0:00:10 > 0:00:14These are the waters off Catalina, a tiny island
0:00:14 > 0:00:1720 miles off the coast of Los Angeles, California.
0:00:22 > 0:00:27These are kelp forests, and they grow here in tremendous abundance
0:00:27 > 0:00:32because the waters here around Catalina are rich in nutrients.
0:00:32 > 0:00:35That's because of the California currents,
0:00:35 > 0:00:37which brings this beautiful, rich, cold water
0:00:37 > 0:00:39up from the depths of the Pacific
0:00:39 > 0:00:44and allows this tremendously rich ecosystem to grow.
0:00:47 > 0:00:51This...remarkable place.
0:00:56 > 0:00:58Oh, look!
0:01:13 > 0:01:18But I'm not here to marvel at these kelp forests.
0:01:18 > 0:01:19Beautiful as they are.
0:01:19 > 0:01:25I'm here to search for a little animal that lives not in this
0:01:25 > 0:01:30forest of nutrients, but out there in the muddy ocean floor.
0:01:47 > 0:01:49There he is, look!
0:01:49 > 0:01:50HE LAUGHS
0:01:50 > 0:01:52Can you see that?!
0:01:55 > 0:01:57Camouflaged in its burrow on the sea floor,
0:01:57 > 0:02:01the mantis shrimp is a seemingly unremarkable creature.
0:02:05 > 0:02:08It's not a real shrimp, but a type of crustacean,
0:02:08 > 0:02:11called a stomatapod.
0:02:11 > 0:02:14I've come to see it because in one way
0:02:14 > 0:02:17the mantis shrimp is truly extraordinary -
0:02:17 > 0:02:19the way it detects the world.
0:02:24 > 0:02:26You see these big...
0:02:26 > 0:02:28eyes that they have to see.
0:02:30 > 0:02:33These are some of the most sophisticated eyes
0:02:33 > 0:02:34in the natural world.
0:02:37 > 0:02:42Each is made up of over 10,000 hexagonal lenses.
0:02:47 > 0:02:51And with twice as many visual pigments as any other animal,
0:02:51 > 0:02:55it can see colours and wavelengths of light that are invisible to me.
0:02:57 > 0:02:59These remarkable eyes
0:02:59 > 0:03:03give the mantis shrimp a unique view of the ocean.
0:03:03 > 0:03:06And this is just one of the many finely-tuned senses
0:03:06 > 0:03:09that have evolved across the planet.
0:03:14 > 0:03:19Sensing, the ability to detect and to react to the world outside,
0:03:19 > 0:03:21is fundamental to life.
0:03:21 > 0:03:25Every living thing is able to respond to its environment.
0:03:26 > 0:03:31In this film, I want to show you how the senses developed,
0:03:31 > 0:03:33how the mechanisms that gather information
0:03:33 > 0:03:36about the outside world evolved,
0:03:36 > 0:03:39how their emergence has helped animals
0:03:39 > 0:03:41thrive in different environments,
0:03:41 > 0:03:45and how the senses have pushed life in new directions,
0:03:45 > 0:03:49and may ultimately have led to our own curiosity
0:03:49 > 0:03:50and intelligence.
0:04:06 > 0:04:08ACOUSTIC GUITAR
0:04:08 > 0:04:13# If you feel lost
0:04:15 > 0:04:19# Lost in the world
0:04:22 > 0:04:27# Just like me
0:04:29 > 0:04:33# Worlds are lost in me
0:04:36 > 0:04:41# Worlds are lost in me. #
0:04:44 > 0:04:46These are the woods of Kentucky,
0:04:46 > 0:04:49the first stop on a journey across America
0:04:49 > 0:04:52that will take me from the far west coast to the Atlantic,
0:04:52 > 0:04:54through the heart of the country.
0:05:00 > 0:05:02It's the animals that I'll find on the way
0:05:02 > 0:05:06that will illuminate the world of the senses,
0:05:06 > 0:05:10and I'm going to start by going deep underground.
0:05:21 > 0:05:24These are the Mammoth Caves in Kentucky.
0:05:24 > 0:05:27With over 300 miles of mapped passages,
0:05:27 > 0:05:32they're the longest cave system in the world.
0:05:42 > 0:05:46But this is also the place to start exploring our own senses.
0:05:47 > 0:05:50We're normally dependent on our sight,
0:05:50 > 0:05:54but down here in the darkness, it's a very different world.
0:05:54 > 0:05:59I have to rely on my other senses to build a picture of my environment.
0:06:01 > 0:06:06It's...completely dark in this cave.
0:06:06 > 0:06:09I can't see anything at all.
0:06:09 > 0:06:13You can see me because we're lighting it with infrared light.
0:06:13 > 0:06:17That's at a wavelength that my eyes are completely insensitive to,
0:06:17 > 0:06:21so as far as I'm concerned, it is pitch black.
0:06:23 > 0:06:25And because it's so dark...
0:06:28 > 0:06:32..your other senses become heightened, particularly hearing.
0:06:33 > 0:06:36It's virtually silent in here.
0:06:39 > 0:06:41But if you listen carefully...
0:06:41 > 0:06:44DRIP OF WATER
0:06:44 > 0:06:49..you can just hear the faint drop of water from somewhere
0:06:49 > 0:06:51deep in the cave system.
0:06:51 > 0:06:55You'd never hear that if the cave were illuminated.
0:06:55 > 0:06:58But you focus on your hearing when it's as dark as this.
0:07:07 > 0:07:09As well as sight and hearing,
0:07:09 > 0:07:12we have of course a range of other senses.
0:07:12 > 0:07:15There's touch, which is a mixture of sensations -
0:07:15 > 0:07:19temperature and pressure and pain -
0:07:19 > 0:07:21and then there are chemical senses,
0:07:21 > 0:07:23so smell and taste,
0:07:23 > 0:07:27and we share those senses with almost every living thing
0:07:27 > 0:07:29on the planet today,
0:07:29 > 0:07:34because they date back virtually to the beginning of life on Earth.
0:07:47 > 0:07:52And even here, in water that's been collected from deep within a cave,
0:07:52 > 0:07:55there are organisms that are detecting and responding
0:07:55 > 0:07:57to their environment
0:07:57 > 0:08:00in the same way that living things have been doing
0:08:00 > 0:08:02for over a billion years.
0:08:29 > 0:08:31Ah.
