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00:00These are the waters of Catalina, a tiny island 20 miles off the coast of Los Angeles, California.
00:18These are kelp forests. They grow here in tremendous abundance because the waters here around Catalina are extremely rich in nutrients.
00:33That's because of the California current, which brings this beautiful, rich, cold water up from the depths of the Pacific and allows this tremendously rich ecosystem to grow.
00:48It's a remarkable place.
00:56Oh, look!
00:58And I'm not here to marvel at these kelp forests, beautiful as they are.
01:20I'm here to search for a little animal that lives not in this forest of nutrients, but out there in the muddy ocean floor.
01:30There he is, look!
01:53Camouflaged in its burrow on the seafloor, the mantis shrimp is a seemingly unremarkable creature.
02:04It's not a real shrimp, but a type of crustacean called a stomatopod.
02:10I've come to see it because in one way, the mantis shrimp is truly extraordinary.
02:17The way it detects the world.
02:23You see those big eyes in the sea?
02:30These are some of the most sophisticated eyes in the natural world.
02:37Each is made up of over 10,000 hexagonal lenses.
02:42And with twice as many visual pigments as any other animal, it can see colors and wavelengths of light that are invisible to me.
02:53These remarkable eyes give the mantis shrimp a unique view of the ocean.
02:59And this is just one of the many finely tuned senses that have evolved across the planet.
03:06Sensing the ability to detect and to react to the world outside is fundamental to life.
03:13Every living thing is able to respond to its environment.
03:18In this film, I want to show you how the senses developed.
03:23How the mechanisms that gather information about the outside world evolved.
03:27How their emergence has helped animals thrive in different environments.
03:32And how the senses have pushed life in new directions and may ultimately have led to our own curiosity and intelligence.
04:27These are the woods of Kentucky.
04:47The first stop on a journey across America that will take me from the far west coast to the Atlantic through the heart of the country.
04:58It's the animals that I'll find on the way that will illuminate the world of the senses.
05:06And I'm going to start by going deep underground.
05:10These are the mammoth caves in Kentucky.
05:25With over 300 miles of mapped passages, they're the longest cave system in the world.
05:41But this is also the place to start exploring our own senses.
05:47We're normally dependent on our sight, but down here in the darkness, it's a very different world.
05:54And I have to rely on my other senses to build a picture of my environment.
06:00It's completely dark in this cave. I can't see anything at all.
06:09You can see me because we're lighting it with infrared light.
06:14And that's at a wavelength that my eyes are completely insensitive to.
06:18So as far as I'm concerned, it is pitch black.
06:23And because it's so dark, your other senses become heightened, particularly hearing.
06:33It's virtually silent in here.
06:39But if you listen carefully, you can just hear the faint drop of water from somewhere deep in the cave system.
06:51You'd never hear that if the cave were illuminated.
06:55But you focus on your hearing when it's as dark as this.
07:00Now, as well as sight and hearing, we have, of course, a range of other senses.
07:06There's touch, which is really a mixture of sensations, temperature and pressure and pain.
07:13And then there are chemical senses, so smell and taste.
07:18And we share those senses with almost every living thing on the planet today.
07:23Because they date back virtually to the beginning of life on Earth.
07:39And even here, in water that's been collected from deep within a cave,
07:44there are organisms that are still alive.
07:47And even here, in water that's been collected from deep within a cave,
07:52there are organisms that are detecting and responding to their environment
07:57in the same way that living things have been doing for over a billion years.
08:18Ah, there it is.
08:22Now that is a paramecium.
08:25It may look like a simple animal, but in fact it's a member of a group of organisms called protists.
08:33And you'd have to go back around two billion years to find a common ancestor.
08:38Paramecia have probably changed little in the last billion years.
08:45And although they appear simple, these tiny creatures display some remarkably complex behaviour.
08:52They have a very unique sense of smell.
08:56They have a very unique sense of taste.
09:00They have a very unique sense of smell.
09:03Although they appear simple, these tiny creatures display some remarkably complex behaviour.
09:11You can even see them responding to their environment.
09:16The cell swims around, powered by a cohort of cilia.
