > 0:41:26And at the front, a fist with a hook on it.
0:41:26 > 0:41:30That is the sign that this is a fully mature male,
0:41:30 > 0:41:34because it uses that in breeding.
0:41:34 > 0:41:38On a few nights in the spring,
0:41:38 > 0:41:43when the moon and the tides are just right, and this is one of them,
0:41:43 > 0:41:48these antique animals crawl up out of the sea to nest here on the beach.
0:41:48 > 0:41:52This male is one of the advance guard,
0:41:52 > 0:41:53but as the night wears on,
0:41:53 > 0:41:56there should be hundreds and thousands of them.
0:42:14 > 0:42:18Horseshoe crabs are found along the eastern seaboard of North America.
0:42:18 > 0:42:20This beach in Delaware Bay
0:42:20 > 0:42:23is the best place to see them in large numbers.
0:42:23 > 0:42:27Here, at least, we can get some idea of what things may have been like
0:42:27 > 0:42:30when their distant relatives, the trilobites,
0:42:30 > 0:42:32swarmed in the seas of long ago.
0:43:05 > 0:43:07At the centre of each mass is a large female,
0:43:07 > 0:43:11and directly behind her, attached by his claws,
0:43:11 > 0:43:13is a male who will fertilise the eggs.
0:43:13 > 0:43:17Other unsuccessful males crowd around.
0:43:23 > 0:43:26The egg mass is laid several inches down in the sand
0:43:26 > 0:43:30and remains there while the tiny larvae develop inside.
0:43:30 > 0:43:33Because of their shape at this early stage,
0:43:33 > 0:43:35they're known as trilobite larvae.
0:43:42 > 0:43:46At the next high tide, a month after the eggs were laid,
0:43:46 > 0:43:48the sea reaches them again.
0:43:52 > 0:43:55The eggs rupture and the larvae swim free.
0:43:55 > 0:43:58Thousands will get eaten within hours.
0:43:58 > 0:44:03But a few will survive to continue this very ancient line.
0:44:07 > 0:44:11Swimming with them are creatures related to those segmented animals
0:44:11 > 0:44:13in the British Columbian shales.
0:44:13 > 0:44:15These survived unobtrusively
0:44:15 > 0:44:18throughout the reign of the trilobites
0:44:18 > 0:44:21but have since come into their own - the crustaceans.
0:44:21 > 0:44:26This is one of them. A copepod with its remarkable simple eye.
0:44:26 > 0:44:31But there are about 3,500 species of crustacean today.
0:44:31 > 0:44:34Most of them have adopted a floating way of life
0:44:34 > 0:44:38and are the staple food of many kinds of fish, as well as of whales.
0:44:45 > 0:44:50They have many lifestyles. Some of them are completely unknown.
0:44:50 > 0:44:53This creature has, on a number of times,
0:44:53 > 0:44:55been seen holding a tiny jellyfish.
0:44:55 > 0:44:59Is it using the jellyfish's stinging cells as protection?
0:45:02 > 0:45:05Or is there some other relationship between the two?
0:45:05 > 0:45:07Whatever their way of life,
0:45:07 > 0:45:10all crustaceans have one problem in common:
0:45:10 > 0:45:13the same one the trilobites had.
0:45:13 > 0:45:15Their external skeleton won't expand.
0:45:15 > 0:45:19So if the animal is to grow, it must be shed.
0:45:20 > 0:45:24First, it extracts some of the important salts from its skeleton
0:45:24 > 0:45:27and reabsorbs them into its bloodstream.
0:45:27 > 0:45:30Then it begins to moult.
0:46:00 > 0:46:04Its new skeleton is soft and crumpled, but it quickly expands.
0:46:04 > 0:46:07For a while, the animal is vulnerable,
0:46:07 > 0:46:09but as the salts are slowly fed back into it,
0:46:09 > 0:46:11the shell hardens.
0:46:17 > 0:46:19In spite of its problems,
0:46:19 > 0:46:22an external skeleton as developed by the crustaceans
0:46:22 > 0:46:26is clearly a very effective and efficient way of building a body.
0:46:26 > 0:46:30And nothing could demonstrate its potential better
0:46:30 > 0:46:33than creatures that live in this bay off the coast of Japan.
0:46:33 > 0:46:37Because down on the sea bottom, 600 metres down,
0:46:37 > 0:46:40there live the largest crabs in the world.
0:46:40 > 0:46:43And this boat is fishing for them right now.
0:47:10 > 0:47:13Without the support of water, its long legs flop.
0:47:13 > 0:47:17The muscles are not strong enough to hold them rigid in air.
0:47:34 > 0:47:39Each leg is a tube down which a strand of muscle runs.