0:08:31 > 0:08:33And there it is.
0:08:33 > 0:08:35Now that is a paramecium.
0:08:35 > 0:08:39It may look like a simple animal, but in fact
0:08:39 > 0:08:42it's a member of a group of organisms called protists.
0:08:42 > 0:08:46You'd have to go back around two billion years
0:08:46 > 0:08:51to find a common ancestor between me and a paramecium.
0:08:55 > 0:08:59Paramecia have probably changed little in the last billion years.
0:09:02 > 0:09:04Although they appear simple,
0:09:04 > 0:09:09these tiny creatures display some remarkably complex behaviour.
0:09:11 > 0:09:15You can even see them responding to their environment.
0:09:15 > 0:09:20The cell swims around, powered by a cohort of cilia,
0:09:20 > 0:09:23tiny hairs embedded in the cell membrane.
0:09:28 > 0:09:32If it bumps into something, the cilia change direction
0:09:32 > 0:09:34and it reverses away.
0:09:36 > 0:09:40They're clearly demonstrating a sense of touch.
0:09:43 > 0:09:47Even though they're single-celled organisms,
0:09:47 > 0:09:50they have no central nervous system,
0:09:50 > 0:09:53they can still do what all life does.
0:09:53 > 0:09:57They can sense their environment and they can react to it,
0:09:57 > 0:10:00and they do that using electricity.
0:10:09 > 0:10:13The mechanism that powers the paramecium's touch response
0:10:13 > 0:10:17lies at the heart of all sensing animals.
0:10:17 > 0:10:21It's based on an electrical phenomenon found throughout nature.
0:10:24 > 0:10:28An electric current is a flow of electric charge,
0:10:28 > 0:10:31and for that to happen, you need an imbalance between
0:10:31 > 0:10:33positive and negative charges.
0:10:33 > 0:10:37Now, usually in nature, things are electrically neutral,
0:10:37 > 0:10:41the positive and negative charges exactly balance out,
0:10:41 > 0:10:46but there are natural phenomena in which there is a separation
0:10:46 > 0:10:50of electric charge. A thunderstorm, for example.
0:10:51 > 0:10:54As thunder clouds build,
0:10:54 > 0:10:57updraughts within them separate charge.
0:10:57 > 0:11:01The lighter ice and water crystals become positively charged
0:11:01 > 0:11:02and are carried upwards,
0:11:02 > 0:11:07while the heavier, negatively charged crystals sink to the bottom.
0:11:08 > 0:11:11This can create a potential difference,
0:11:11 > 0:11:14a voltage between the cloud and the ground
0:11:14 > 0:11:17of as much as 100 million volts.
0:11:20 > 0:11:24Now, nature abhors a gradient. It doesn't like an imbalance,
0:11:24 > 0:11:29and it tries to correct it by having an electric current flow.
0:11:29 > 0:11:32In the case of a thunderstorm, that's a bolt of lightning.
0:11:52 > 0:11:56And it's the same process that governs the paramecium's behaviour,
0:11:56 > 0:11:59but on a tiny scale.
0:11:59 > 0:12:02In common with virtually all other cells,
0:12:02 > 0:12:04and certainly all animal cells,
0:12:04 > 0:12:08the paramecium maintains a potential difference
0:12:08 > 0:12:10across its cell membrane.
0:12:10 > 0:12:14It does that in common with a thunderstorm by charge separation.
0:12:16 > 0:12:19By manipulating the number of position ions
0:12:19 > 0:12:23inside and outside its membrane, the paramecium creates
0:12:23 > 0:12:27a potential difference of just 40 millivolts.
0:12:28 > 0:12:33So when a paramecium is just sat there, not bumping into anything,
0:12:33 > 0:12:37floating in this liquid, then it's like a little battery.
0:12:37 > 0:12:41It's maintaining the potential difference across its cell membrane,
0:12:41 > 0:12:45and it can use that to sense its surroundings.
0:12:46 > 0:12:50When it bumps into something, its cell membrane deforms,
0:12:50 > 0:12:55opening channels that allow positive ions to flood back
0:12:55 > 0:12:56across the membranes.
0:12:57 > 0:12:59As the potential difference falls,
0:12:59 > 0:13:03it sets off an electrical pulse that triggers
0:13:03 > 0:13:06the cilia to start beating in the opposite direction.
0:13:08 > 0:13:13That electrical pulse spreads round the whole cell in a wave
0:13:13 > 0:13:15called an action potential.
0:13:16 > 0:13:20And the paramecium reverses out of trouble.
0:13:22 > 0:13:27This ability to precisely control flows of electric charge
0:13:27 > 0:13:32across a membrane is not unique to the paramecium.
0:13:32 > 0:13:36It actually lies at the heart of all animal senses.
0:13:36 > 0:13:40In fact, every time I sense anything in the world,
0:13:40 > 0:13:43with my eyes, with my ears, with my fingers,
0:13:43 > 0:13:47at some point between that sensation and my brain,
0:13:47 > 0:13:50something very similar to that will happen.
0:14:02 > 0:14:06Although the same electrical mechanism underpins all sensing,
0:14:06 > 0:14:11every animal has a different suite of sensory capabilities
0:14:11 > 0:14:15that is beautifully adapted to the environment it lives in.
0:14:22 > 0:14:24This is the Big Black River,
0:14:24 > 0:14:28a tributary of the mighty Mississippi in America's deep south.
0:14:33 > 0:14:39And these dark and murky waters are home to a ferocious predator.
0:14:43 > 0:14:46Even though it's impossible to see more than a couple of inches
0:14:46 > 0:14:48through the water,
0:14:48 > 0:14:52this predator has found a way to track down and catch its prey
0:14:52 > 0:14:54with terrifying efficiency.
0:15:16 > 0:15:17To help me catch one,
0:15:17 > 0:15:21I've enlisted the support of wildlife biologist Don Jackson.
0:15:41 > 0:15:45- You go... Wrestle it. - I'll wrestle it now.
0:15:47 > 0:15:50- He's going over right here.- Is he?
0:15:57 > 0:15:59There you go.
0:15:59 > 0:16:03He can bite. Argh!
0:16:03 > 0:16:06I'll show you the mouth of this thing.
0:16:06 > 0:16:11Hang on... So you can see what the prey sees when he comes.
0:16:11 > 0:16:15Anything that'll fit in that mouth, he'll grab it!