09:20Tiny hairs embedded in the cell membrane.
09:24If it bumps into something, the cilia change direction and it reverses away.
09:33They're clearly demonstrating a sense of touch.
09:41Even though they're single-celled organisms, they have no central nervous system,
09:47they can still do what all life does.
09:50They can sense their environment, and they can react to it.
09:54And they do that using electricity.
10:06The mechanism that powers the paramecium's touch response lies at the heart of all sensing in animals.
10:13And it's based on an electrical phenomenon, phytonutrients.
10:17And it's based on an electrical phenomenon found throughout nature.
10:24An electric current is a flow of electric charge.
10:28And for that to happen, you need an imbalance between positive and negative charges.
10:33Now, usually in nature, things are electrically neutral.
10:37The positive and negative charges exactly balance out.
10:41But there are natural phenomena in which there is a separation of electric charge.
10:47A thunderstorm, for example.
10:51As thunderclouds build, updrafts within them separate charge.
10:57The lighter ice and water crystals become positively charged and are carried upwards,
11:02while the heavier, negatively charged crystals sink to the bottom.
11:08This can create a potential difference,
11:11a voltage between the cloud and the ground of as much as 100 million volts.
11:18Now, nature abhors a gradient.
11:23It doesn't like an imbalance, and it tries to correct it by having an electric current flow.
11:29In the case of a thunderstorm, that's a bolt of lightning.
11:48And it's the same process that governs the paramecium's behaviour, but on a tiny scale.
11:56In common with virtually all other cells, and certainly all animal cells,
12:01the paramecium maintains a potential difference across its cell membrane.
12:07And it does that, in common with a thunderstorm, by charge separation.
12:11By manipulating the number of positive ions inside and outside its membrane,
12:16the paramecium creates a potential difference of just 40 millivolts.
12:23So when a paramecium is just sat there, not bumping into anything, floating in this liquid,
12:29then it's like a little battery.
12:32It's maintaining a potential difference across its cell membrane.
12:35And it can use that to sense its surroundings.
12:40When it bumps into something, its cell membrane deforms,
12:44opening channels that allow positive ions to flood back across the membrane.
12:50As the potential difference falls, it sets off an electrical pulse
12:55that triggers the cilia to start beating in the opposite direction.
12:59That electrical pulse spreads around the whole cell in a wave called an action potential.
13:07And the paramecium reverses out of trouble.
13:13Now, this ability to precisely control flows of electric charge across a membrane
13:20is not something that can be done by humans.
13:22Now, this ability to precisely control flows of electric charge across a membrane
13:29is not unique to the paramecium.
13:32It actually lies at the heart of all animal senses.
13:36In fact, every time I sense anything in the world with my eyes, with my ears, or with my fingers,
13:43at some point between that sensation and my brain,
13:48something very similar to that will happen.
13:52And that's what the paramecium does.
14:07Although the same electrical mechanism underpins all sensing,
14:12every animal has a different suite of sensory capabilities
14:17that is beautifully adapted to the environment it lives in.
14:22This is the Big Black River, a tributary of the mighty Mississippi in America's deep south.
14:33And these dark and murky waters are home to a ferocious predator.
14:43Even though it's impossible to see more than a couple of inches through the water,
14:47this predator has found a way to track down and catch its prey with terrifying efficiency.
14:53To help me catch one, I've enlisted the support of wildlife biologist, Don Jackson.
14:58How do you feel?
15:01Do you feel any pain?
15:03It's okay, it was just a dream.
15:06We had to renew our contract.
15:09How do you feel?
15:12I feel a little better.
15:15Oh!
15:17What's happened to you?
15:20Nothing.
15:23Let me go and see what happened!
15:25What is it?
15:34That's the...
15:37What is it?
15:41You're going to wrestle it to me.
15:44Wrestle it now.
15:47It's going to go right here.
15:48Is it?
15:56There you go.
15:57And he can bite.
16:02We'll show you the mouth of this thing.
16:05Head on.
16:06So you can see what the prey sees when he comes.
16:11Anything that will fit in that mouth, he'll grab it.
16:13Yeah.