0:47:39 > 0:47:42The muscle is attached to a projection from the next joint
0:47:42 > 0:47:46so that when the muscle contracts, the joint moves.
0:47:46 > 0:47:49It's rather like the arm of an industrial crane,
0:47:49 > 0:47:52which has an outer network of steel girders
0:47:52 > 0:47:55down which a wire hawser runs.
0:47:55 > 0:47:59Of course, the one sort of joint you can't put on such a system
0:47:59 > 0:48:03is a ball-and-socket joint which gives a sort of universal movement
0:48:03 > 0:48:06that I can have in my shoulder or my thigh.
0:48:06 > 0:48:07But the crab deals with that
0:48:07 > 0:48:10by having each joint working in different planes.
0:48:10 > 0:48:12So one way or another,
0:48:12 > 0:48:17it can reach almost anything within its immediate neighbourhood
0:48:17 > 0:48:22and convey it with its pincers to the mouth, where it's chewed up.
0:48:23 > 0:48:28Its body is protected by this heavy armour of shell
0:48:28 > 0:48:30and the crab can tell what's going on around it
0:48:30 > 0:48:34because through the armour there project tiny little sensory bristles.
0:48:34 > 0:48:36This creature is indeed spectacular,
0:48:36 > 0:48:40but every now and again from the waters of this bay,
0:48:40 > 0:48:43the fishermen bring up a real giant.
0:48:43 > 0:48:47And creatures like this are over 11 feet across.
0:48:50 > 0:48:54Most crustaceans, however, are of a more modest size.
0:48:54 > 0:48:57Apart from the myriads of tiny ones in the ocean,
0:48:57 > 0:49:01there are vast numbers of small crabs and prawns and shrimps,
0:49:01 > 0:49:04all with specialised ways of life.
0:49:04 > 0:49:06All these, for example,
0:49:06 > 0:49:09come from just one small patch of the Great Barrier Reef.
0:49:17 > 0:49:22Crustaceans use pigments for camouflage in the most elegant way.
0:49:31 > 0:49:34Some, in fact, are very difficult to see at all
0:49:34 > 0:49:36unless photographed in close-up.
0:49:42 > 0:49:46The crustaceans show clearly what advantages can come
0:49:46 > 0:49:49from having a body divided into segments.
0:49:49 > 0:49:53Each can bear appendages, and the crustaceans have modified them
0:49:53 > 0:49:56into many different tools.
0:50:00 > 0:50:03Sometimes they're used for respiration,
0:50:03 > 0:50:05sometimes for reproduction,
0:50:05 > 0:50:10some as antennae, mouth parts, food manipulators, pincers
0:50:10 > 0:50:12and, of course, legs.
0:50:30 > 0:50:34An external jointed skeleton has one quality I've not yet mentioned.
0:50:34 > 0:50:39Mechanically, it works just as well on land as it does in water.
0:50:39 > 0:50:41So from that point of view you might say
0:50:41 > 0:50:44the crustaceans are pre-adapted to life on land,
0:50:44 > 0:50:48and indeed, one group has made the move.
0:50:48 > 0:50:50Quite formidable animals they are too.
0:50:50 > 0:50:54This is a rubber crab.
0:50:54 > 0:50:57I must handle him with some care
0:50:57 > 0:51:01because you can get quite a nip from these pincers.
0:51:01 > 0:51:06He uses them to cut down young coconuts, on which it feeds.
0:51:06 > 0:51:10It's said that he can even hammer a hole into a mature coconut,
0:51:10 > 0:51:13though no-one's actually seen him do it.
0:51:13 > 0:51:20He breathes through a chamber at the back of the shell here.
0:51:20 > 0:51:22It doesn't contain gills,
0:51:22 > 0:51:27but the oxygen is absorbed through the puckered lining of the chamber.
0:51:27 > 0:51:32So here's a creature that can breathe on land, move on land, eat on land.
0:51:32 > 0:51:36It's true, it has to go back into the sea in order to breed,
0:51:36 > 0:51:41but otherwise, it's a fully operational land-living animal.
0:51:52 > 0:51:55Other descendants of sea-living invertebrates
0:51:55 > 0:51:57have also made the move onto land at various times.
0:51:57 > 0:51:59Snails, for example.
0:51:59 > 0:52:01Though robbed of the support of water,
0:52:01 > 0:52:04they're never able to grow their shells on land
0:52:04 > 0:52:06as big as they do in the sea.
0:52:06 > 0:52:10It's the segmented animals that have adapted best to land.
0:52:10 > 0:52:14And of all those, it's the ones that did it first
0:52:14 > 0:52:16who have been most spectacularly successful.
0:52:16 > 0:52:22The insects. They emerged some 400 million years ago
0:52:22 > 0:52:27and they wrote the next great chapter in the history of life on Earth.