0:16:15 > 0:16:19You can hold him if you just want to put your hand all the way under him.
0:16:19 > 0:16:22- Come all the way. All the way. Hold him up close to you.- Yeah.
0:16:24 > 0:16:27- How about that?- I've got him. Yeah.
0:16:30 > 0:16:32This is the top predator in this river.
0:16:32 > 0:16:37This is a, what? A 25-pound flathead catfish.
0:16:37 > 0:16:40You see those protrusions from his head?
0:16:40 > 0:16:42Those are barbels.
0:16:42 > 0:16:45They sense a vibration in the mud, on the river bed,
0:16:45 > 0:16:48but the most interesting thing about the catfish
0:16:48 > 0:16:52is that she really is, in some ways, one big tongue.
0:16:52 > 0:16:57There are taste sensors covering every part of her body,
0:16:57 > 0:17:00and she can build up a 3D picture of the river
0:17:00 > 0:17:04by detecting the chemical scents of animals.
0:17:04 > 0:17:06So, her eyes are not much use.
0:17:06 > 0:17:08As you can see, this river's extremely muddy,
0:17:08 > 0:17:11but it's the sense of taste that does the job of
0:17:11 > 0:17:13building up a picture of the world,
0:17:13 > 0:17:16and that's how he hunts, and he weighs a ton.
0:17:21 > 0:17:25I can feel those teeth. Ow!
0:17:25 > 0:17:27I'm going to let go.
0:17:27 > 0:17:29All right, you. Go on.
0:17:35 > 0:17:38The sensory world of the catfish is a remarkable one.
0:17:39 > 0:17:43Its map of its universe is built from the thousands of chemicals
0:17:43 > 0:17:44it can detect in the water.
0:17:46 > 0:17:50A swirling mix of tastes and concentrations,
0:17:50 > 0:17:53flavours and gradients.
0:17:53 > 0:17:55It's a world we can hardly imagine.
0:18:00 > 0:18:03There's an interesting almost philosophical point here
0:18:03 > 0:18:07because it's easy to imagine that we humans perceive the world
0:18:07 > 0:18:11in some kind of objective way, but that's not the case at all.
0:18:11 > 0:18:12Think about the catfish.
0:18:12 > 0:18:16The catfish sees the world as a kind of swarm of chemicals
0:18:16 > 0:18:19in the river, or vibrations on the river bed,
0:18:19 > 0:18:23whereas we see the world as reflected light off the forest,
0:18:23 > 0:18:26and I can hear the sounds of animals out there
0:18:26 > 0:18:28somewhere in the undergrowth.
0:18:28 > 0:18:31The catfish sees the world completely differently.
0:18:31 > 0:18:35So the way you perceive the world is determined by
0:18:35 > 0:18:37your environment,
0:18:37 > 0:18:41and no two animals see the world in the same way.
0:18:52 > 0:18:57Like every animal, we have evolved the senses that enable us to live
0:18:57 > 0:18:59in our environment.
0:19:05 > 0:19:08But as well as equipping us for the present,
0:19:08 > 0:19:11those senses can also tell us about our past.
0:19:17 > 0:19:21Now we have a sense of touch like the paramecium,
0:19:21 > 0:19:24and we have the chemical senses, taste and smell,
0:19:24 > 0:19:29like the catfish, but for us, the dominant senses
0:19:29 > 0:19:31are hearing and sight,
0:19:31 > 0:19:33and to understand them,
0:19:33 > 0:19:36we first have to understand their evolutionary history.
0:19:49 > 0:19:53And that's why I'm in the Mojave Desert in California,
0:19:53 > 0:19:56to track down an animal that can tell us something
0:19:56 > 0:19:59about the origins of our own senses.
0:20:11 > 0:20:14The creature I'm looking for is easiest to find in the dark,
0:20:14 > 0:20:17using ultra-violet light.
0:20:26 > 0:20:28Oh!
0:20:28 > 0:20:29HE LAUGHS
0:20:29 > 0:20:32Whoa!
0:20:32 > 0:20:35Man! Did you see that?
0:20:38 > 0:20:41Look at that. Absolutely bizarre.
0:20:41 > 0:20:44It's glowing absolutely bright green.
0:20:44 > 0:20:48Nobody has any idea what evolutionary advantage
0:20:48 > 0:20:49that confers.
0:20:51 > 0:20:54Although they now live in some of the driest,
0:20:54 > 0:20:57most hostile environments on Earth,
0:20:57 > 0:21:02like here in the desert, scorpions evolved as aquatic predators
0:21:02 > 0:21:06before emerging onto the land about 380 million years ago.
0:21:09 > 0:21:13They've adapted to be able to survive the extreme heat,
0:21:13 > 0:21:16and can go for over a year without food or water.
0:21:18 > 0:21:21Despite their fearsome reputation,
0:21:21 > 0:21:2698% of scorpion species have a sting that is no worse than a bee's.
0:21:29 > 0:21:32Perhaps the most fascinating thing about scorpions
0:21:32 > 0:21:34from an evolutionary perspective
0:21:34 > 0:21:37is the way that they catch their prey.
0:21:37 > 0:21:43You see that he spreads his legs out on the surface of the sand.
0:21:43 > 0:21:47And that's because he uses his legs to detect vibrations.
0:21:52 > 0:21:57Scorpions hunt insects like this beetle.
0:21:57 > 0:22:00It's almost impossible to see them in the dark,
0:22:00 > 0:22:04so the scorpion has evolved another way to track them down,
0:22:04 > 0:22:07by adapting its sense of touch.
0:22:12 > 0:22:15As the insect's feet move across the sand,
0:22:15 > 0:22:19they set off tiny waves of vibration through the ground.
0:22:20 > 0:22:24If just a single grain of sand is disturbed
0:22:24 > 0:22:25within range of the scorpion,
0:22:25 > 0:22:30it will sense it through the tips of its legs.
0:22:32 > 0:22:38They can detect vibrations that are around the size of a single atom
0:22:38 > 0:22:40as they sweep past.
0:22:47 > 0:22:49By measuring the time delay,
0:22:49 > 0:22:52between the waves arriving at each of its feet,
0:22:52 > 0:22:56the scorpion can calculate the precise direction
0:22:56 > 0:22:58and distance to its prey.
0:23:39 > 0:23:43And that ability to detect vibrations and use them
0:23:43 > 0:23:45to build up a picture of our surroundings
0:23:45 > 0:23:49is something that we share with scorpions.