16:14And you can hold him if you just want to put your hands all the way under him.
16:17Come all the way.
16:18Come all the way.
16:19Hold him up close to you.
16:20Yeah.
16:23How about that?
16:24How about that?
16:25You've got him.
16:26Yeah.
16:30This is the top predator in this river.
16:32This is a, what, a 25-pound flathead catfish.
16:37You see those protrusions from his head.
16:40Those are barbels.
16:41They sense vibration in the mud on the riverbed.
16:45But the most interesting thing about the catfish
16:48is that she really is, in some ways, one big tong.
16:52There are taste sensors covering every part of her body.
16:56And she can build up a three-dimensional picture of the river
16:59by detecting the chemical scents of animals.
17:03So her eyes are not much use.
17:05As you can see, this river's extremely muddy.
17:07But it's the sense of taste that does the job of building up a picture of the world.
17:13And that's how he hunts, and he weighs a ton.
17:17Oh, I can feel those teeth.
17:19Ow!
17:21I'm going to let go.
17:23All right, you. Go on.
17:30The sensory world of the catfish is a remarkable one.
17:35Its map of its universe is built from the thousands of chemicals
17:39it can detect in the water.
17:42A swirling mix of tastes and concentrations, flavours and gradients.
17:48It's a world we can hardly imagine.
17:56There's an interesting, almost philosophical point here,
17:59because it's easy to imagine that we humans perceive the world
18:02in some kind of objective way.
18:04But that's not the case at all.
18:06If you think about the catfish,
18:08the catfish sees the world as a kind of swarm of chemicals in the river,
18:13or vibrations on the riverbed.
18:15Whereas we see the world as reflected light off the forest,
18:19and I can hear the sounds of animals out there somewhere in the undergrowth.
18:24The catfish sees the world completely differently.
18:27So the way you perceive the world is determined by your environment.
18:32And no two animals see the world in the same way.
18:36And no two animals see the world in the same way.
18:52Like every animal, we have evolved the senses
18:55that enable us to live in our environment.
19:00But as well as equipping us for the present,
19:03those senses can also tell us about our past.
19:14Now, we have a sense of touch, like the paramecium,
19:17and we have the chemical senses of taste and smell, like the catfish.
19:22But for us, the dominant senses are here.
19:26For us, the dominant senses are hearing and sight.
19:31And to understand them,
19:33we first have to understand our evolutionary history.
19:49And that's why I'm in the Mojave Desert in California,
19:53to track down an animal that can tell us something
19:56about the origins of our own senses.
20:10The creature I'm looking for is easiest to find in the dark,
20:14using ultraviolet light.
20:24Whoa!
20:26Man!
20:28Did you see that?
20:33Look at that. Absolutely bizarre.
20:36It's glowing absolutely bright green.
20:39Nobody has any idea what evolutionary advantage that confers.
20:47Although they now live in small groups,
20:51although they now live in some of the driest,
20:54most hostile environments on Earth, like here in the desert,
20:58scorpions evolved as aquatic predators
21:01before emerging onto the land about 380 million years ago.
21:09They've adapted to be able to survive the extreme heat
21:12and can go for over a year without food or water.
21:16And despite their fearsome reputation,
21:1998% of scorpion species have a sting that is no worse than a bee's.
21:27But perhaps the most fascinating thing about scorpions
21:30from an evolutionary perspective
21:32is the way that they catch their prey.
21:35You see that he spreads his legs out on the surface of the sand,
21:40and that's because he uses his legs to detect
21:44vibrations.
21:50Scorpions hunt insects like this beetle.
21:54It's almost impossible to see them in the dark,
21:57so the scorpion has evolved another way to track them down,
22:02by adapting its sense of touch.
22:09As the insect's feet move across the sand,
22:12they set off tiny waves of vibration through the ground.
22:17If just a single grain of sand is disturbed within range of the scorpion,
22:22it will sense it through the tips of its legs.
22:29They can detect vibrations that are around the size of a single atom
22:35as they sweep past.
22:38By measuring the time delay between the waves arriving at each of its feet,
22:44the scorpion can calculate the precise direction and distance to its prey.
22:50And by measuring the time delay between the waves arriving at each of its feet,
22:56the scorpion can calculate the precise direction and distance to its prey.
23:21SCORPION
23:38And that ability to detect vibrations and use them to build up a picture of our surroundings
23:45is something that we share with scorpions.
23:51While the scorpion has adapted its sense of touch to detect vibrations in the ground,
23:58we use a very similar system to detect the tiny vibrations in air that we call sound.
24:06And like the scorpions, ours is a remarkably sensitive system.
24:12Our ears can hear sounds over a huge range.
24:21We can detect sound waves of very low frequency at the base end of the spectrum.
24:31But we can also hear much higher pitched sounds,
24:35sounds with frequencies hundreds or even a thousand times greater.
24:42And we can detect huge changes in sound intensity.
24:50From the delicate buzzing created by an insect's flapping wings,
24:59to the roar of an engine, which can be a hundred million times louder.
25:06SCORPION
25:13The story of how we developed our ability to hear is one of the great examples of evolution in action.
25:21Because the first animals to crawl out of the water onto the land
25:25would have had great difficulty hearing anything in their new environment.
25:36These are the Everglades.
25:43A vast area of swamps and wetlands that has covered the southern tip of Florida for over 4,000 years.
26:06Through the creatures we find here, like the American alligator, a member of the crocodile family,
26:13we can trace the story of how our hearing developed as we emerged onto the land.
26:24And it starts below the water, with the fish.
26:30If you're a fish, then hearing isn't a problem.
26:34You live in water and you're made of water, so sound has no problem at all travelling from the outside to the inside.
26:41But when life emerged from the oceans onto the land, then hearing became a big problem.
26:49See, sound doesn't travel well from air into water.
26:53If I make a noise now, then over 99.9% of the sound is reflected back off the surface of the water.
27:04It's because of that reflection that underwater you can hear very little from above the surface.
27:10And it's exactly the same problem that our ears face, because they too are filled with fluid.
27:18So, if evolution hadn't found an ingenious solution to the problem of getting sound from air into water,
27:27then I wouldn't be able to hear anything at all.
27:32And that solution relies on some of the most delicate moving parts in the human body.
27:39If I just drop them...
27:42Hang on a second.
27:44Oh, I've done it again.
27:46Bloody hell. Idiot.
27:49I just flipped out.
27:54These are the smallest three bones in the human body.
27:58They're called the malleus, the incus and the stapes.
28:02And they sit between the eardrum and the entrance to your inner ear, to the place where the fluid sits.
28:12The bones help to channel sound into the ear through two mechanisms.
28:19First, they act as a series of levers, magnifying the movement of the eardrum.
28:26And second, because the surface area of the eardrum is 17 times greater than the footprint of the stapes,
28:33the vibrations are passed into the inner ear with much greater force.
28:39And that has a dramatic effect.
28:42Rather than 99.9% of the sound energy being reflected away,
28:49it turns out that with this arrangement, 60% of the sound energy is passed from the eardrum into the inner ear.
29:00Now, this set-up is so intricate and so efficient
29:03that it almost looks as if these bones could only ever have been for this purpose.
29:09But in fact, you can see their origin if you look way back in our evolutionary history.
29:19In order to understand where that collection of small bones in our ears came from,
29:25you have to go back in our evolutionary family tree, way beyond the fish that we see today.
29:31In fact, back around 530 million years,
29:35to when the oceans were populated with jawless fish called Aenathans.
29:40They're similar to the modern lamprey.
29:42Now, they didn't have a jaw, but they had gills supported by gill arches.
29:48Now, over a period of around 50 million years,
29:52the most forward of those gill arches migrated forward in the head to form jaws.
30:02And you see fish like these, the first jawed fish in the world,
30:08And you see fish like these, the first jawed fish in the fossil record,
30:13around 460 million years ago.
30:16And there, at the back of the jaw, there is that bone, the higher mandibular,
30:22supporting the rear of the jaw.
30:25Then, around 400 million years ago,
30:28the first vertebrates made the journey from the sea to the land.
30:32Their fins became legs.