0:23:53 > 0:23:56While the scorpion has adapted its sense of touch
0:23:56 > 0:24:00to detect vibrations in the ground,
0:24:00 > 0:24:04we use a very similar system to detect the tiny vibrations in air
0:24:04 > 0:24:07that we call sound.
0:24:08 > 0:24:12And like the scorpions, ours is a remarkably sensitive system.
0:24:14 > 0:24:17Our ears can hear sounds over a huge range.
0:24:22 > 0:24:25We can detect sound waves of very low frequency
0:24:25 > 0:24:28at the bass end of the spectrum.
0:24:31 > 0:24:35But we can also hear much higher-pitched sounds,
0:24:35 > 0:24:39sounds with frequencies hundreds or even a thousand times greater.
0:24:43 > 0:24:47And we can detect huge changes in sound intensity...
0:24:50 > 0:24:55..from the delicate buzzing created by an insect's flapping wings...
0:24:59 > 0:25:04..to the roar of an engine, which can be 100 million times louder.
0:25:13 > 0:25:16The story of how we developed our ability to hear
0:25:16 > 0:25:20is one of the great examples of evolution in action...
0:25:22 > 0:25:25..because the first animals to crawl out of the water onto the land
0:25:25 > 0:25:28would have had great difficulty hearing anything
0:25:28 > 0:25:31in their new environment.
0:25:39 > 0:25:41These are the Everglades.
0:25:45 > 0:25:49A vast area of swamps and wetlands that has covered the southern tip
0:25:49 > 0:25:52of Florida for over 4,000 years.
0:26:07 > 0:26:10Through the creatures we find here,
0:26:10 > 0:26:14like the American alligator, a member of the crocodile family,
0:26:14 > 0:26:18we can trace the story of how our hearing developed
0:26:18 > 0:26:20as we emerged onto the land.
0:26:25 > 0:26:29And it starts below the water, with the fish.
0:26:31 > 0:26:34If you're a fish, then hearing isn't a problem.
0:26:34 > 0:26:37You live in water and you're made of water,
0:26:37 > 0:26:40so sound has no problem at all travelling from the outside
0:26:40 > 0:26:41to the inside,
0:26:41 > 0:26:46but when life emerged from the oceans onto the land,
0:26:46 > 0:26:49then hearing became a big problem.
0:26:49 > 0:26:53See, sound doesn't travel well from air into water.
0:26:53 > 0:26:55If I make a noise now...
0:26:56 > 0:26:59..over 99.9% of the sound
0:26:59 > 0:27:03is reflected back off the surface of the water.
0:27:04 > 0:27:07It's because of that reflection that underwater
0:27:07 > 0:27:11you can hear very little from above the surface.
0:27:11 > 0:27:14And it's exactly the same problem our ears face,
0:27:14 > 0:27:17because they too are filled with fluid.
0:27:19 > 0:27:24So, if evolution hadn't found an ingenious solution to the problem
0:27:24 > 0:27:27of getting sound from air into water,
0:27:27 > 0:27:30then I wouldn't be able to hear anything at all.
0:27:33 > 0:27:37And that solution relies on some of the most delicate moving parts
0:27:37 > 0:27:39in the human body.
0:27:41 > 0:27:44Have I just dropped them? Hang on a second.
0:27:44 > 0:27:49Oh, I've done it again! Bloody hell! Idiot!
0:27:49 > 0:27:51Just flipped out!
0:27:54 > 0:27:58These are the smallest three bones in the human body,
0:27:58 > 0:28:02called the malleus, the incus and the stapes,
0:28:02 > 0:28:08and they sit between the eardrum and the entrance to your inner ear,
0:28:08 > 0:28:12to the place where the fluid sits.
0:28:12 > 0:28:17The bones help to channel sound into the ear through two mechanisms.
0:28:19 > 0:28:22First, they act as a series of levers,
0:28:22 > 0:28:26magnifying the movement of the eardrum.
0:28:29 > 0:28:33And second, because the surface area of the eardrum is 17 times
0:28:33 > 0:28:36greater than the footprint of the stapes,
0:28:36 > 0:28:39the vibrations are passed into the inner ear
0:28:39 > 0:28:41with much greater force.
0:28:41 > 0:28:45And that has a dramatic effect.
0:28:45 > 0:28:50Rather than 99.9% of the sound energy being reflected away,
0:28:50 > 0:28:53it turns out that with this arrangement,
0:28:53 > 0:28:5960% of the sound energy is passed from the eardrum into the inner ear.
0:29:01 > 0:29:04Now, this setup is so intricate and so efficient,
0:29:04 > 0:29:07it almost looks as if those bones could only ever
0:29:07 > 0:29:10have been for this purpose,
0:29:10 > 0:29:14but in fact, you can see their origin if you look
0:29:14 > 0:29:17way back in our evolutionary history.
0:29:24 > 0:29:28In order to understand where that collection of small bones
0:29:28 > 0:29:30in our ears came from,
0:29:30 > 0:29:33you have to go back in our evolutionary family tree
0:29:33 > 0:29:36way beyond the fish that we see today.
0:29:36 > 0:29:39In fact, back around 530 million years
0:29:39 > 0:29:44to when the oceans were populated with jawless fish, called agnathans.
0:29:44 > 0:29:47They're similar to the modern lamprey.
0:29:47 > 0:29:49Now, they didn't have a jaw,
0:29:49 > 0:29:54but they had gills supported by gill arches.
0:29:55 > 0:30:00Now, over a period of 50 million years, the most forward of those
0:30:00 > 0:30:07gill arches migrated forward in the head to form jaws.
0:30:09 > 0:30:11And you see fish like these,
0:30:11 > 0:30:14the first jawed fish in the fossil record,
0:30:14 > 0:30:16around 460 million years ago.
0:30:16 > 0:30:21And, there, at the back of the jaw, there is that bone,
0:30:21 > 0:30:25the hyomandibular, supporting the rear of the jaw.
0:30:26 > 0:30:30Then, around 400 million years ago, the first vertebrates
0:30:30 > 0:30:33made the journey from the sea to the land.
0:30:33 > 0:30:34Their fins became legs,
0:30:34 > 0:30:39but in their skull and throat, other changes were happening.