30:35But in their skull and throat, other changes were happening.
30:39The gills were no longer needed to breathe the oxygen in the atmosphere.
30:44And so, they faded away and became different structures in the head and throat.
30:50And that bone, the higher mandibular, became smaller and smaller,
30:56until its function changed.
31:00It now was responsible for picking up vibrations in the jaw
31:05and transmitting them to the inner ear of the reptiles.
31:09And that is still true today of our friends over there, the crocodiles.
31:25Once more with alligator.
31:30But even then, the process continued.
31:34Around 210 million years ago, the first mammals evolved.
31:39And unlike our friends the reptiles here,
31:43mammals have a jaw that's made of only one bone.
31:47A reptile's jaw is made of several bones fused together.
31:52So, that freed up two bones, which moved and shrank,
32:01and eventually became the malleus, the incus and stapes.
32:09So, this is the origin of those three tiny bones
32:12that are so important to mammalian hearing.
32:16It's quite big, isn't it?
32:46I think this is a most wonderful example of the blind, undirected ingenuity of evolution.
32:58That it's taken the bones in gills of fish
33:02and converted them into the intricate structures inside my ears
33:06that efficiently allow sound to be transmitted from air into fluid.
33:11It's a remarkable thought
33:14that to fully understand the form and function of my ears,
33:17you have to understand my distant evolutionary past in the oceans of ancient Earth.
33:25We're hunting for a man-fish-european.
33:46All sensing has evolved to fulfil one simple function,
33:50to provide us with the specific information we need to survive.
34:02And nowhere is that clearer than in the sense of vision.
34:14Almost all animals can see.
34:1796% of animal species have eyes.
34:21But what those eyes see varies enormously.
34:27So, with an animal like the mantis shrimp,
34:30you have to ask what it is about its way of life
34:33that demands such a complex visual system.
34:38I've got to be very quick and very careful with this.
34:42Let him out.
34:46The complex structure of the mantis shrimp's eyes
34:50give it incredibly precise depth perception.
34:55We have binocular vision.
34:58We look with two eyes from slightly different angles
35:01and judge distance by comparing the eyes.
35:06Each of the mantis shrimp's eyes has trinocular vision.
35:11Each eye takes three separate images of the same object.
35:16Comparing all three gives them exceptionally precise range-finding.
35:22And they need that information to hunt their prey.
35:28Despite appearances, he's a dangerous animal.
35:32He has one of the hardest punches in nature.
35:36Those yellow appendages you can see on the front of his body
35:40are called raptoral appendages.
35:42They're actually highly evolved front legs
35:45and they can punch with tremendous force.
35:49The mantis shrimp's punch is one of the fastest movements
35:52in the animal world.
35:56Slowed down by over a thousand times,
35:59we can clearly see its power.
36:03It can release its legs with the force of a bullet.
36:08In the wild, they use that same force
36:11to attack the prey with their claws.
36:16In the wild, they use that punch
36:19to break through the shells of their prey.
36:22But it could easily break my finger.
36:26The need to precisely deploy this formidable weapon
36:30is one of the reasons the mantis shrimp
36:33has developed its complex range-finding ability.
36:39And that punch can also help explain
36:42their sophisticated colour vision.
36:45Because the coloured flashes on their body
36:48warn other mantis shrimp that they may be about to attack,
36:52while other colour signals have a quite different meaning.
36:57And yet reading these signals in the ocean
37:00can be surprisingly difficult.
37:05In the deep ocean, colours shift from minute to minute,
37:09from hour to hour, with changing lighting conditions,
37:12changing conditions in the ocean.
37:14But it's thought that even though the light quality
37:17can change tremendously, the mantis shrimp
37:19can still identify specific colours very accurately
37:23because of their sophisticated eyes.
37:29The mantis shrimp's eyes are beautifully tuned to their needs,
37:34but they're very different from our eyes.
37:37With their thousands of lenses and complex colour vision,
37:41they have a completely different way of viewing the world.
37:46And yet there's strong evidence that the mantis shrimp's eyes
37:50and ours share a common origin.
37:56Because on a molecular level,
37:58every eye in the world works in the same way.