0:30:39 > 0:30:42The gills were no longer needed
0:30:42 > 0:30:45to breathe the oxygen in the atmosphere,
0:30:45 > 0:30:47and so they faded away
0:30:47 > 0:30:51and became different structures in the head and throat,
0:30:51 > 0:30:57and that bone, the hyomandibular, became smaller and smaller,
0:30:57 > 0:31:00until its function changed.
0:31:00 > 0:31:05It now was responsible for picking up vibrations in the jaw
0:31:05 > 0:31:09and transmitting them to the inner ear of the reptiles.
0:31:09 > 0:31:16And that is still true today of our friends over there...
0:31:17 > 0:31:19the crocodiles.
0:31:25 > 0:31:27Once more with alligator.
0:31:30 > 0:31:33But even then, the process continued.
0:31:34 > 0:31:39Around 210 million years ago, the first mammals evolved,
0:31:39 > 0:31:43and unlike our friends, the reptiles here,
0:31:43 > 0:31:47mammals have a jaw that's made of only one bone.
0:31:47 > 0:31:52A reptile's jaw is made of several bones fused together,
0:31:52 > 0:31:56so that freed up two bones,
0:31:56 > 0:31:58which moved,
0:31:58 > 0:32:01and shrank,
0:32:01 > 0:32:05and eventually became the malleus,
0:32:05 > 0:32:09the incus and stapes.
0:32:09 > 0:32:12So this is the origin of those three tiny bones
0:32:12 > 0:32:16that are so important to mammalian hearing.
0:32:21 > 0:32:22He's quite big, isn't he?
0:32:52 > 0:32:55I think this is a most wonderful example of the blind,
0:32:55 > 0:32:58undirected ingenuity of evolution,
0:32:58 > 0:33:02that it's taken the bones in gills of fish
0:33:02 > 0:33:06and converted them into the intricate structures inside my ears
0:33:06 > 0:33:12that efficiently allow sound to be transmitted from air into fluid.
0:33:12 > 0:33:14It's a remarkable thought
0:33:14 > 0:33:17that to fully understand the form and function of my ears,
0:33:17 > 0:33:22you have to understand my distant evolutionary past
0:33:22 > 0:33:24in the oceans of ancient earth.
0:33:42 > 0:33:44We're hunting for the mantis shrimp.
0:33:46 > 0:33:50'All sensing has evolved to fulfil one simple function - to provide us
0:33:50 > 0:33:54'with the specific information we need to survive.'
0:33:54 > 0:33:56There he is!
0:33:59 > 0:34:02I might try and grab him.
0:34:02 > 0:34:06'And nowhere is that clearer than in the sense of vision.'
0:34:10 > 0:34:12He's quite tricky to catch!
0:34:14 > 0:34:16'Almost all animals can see.'
0:34:17 > 0:34:20'96% of animal species have eyes.'
0:34:22 > 0:34:25'But what those eyes can see varies enormously.'
0:34:27 > 0:34:31'So with an animal like the mantis shrimp, you have to ask what it is
0:34:31 > 0:34:36'about its way of life that demands such a complex visual system.'
0:34:42 > 0:34:46Got to be very quick and very careful with this.
0:34:46 > 0:34:48Let him out.
0:34:51 > 0:34:54The complex structure of the mantis shrimp's eyes
0:34:54 > 0:34:57give it incredibly precise depth perception.
0:34:59 > 0:35:02We have binocular vision.
0:35:02 > 0:35:05We look with two eyes from slightly different angles,
0:35:05 > 0:35:09and judge distance by comparing the differences between the two images.
0:35:11 > 0:35:15Each of the mantis shrimp's eyes has trinocular vision.
0:35:17 > 0:35:21Each eye takes three separate images of the same object.
0:35:22 > 0:35:27Comparing all three gives them exceptionally precise range-finding,
0:35:27 > 0:35:31and they need that information to hunt their prey.
0:35:35 > 0:35:37Despite appearances,
0:35:37 > 0:35:43it is a dangerous animal. He has one of the hardest punches in nature.
0:35:43 > 0:35:46Those yellow appendages you can see on the front of his body
0:35:46 > 0:35:48are called raptoral appendages.
0:35:48 > 0:35:50They're actually highly evolved from legs,
0:35:50 > 0:35:54and they can punch with tremendous force.
0:35:57 > 0:35:59The mantis shrimp's punch
0:35:59 > 0:36:01is one of the fastest movements in the animal world.
0:36:05 > 0:36:09Slowed down by over a thousand times, we can clearly see its power.
0:36:11 > 0:36:14It can release its legs with the force of a bullet.
0:36:17 > 0:36:19In the wild,
0:36:19 > 0:36:23they use that punch to break through the shells of their prey.
0:36:23 > 0:36:25But it could easily break my finger.
0:36:28 > 0:36:31The need to precisely deploy this formidable weapon
0:36:31 > 0:36:34is one of the reasons the mantis shrimp has developed
0:36:34 > 0:36:36its complex range-finding ability.
0:36:42 > 0:36:47And that punch can also help explain their sophisticated colour vision.
0:36:48 > 0:36:52Because the coloured flashes on their body warn other mantis shrimp
0:36:52 > 0:36:54that they may be about to attack.
0:36:55 > 0:36:58While other colour signals have a quite different meaning.
0:37:01 > 0:37:05Yet reading these signals in the ocean can be surprisingly difficult.
0:37:07 > 0:37:11In the deep ocean, colours shift from minute to minute,
0:37:11 > 0:37:14from hour to hour, with changing lighting conditions,
0:37:14 > 0:37:16changing conditions in the ocean,
0:37:16 > 0:37:17but it's thought that
0:37:17 > 0:37:20even though the light quality can change tremendously,
0:37:20 > 0:37:25the mantis shrimp can still identify specific colours very accurately,
0:37:25 > 0:37:28because of those sophisticated eyes.
0:37:32 > 0:37:36The mantis shrimp's eyes are beautifully tuned to their needs.
0:37:36 > 0:37:40But they're very different from our eyes.
0:37:40 > 0:37:43With their thousands of lenses and their complex colour vision,
0:37:43 > 0:37:46they have a completely different way of viewing the world.
0:37:48 > 0:37:51And yet there's strong evidence that the mantis shrimp's eyes
0:37:51 > 0:37:54and ours share a common origin.
0:37:57 > 0:37:59Because on a molecular level,
0:37:59 > 0:38:02every eye in the world works in the same way.
0:38:16 > 0:38:18In order to form an image of the world,
0:38:18 > 0:38:22then obviously the first thing you have to do is detect light,
0:38:22 > 0:38:28and I have a sample here of the molecules that do that,
0:38:28 > 0:38:31that detect light in my eye.
0:38:31 > 0:38:34It's actually, specifically, the molecules that's in the black
0:38:34 > 0:38:38and white receptor cells in my eyes, the rods.
0:38:38 > 0:38:40It's called rhodopsin.
0:38:40 > 0:38:43And the moment I expose this to light,
0:38:43 > 0:38:46you'll see an immediate physical change.
0:38:50 > 0:38:52There you go.
0:38:52 > 0:38:54Did you see that? It was very quick.
0:38:54 > 0:38:58It came out very pink indeed, and it immediately went yellow.
0:38:58 > 0:39:02This subtle shift in colour is caused by the rhodopsin molecule
0:39:02 > 0:39:05changing shape as it absorbs the light.
0:39:06 > 0:39:08In my eyes, what happens is
0:39:08 > 0:39:12that change in structure triggers an electrical signal
0:39:12 > 0:39:15which ultimately goes all the way to my brain,
0:39:15 > 0:39:17which forms an image of the world.
0:39:20 > 0:39:21It is this chemical reaction
0:39:21 > 0:39:24that's responsible for all vision on the planet.
0:39:27 > 0:39:32Closely related molecules lie at the heart of every animal eye.
0:39:33 > 0:39:37That tells us that this must be a very ancient mechanism.
0:39:42 > 0:39:46To find its origins, we must find a common ancestor
0:39:46 > 0:39:49that links every organism that uses rhodopsin today.
0:39:50 > 0:39:52We know that common ancestor must have lived
0:39:52 > 0:39:56before all animals' evolutionary lines diverged.
0:39:58 > 0:40:00But it may have lived at any time before then.
0:40:03 > 0:40:06So what is that common ancestor?
0:40:06 > 0:40:10Well, here's where we approach the cutting edge of scientific research.
0:40:10 > 0:40:13The answer is that we don't know for sure,
0:40:13 > 0:40:17but a clue might be found here,
0:40:17 > 0:40:20in these little green blobs,
0:40:20 > 0:40:26which are actually colonies of algae, algae called volvox.
0:40:28 > 0:40:31We have very little in common with algae.
0:40:31 > 0:40:35We've been separated in evolutionary terms for over one billion years.
0:40:36 > 0:40:39But we do share one surprising similarity.
0:40:41 > 0:40:45These volvox have light-sensitive cells that control their movement.
0:40:47 > 0:40:49And the active ingredient of those cells
0:40:49 > 0:40:52is a form of rhodopsin so similar to our own
0:40:52 > 0:40:55that it's thought they may share a common origin.
0:41:00 > 0:41:01What does that mean?
0:41:03 > 0:41:06Does it mean that we share a common ancestor with the algae,
0:41:06 > 0:41:11and in that common ancestor, the seeds of vision can be found?
0:41:13 > 0:41:18To find a source that may have passed this ability to detect light
0:41:18 > 0:41:19to both us and the algae,
0:41:19 > 0:41:23we need to go much further back down the evolutionary tree.
0:41:27 > 0:41:30To organisms like cyanobacteria.
0:41:31 > 0:41:35They were among the first living things to evolve on the planet,
0:41:35 > 0:41:38and it's thought that the original rhodopsins may have developed
0:41:38 > 0:41:41in these ancient photosynthetic cells.
0:41:44 > 0:41:48So the origin of my ability to see
0:41:48 > 0:41:53may have been well over a billion years ago,
0:41:53 > 0:41:58in an organism as seemingly simple as a cyanobacteria.
0:42:08 > 0:42:09The basic chemistry of vision
0:42:09 > 0:42:12may have been established for a long time,
0:42:12 > 0:42:15but it's a long way from that chemical reaction
0:42:15 > 0:42:19to a fully functioning eye that can create an image of the world.
0:42:22 > 0:42:25The eye is a tremendously complex piece of machinery,
0:42:25 > 0:42:28built from lots of interdependent parts,
0:42:28 > 0:42:33and it seems very difficult to imagine how that could have evolved
0:42:33 > 0:42:36in a series of small steps, but actually,
0:42:36 > 0:42:38we understand that process very well indeed.
0:42:39 > 0:42:41I can show you, by building an eye.
0:42:53 > 0:42:55The first step in building an eye
0:42:55 > 0:42:59would need to take some kind of light-sensitive pigment,
0:42:59 > 0:43:02rhodopsin, for example, and build it on to a membrane.
0:43:02 > 0:43:07So imagine this is such a membrane, with the pigment cells attached,
0:43:07 > 0:43:10then immediately you have something that can detect
0:43:10 > 0:43:15the difference between dark and light.
0:43:15 > 0:43:17Now, the advantage of this arrangement
0:43:17 > 0:43:19is that it's very sensitive to light.
0:43:19 > 0:43:23There's no paraphernalia in front of the retina to block light,
0:43:23 > 0:43:26but the disadvantage, as you can see,
0:43:26 > 0:43:29is that there is no image formed at all.
0:43:29 > 0:43:33It just allows you to tell the difference between light and dark.
0:43:33 > 0:43:39But you can improve that a lot by adding an aperture,
0:43:39 > 0:43:45a small hole in front of the retina, so this is a movable aperture,
0:43:45 > 0:43:49just like the sort of thing you've got in your camera,
0:43:49 > 0:43:53And now, we see that the image gets sharper.
0:43:56 > 0:43:59But the problem is that in order to make it sharper,
0:43:59 > 0:44:01we have to narrow down the aperture,
0:44:01 > 0:44:04and that means that you get less and less light,
0:44:04 > 0:44:07so this eye becomes less and less sensitive.
0:44:08 > 0:44:12So there's one more improvement that nature made,
0:44:12 > 0:44:17which is to replace the pinhole, the simple aperture...
0:44:19 > 0:44:20With a lens.
0:44:26 > 0:44:28Look at that.
0:44:29 > 0:44:32A beautifully sharp image.
0:44:35 > 0:44:38The lens is the crowning glory of the evolution of the eye.
0:44:40 > 0:44:44By bending light onto the retina, it allows the aperture to be opened,
0:44:44 > 0:44:49letting more light into the eye, and a bright, detailed image is formed.