38:15In order to form an image of the world,
38:17then obviously the first thing you have to do is detect light.
38:22And I have a sample here of the molecules that do that,
38:28that detect light in my eye.
38:31It's actually specifically the molecules
38:33that's in the black and white receptor cells in my eyes, the rods.
38:38It's called redoxin.
38:40And the moment I expose this to light,
38:43you'll see an immediate physical change.
38:48There you go.
38:50Did you see that? It was very quick.
38:52It came out very pink indeed, and it immediately went yellow.
38:56This subtle shift in colour is caused by the redoxin molecule
39:01changing shape as it absorbs the light.
39:04In my eye, what happens is that change in structure
39:09triggers an electrical signal,
39:11which ultimately goes all the way to my brain,
39:14which forms an image of the world.
39:18It's this chemical reaction
39:20that's responsible for all vision on the planet.
39:26Closely related molecules lie at the heart of every animal eye.
39:31And that tells us that this must be a very ancient mechanism.
39:40To find its origins,
39:42we must find a common ancestor
39:44that links every organism that uses redoxin today.
39:48We know that common ancestor must have lived
39:51before all animals' evolutionary lines diverged.
39:56But it may have lived at any time before then.
40:02So what is that common ancestor?
40:04Well, here's where we approach the cutting edge of scientific research.
40:08The answer is that we don't know for sure.
40:11But a clue might be found here,
40:15in these little green blobs,
40:18which are actually colonies of algae,
40:22algae called volvox.
40:26We have very little in common with algae.
40:29We've been separated in evolutionary terms for over a billion years.
40:34But we do share one surprising similarity.
40:38These volvox have light-sensitive cells that control their movement.
40:43And the active ingredient of those cells is a form of redoxin,
40:48so similar to our own that it's thought they may share a common origin.
40:57What does that mean?
40:59Does it mean that we share a common ancestor with the algae,
41:04and in that common ancestor, the seeds of vision can be found?
41:11To find a source that may have passed this ability to detect light
41:15to both us and the algae,
41:17we need to go much further back down the evolutionary train,
41:25to organisms like cyanobacteria.
41:28They were among the first living things to evolve on the planet.
41:32And it's thought that the original redoxins
41:34may have developed in these ancient photosynthetic cells.
41:41So the origin of my ability to see
41:45may have been well over a billion years ago
41:49in an organism as seemingly simple as a cyanobacterium.
41:56The basic chemistry of vision may have been established for a long time,
42:01but it's a long way from that chemical reaction
42:04to a fully-functioning eye that can create an image of the world.
42:12The eye is a tremendously complex piece of machinery
42:15and it's thought to have evolved over a million years.
42:20The eye is a tremendously complex piece of machinery
42:23built from lots of interdependent parts,
42:26and it seems very difficult to imagine
42:28how that could have evolved in a series of small steps.
42:32But actually, we understand that process very well indeed.
42:36I can show you by building an eye.
42:50The first step in building an eye
42:53would be to take some kind of light-sensitive pigment,
42:56redoxin, for example, and build it onto a membrane.
43:01So imagine this is such a membrane with the pigment cells attached.
43:05Then immediately you have something that can detect
43:09the difference between dark and light.
43:13Now, the advantage of this arrangement
43:15is that it's very sensitive to light.
43:17There's no paraphernalia in front of the retina to block light.
43:22But the disadvantage, as you can see,
43:25is that there's no image formed at all.
43:27It just allows you to tell the difference between light and dark.
43:31But you can improve that a lot by adding an aperture,
43:38a small hole in front of the retina.
43:41So this is a movable aperture,
43:44just like the sort of thing you've got in your camera.
43:48And now you can see that the image gets sharper.
43:55But the problem is that in order to make it sharper,
43:58you have to narrow down the aperture,
44:01and that means that you get less and less light,
44:04so this eye becomes less and less sensitive.
44:07So there's one more improvement that nature made,
44:11which is to replace the pinhole, the simple aperture,
44:17with a lens.
44:25Look at that.
44:28A beautifully sharp image.
44:34The lens is the crowning glory of the evolution of the eye.