0:45:04 > 0:45:08Our eyes are called camera eyes, because, like a camera,
0:45:08 > 0:45:10they consist of a single lens
0:45:10 > 0:45:13that bends the light onto the photoreceptor
0:45:13 > 0:45:16to create a high-quality image of the world.
0:45:19 > 0:45:21But that has a potential drawback,
0:45:21 > 0:45:23because to make sense of all that information,
0:45:23 > 0:45:25we need to be able to process it.
0:45:27 > 0:45:29Each one of my eyes contains
0:45:29 > 0:45:32over 100 million individual photoreceptor cells.
0:45:32 > 0:45:34That's about five or ten times the number
0:45:34 > 0:45:36in the average digital camera.
0:45:36 > 0:45:38So if my visual system works
0:45:38 > 0:45:43by just taking a series of individual still images of the world
0:45:43 > 0:45:46and transmitting all that information to my brain,
0:45:46 > 0:45:48then my brain would be overwhelmed.
0:45:48 > 0:45:52It's just not practical, so that's NOT what animals do.
0:45:52 > 0:45:55Instead, their visual systems have evolved
0:45:55 > 0:45:59to extract only the information that is necessary.
0:46:04 > 0:46:07And this is wonderfully illustrated in the toad.
0:46:10 > 0:46:14The toad has eyes that are structurally very similar to ours.
0:46:15 > 0:46:19But much of the time, it's as if it isn't seeing anything at all.
0:46:21 > 0:46:24It seems completely oblivious to its surroundings.
0:46:26 > 0:46:30Until something, like a mealworm, takes its interest.
0:46:32 > 0:46:35If you think about what's important to a toad visually,
0:46:35 > 0:46:39then it's the approach of either pray or predators,
0:46:39 > 0:46:45so the toad's visual system is optimised to detect them,
0:46:45 > 0:46:51So, there, we've put a worm in front of the toad, and did you see that?
0:46:51 > 0:46:54Incredibly quickly, the toad ate the worm.
0:46:55 > 0:46:58As soon as the mealworm wriggles in front of the toad,
0:46:58 > 0:47:00its eyes lock onto the target.
0:47:02 > 0:47:05Then it strikes in a fraction of a second.
0:47:09 > 0:47:11It's an astonishingly precise reaction,
0:47:11 > 0:47:14but it's also a very simple one.
0:47:14 > 0:47:19Because the toad is only focusing on one property of the mealworm -
0:47:19 > 0:47:21the way it moves.
0:47:28 > 0:47:30These 1970s lab tests
0:47:30 > 0:47:35show how a toad will try and eat anything long and thin.
0:47:35 > 0:47:39But only if it moves on its side, like a worm.
0:47:40 > 0:47:44And that's because the toad has neural circuits in its retina
0:47:44 > 0:47:47that only respond to lengthwise motion.
0:47:49 > 0:47:52If, instead, the target is rotated into an upright position,
0:47:52 > 0:47:54the toad doesn't respond at all.
0:48:10 > 0:48:13At first sight, the visual system of the toad
0:48:13 > 0:48:16seems a little bit primitive and imperfect.
0:48:16 > 0:48:20It is true that if you put a toad in a tank full of dead worms,
0:48:20 > 0:48:23it'll starve to death, because they're not moving,
0:48:23 > 0:48:26so it doesn't recognise them as food.
0:48:26 > 0:48:30But it doesn't need to see the world in all the detail that I see it.
0:48:30 > 0:48:33What it needs to focus on is movement,
0:48:33 > 0:48:36because if it can see movement then it can survive,
0:48:36 > 0:48:40because it can avoid predators, and it can eat its prey.
0:48:40 > 0:48:44I suppose, in a sense, if it moves like a worm, in nature,
0:48:44 > 0:48:46then it's likely to be a worm.
0:48:58 > 0:49:01This ability to simplify the visual world
0:49:01 > 0:49:04into the most relevant bits of information
0:49:04 > 0:49:07is something that every animal does.
0:49:07 > 0:49:09We do it all the time.
0:49:09 > 0:49:13We also have visual systems that detect motion.
0:49:13 > 0:49:15Others identify edges and faces.
0:49:17 > 0:49:22But extracting more information takes more processing power.
0:49:22 > 0:49:24That requires a bigger brain.
0:49:25 > 0:49:28And to see the results of this evolutionary drive
0:49:28 > 0:49:30towards greater processing power,
0:49:30 > 0:49:33I've come to the heart of Metropolitan Florida.
0:49:35 > 0:49:38You know, it may not look like it, but underneath this flyover,
0:49:38 > 0:49:40just out in the shallow water,
0:49:40 > 0:49:42is one of the best places in the world
0:49:42 > 0:49:44to find a particularly interesting animal.
0:49:46 > 0:49:48It's an animal that's evolved
0:49:48 > 0:49:51to make the most of the information its eyes can provide.
0:49:58 > 0:50:03Well, what we're going to do is find some octopus.
0:50:05 > 0:50:09And it's, as you say in physics, nontrivial.
0:50:10 > 0:50:13Because they've developed a beautiful way
0:50:13 > 0:50:15of camouflaging themselves.
0:50:19 > 0:50:23They change colour. Their cells and their skin change colour
0:50:23 > 0:50:24to match their surroundings.
0:50:24 > 0:50:27It's an ability that we don't possess, of course.
0:50:27 > 0:50:29It makes them difficult to find.
0:50:41 > 0:50:44There he is, look.
0:50:46 > 0:50:47Ha-ha!
0:50:47 > 0:50:49He went flying into there,
0:50:49 > 0:50:53and a crab and a load of fish are flying out, and look at his ink.
0:50:53 > 0:50:55A defence mechanism. I don't know where he is.
0:50:55 > 0:50:57He's hiding somewhere in there.
0:51:05 > 0:51:06Look at those colours!
0:51:07 > 0:51:09What a remarkable creature.
0:51:11 > 0:51:15'Although the octopus is a mollusc, like slugs and snails,
0:51:15 > 0:51:19'in many ways, it seems more similar to us.'
0:51:19 > 0:51:20Whoa!
0:51:21 > 0:51:25'It's believed to be the most intelligent invertebrate.'
0:51:25 > 0:51:28It's like he's holding his fists up.
0:51:28 > 0:51:29Look at that.