44:39By bending light onto the retina, it allows the aperture to be opened,
44:43letting more light into the eye,
44:46and a bright, detailed image is formed.
45:03Our eyes are called camera eyes,
45:06because like a camera, they consist of a single lens
45:10that bends the light onto the photoreceptor
45:13to create a high-quality image of the world.
45:18But that has a potential drawback,
45:20because to make sense of all that information,
45:23we need to be able to process it.
45:26Each one of my eyes contains
45:28over 100 million individual photoreceptor cells.
45:31I mean, that's about five or ten times the number
45:34in the average digital camera.
45:36So if my visual system worked
45:38by just taking a series of individual still images of the world
45:42and transmitting all that information to my brain,
45:45then my brain would be overwhelmed.
45:47It's just not practical.
45:49So that's not what animals do.
45:51Instead, their visual systems have evolved
45:54to extract only the information that's necessary.
46:04And this is wonderfully illustrated in the toad.
46:09The toad has eyes that are structurally very similar to ours.
46:15But much of the time, it's as if it isn't seeing anything at all.
46:20It seems completely oblivious to its surroundings...
46:26..until something, like a mealworm, takes its interest.
46:32If you think about what's important to a toad visually,
46:36then it's the approach of either prey or predators.
46:40So the toad's visual system is optimised to detect them.
46:45So there, we put a worm in front of the toad.
46:49And did you see that?
46:51Incredibly quickly, the toad ate the worm.
46:55As soon as the mealworm wriggles in front of the toad,
46:59it will lock onto its target.
47:02Then it strikes in a fraction of a second.
47:09It's an astonishingly precise reaction,
47:12but it's also a very simple one,
47:15because the toad is only focusing on one property of the mealworm,
47:19the way it moves.
47:22These 1970s lab tests show
47:25how a toad will try and eat anything long and thin,
47:29but only if it moves on its side, like a worm.
47:35And that's because the toad has neural circuits in its retina
47:39that only respond to lengthwise motion.
47:42If, instead, the target is rotated into an upward motion,
47:48if, instead, the target is rotated into an upright position,
47:52the toad doesn't respond at all.
48:09At first sight, the visual system of the toad
48:12seems a little bit primitive and imperfect.
48:15And it is true that if you put a toad in a tank full of dead worms,
48:19it'll starve to death because they're not moving,
48:22so it doesn't recognise them as food.
48:25But it doesn't need to see the world in all the detail that I see it.
48:29What it needs to focus on is movement,
48:32because if it can see movement, then it can survive,
48:36because it can avoid predators and it can eat its prey.
48:39I suppose, in a sense, if it moves like a worm,
48:43in nature, then it's likely to be a worm.
48:57This ability to simplify the visual world
49:00into the most relevant bits of information
49:03is something that every animal does.
49:06We do it all the time.
49:08We also have visual systems that detect motion.
49:12Others identify edges and faces.
49:17But extracting more information takes more processing power.
49:21That requires a bigger brain.
49:25And to see the results of this evolutionary drive
49:28towards greater processing power,
49:30I've come to the heart of metropolitan Florida.
49:35You know, it may not look like it,
49:37but underneath this flyover, just out in the shallow water,
49:40it's one of the best places in the world
49:42to find particularly interesting animals.
49:46It's an animal that's evolved to make the most
49:49of the information its eyes can provide.
49:59Well, what we're going to do is try and hunt for some octopus.
50:05And it's, as you say in physics, non-trivial.
50:10Because they develop in a beautiful way.
50:13They camouflage in themselves.
50:18They change colour.
50:20They have cells in their skin that change colour
50:23to match their surroundings.
50:25It's an ability that we don't possess, of course.
50:28It makes it difficult to find.
50:41There he is. Look.
50:47He went flying into there.
50:49A crab and a load of fish went flying out.
50:51Look at his ink.
50:53A defence mechanism.
50:55I don't know where he is. He's hiding somewhere in there.
51:05Look at those colours.
51:07What a remarkable creature.
51:11Although the octopus is a mollusk, like slugs and snails,
51:15in many ways it seems more similar to us.