0:51:29 > 0:51:33'Its brain contains about 500 million nerve cells,
0:51:33 > 0:51:35'about the same as a dog's.'
0:51:35 > 0:51:36What are you doing?
0:51:41 > 0:51:43You know, if you want an example of an alien intelligence
0:51:43 > 0:51:44here on earth..
0:51:46 > 0:51:47that must surely be it.
0:51:49 > 0:51:54'And it's used that brain to develop some remarkable abilities.'
0:51:56 > 0:51:59'It's become a skilled mimic.'
0:51:59 > 0:52:01'It can rapidly change not only its colour,
0:52:01 > 0:52:03'but its shape, to match the background.'
0:52:19 > 0:52:22'Some species even do impressions of other animals.'
0:52:29 > 0:52:34'They become cunning predators, and adept problem-solvers.'
0:52:36 > 0:52:39'They've even been reported to use tools.'
0:52:41 > 0:52:44'All these skills are signs of great intelligence,
0:52:44 > 0:52:48'but they also rely on an acute sense of vision.'
0:52:50 > 0:52:54Look at those big eyes surveying the surroundings.
0:52:55 > 0:52:58Checking us out.
0:52:58 > 0:53:03Camera eyes, just like mine, and they're vitally important
0:53:03 > 0:53:06for allowing the octopus to live the lifestyle it does,
0:53:06 > 0:53:11so a visual animal in the same way that I'm a visual animal.
0:53:14 > 0:53:17'The octopus is one of the only invertebrates
0:53:17 > 0:53:19'to have complex camera eyes.'
0:53:22 > 0:53:26'Like our eyes, they capture detailed images of the world.'
0:53:27 > 0:53:29'And their brains have evolved
0:53:29 > 0:53:32'to be able to extract the most information from those images.'
0:53:36 > 0:53:40'The optic lobes make up about 30% of the octopus' brain.'
0:53:41 > 0:53:43'The only other group
0:53:43 > 0:53:46'that is known to devote so much of its brain to visual processing
0:53:46 > 0:53:48'is our group.
0:53:48 > 0:53:53'The primates - the most intelligent vertebrates.'
0:53:55 > 0:53:57I think it's a fascinating thought
0:53:57 > 0:54:00that that intelligence is a result
0:54:00 > 0:54:04of the need to process all the information
0:54:04 > 0:54:06from those big, complex eyes.
0:54:10 > 0:54:13'What's so compelling about the octopus' intelligence
0:54:13 > 0:54:16'is that it evolved completely separately to ours.'
0:54:19 > 0:54:22'We last shared a common ancestor 600 million years ago.'
0:54:23 > 0:54:27'An ancestor that had neither eyes nor a brain.'
0:54:29 > 0:54:32'But we've both evolved sophisticated camera eyes,
0:54:32 > 0:54:35'and large, intelligent brains.'
0:54:37 > 0:54:42'It suggests a tantalising link between sensory processing
0:54:42 > 0:54:44'and the evolution of intelligence.'
0:54:55 > 0:54:59Sensing has played a key role in the evolution of life on Earth.
0:55:04 > 0:55:05The first organisms
0:55:05 > 0:55:09were able to detect and respond to their immediate environment,
0:55:09 > 0:55:11as paramecia do today.
0:55:15 > 0:55:19But as animals evolved, and their environments became more complex,
0:55:19 > 0:55:22their senses evolved with them.
0:55:23 > 0:55:27Developing the mechanisms to let them decode vibrations
0:55:27 > 0:55:28and detect light.
0:55:29 > 0:55:32Allowing them to build three-dimensional pictures
0:55:32 > 0:55:34of their environments,
0:55:34 > 0:55:42and stimulating the growth of brains that could handle all that data.
0:55:49 > 0:55:50But for one species,
0:55:50 > 0:55:53the desire to gather more and more sensory information
0:55:53 > 0:55:55has become overwhelming.
0:56:01 > 0:56:03That species is us.
0:56:19 > 0:56:22This is the closest thing to hallowed ground that exists
0:56:22 > 0:56:24in a subject that has no saints,
0:56:24 > 0:56:28because that telescope is the one that Edwin Hubble used
0:56:28 > 0:56:30to expand our horizons, I would argue,
0:56:30 > 0:56:34more than anyone else before or since.
0:56:45 > 0:56:50In 1923, Edwin Hubble took this photograph of the Andromeda galaxy.
0:56:50 > 0:56:52You can see his handwriting on the photograph.
0:56:52 > 0:56:56He did it by sitting here night after night for over a week,
0:56:56 > 0:56:58exposing this photographic plate.
0:56:58 > 0:56:59Now, at the time,
0:56:59 > 0:57:03it was thought that this misty patch you see in the night sky
0:57:03 > 0:57:07was just a cloud, maybe a gas cloud in our own galaxy,
0:57:07 > 0:57:09but Hubble, because of the power of this telescope,
0:57:09 > 0:57:13identified individual stars, and crucially,
0:57:13 > 0:57:18he found that it was way outside our own galaxy.
0:57:18 > 0:57:19In other words,
0:57:19 > 0:57:23Hubble had discovered this is a distant island of stars.
0:57:23 > 0:57:26We now know it's over two million light years away,
0:57:26 > 0:57:29composed of a trillion suns like ours.
0:57:37 > 0:57:40Hubble demonstrated that there's more to the universe
0:57:40 > 0:57:41than our own galaxy.
0:57:42 > 0:57:46He extended the reach of our senses further than we could have imagined.
0:57:48 > 0:57:49With the help of the telescope,
0:57:49 > 0:57:55we could perceive and comprehend worlds billions of light years away.
0:58:02 > 0:58:04There's a wonderful feedback at work here,
0:58:04 > 0:58:08because the increasing amounts of data delivered by our senses
0:58:08 > 0:58:10drove the evolution of our brains,
0:58:10 > 0:58:14and those increasingly sophisticated brains became curious
0:58:14 > 0:58:16and demanded more and more data.
0:58:18 > 0:58:20And so we built telescopes
0:58:20 > 0:58:23that were able to extend our senses beyond the horizon
0:58:23 > 0:58:27and showed us a universe that's billions of years old
0:58:27 > 0:58:30and contains trillions of stars and galaxies.
0:58:32 > 0:58:37Our insatiable quest for information is the making of us.
0:59:02 > 0:59:06Subtitles by Red Bee Media