51:18Whoa!
51:21It's believed to be the most intelligent invertebrate.
51:24Like he's holding his fists up.
51:27Look at that.
51:29Its brain contains about 500 million nerve cells,
51:33about the same as a dog's.
51:35What are you doing?
51:40You know, if you do want an example of an alien intelligence here on Earth,
51:45that must surely be it.
51:48And it's used that brain to develop some remarkable abilities.
51:56It's become a skilled mimic.
51:58It can rapidly change not only its colour,
52:01but its shape to match the background.
52:11Whoa!
52:18Some species even do impressions of other animals.
52:29They've become culling predators and adept problem solvers.
52:36They've even been reported to use tools.
52:41All these skills are signs of great intelligence.
52:45But they also rely on an acute sense of vision.
52:49Look at those big eyes.
52:52There you go, that's random.
52:55Checking us out.
52:58Camera eyes, just like mine,
53:00and they're vitally important for allowing the octopus to live the lifestyle it does.
53:06It's a visual animal in the same way that I'm a visual animal.
53:14The octopus is one of the only invertebrates to have complex camera eyes.
53:22Like our eyes, they capture detailed images of the world,
53:27and their brains have evolved to be able to extract the most information from those images.
53:34The optic lobes make up about 30% of the octopus's brain.
53:40The only other group that is known to devote so much of its brain to visual processing is our group,
53:47the primates, the most intelligent vertebrates.
53:54I think it's a fascinating thought
53:57that intelligence is a result of the need to process all the information
54:03from those big, complex eyes.
54:09What's so compelling about the octopus's intelligence
54:13is that it evolves completely separately to ours.
54:18We last shared a common ancestor 600 million years ago,
54:23an ancestor that had neither eyes nor a brain.
54:29But we've both evolved sophisticated camera eyes and large, intelligent brains.
54:37It suggests a tantalising link between sensory processing and the evolution of intelligence.
54:46SENSING
54:55Sensing has played a key role in the evolution of life on Earth.
55:04The first organisms were able to detect and respond to their immediate environment,
55:09as Paramecia do today.
55:14But as animals evolved and their environments became more complex,
55:19their senses evolved with them,
55:23developing the mechanisms to let them decode vibrations and detect light,
55:29allowing them to build three-dimensional pictures of their environments
55:35and stimulating the growth of brains that could handle all that data.
55:44SENSING
55:49But for one species, the desire to gather more and more sensory information
55:54has become overwhelming.
56:01That species is us.
56:06SENSING
56:19This is the closest thing to hallowed ground that exists in a subject that has no sense,
56:24because that telescope is the one that Edwin Hubble used to expand our horizons,
56:29I would argue, more than anyone else before or since.
56:35SENSING
56:45In 1923, Edwin Hubble took this photograph of the Andromeda Galaxy.
56:50You can see his handwriting on the photograph.
56:52He did it by sitting here night after night for over a week, exposing this photographic plate.
56:58Now, at the time, it was thought that this misty patch you see in the night sky
57:03was just a cloud, maybe a gas cloud in our own galaxy.
57:07But Hubble, because of the power of this telescope, identified individual stars,
57:12and crucially, he found that it was way outside our own galaxy.
57:17In other words, Hubble had discovered this is a distant island of stars.
57:23We now know it's over two million light-years away, composed of a trillion suns like ours.
57:31SENSING
57:37Hubble demonstrated that there's more to the universe than our own galaxy.
57:42He extended the reach of our senses further than we could have imagined.
57:48With the help of the telescope, we could perceive and comprehend worlds billions of light-years away.
57:57SENSING
58:02There's a wonderful feedback at work here,
58:04because the increasing amounts of data delivered by our senses drove the evolution of our brains,
58:10and those increasingly sophisticated brains became curious and demanded more and more data.
58:18And so, we built telescopes that were able to extend our senses beyond the horizon
58:24and showed us a universe that's billions of years old and contains trillions of stars and galaxies.
58:32Our insatiable quest for information is the making of us.
58:38SENSING
58:54Hubblecast is produced by ESA and the European Space Agency
58:57Transcription by ESO, translated by —