0:00:19 > 0:00:24Every living thing that we know to exist is found on this one rock...
0:00:28 > 0:00:32..it became a home to life almost four billion years ago,
0:00:32 > 0:00:36and today hosts an incredibly diverse natural world...
0:00:40 > 0:00:44..in this programme, I want to show you how living things
0:00:44 > 0:00:47evolved some of their most important abilities...
0:00:48 > 0:00:52..and how the laws of physics govern the lives of all things -
0:00:52 > 0:00:54from the very biggest...
0:00:54 > 0:00:56to the very smallest.
0:01:18 > 0:01:21These are the Mammoth Caves, in Kentucky.
0:01:21 > 0:01:24With over 300 miles of mapped passages,
0:01:24 > 0:01:27they're the longest cave system in the world...
0:01:32 > 0:01:36..but this is also the place to start exploring our own senses.
0:01:37 > 0:01:39We're normally dependent on our sight
0:01:39 > 0:01:44but down here, in the darkness, it's a very different world
0:01:44 > 0:01:46and I have to rely on my other senses
0:01:46 > 0:01:48to build a picture of my environment.
0:01:50 > 0:01:52Now, it's...
0:01:52 > 0:01:55completely dark in this cave.
0:01:55 > 0:01:58I can't see anything...at all.
0:01:58 > 0:02:03You can see me because we're lighting it with infrared light
0:02:03 > 0:02:06and that's a wavelength that my eyes are completely insensitive to.
0:02:06 > 0:02:10So, as far as I'm concerned, it is pitch black.
0:02:12 > 0:02:14And because it's so dark...
0:02:17 > 0:02:21..your other senses become heightened - particularly hearing...
0:02:22 > 0:02:25..it's virtually silent in here...
0:02:27 > 0:02:28..but, if you listen carefully,
0:02:28 > 0:02:32you can just hear the faint drop
0:02:32 > 0:02:35of water, from somewhere deep in the cave system.
0:02:35 > 0:02:40You'd never hear that if the cave were illuminated...
0:02:40 > 0:02:43but you focus on your hearing when it's as dark as this.
0:02:50 > 0:02:52Now, as well as sight and hearing,
0:02:52 > 0:02:54we have, of course, a range of other senses.
0:02:54 > 0:02:58There's touch, which is really a mixture of sensations,
0:02:58 > 0:03:01temperature, and pressure, and pain.
0:03:01 > 0:03:06And then there are chemical senses - so, smell and taste -
0:03:06 > 0:03:10and we share those senses with almost every living thing
0:03:10 > 0:03:11on the planet today
0:03:11 > 0:03:16because they date back virtually to the beginning of life on earth.
0:03:25 > 0:03:30And even here, in water that's been collected from deep within a cave,
0:03:30 > 0:03:31there are organisms
0:03:31 > 0:03:35that are detecting and responding to their environment
0:03:35 > 0:03:39in the same way that living things have been doing
0:03:39 > 0:03:40for over a billion years.
0:03:54 > 0:03:55Ah...
0:03:56 > 0:03:57..there it is.
0:04:00 > 0:04:02That is a paramecium.
0:04:02 > 0:04:06It may look like a simple animal
0:04:06 > 0:04:10but, in fact, it's a member of a group of organisms called protists
0:04:10 > 0:04:13and you'd have to go back around two billion years
0:04:13 > 0:04:18to find a common ancestor between me and the paramecium.
0:04:23 > 0:04:27Paramecia have probably changed little in the last billion years...
0:04:30 > 0:04:32..and, although they appear simple,
0:04:32 > 0:04:36these tiny creatures display some remarkably complex behaviour.
0:04:39 > 0:04:42You can even see them responding to their environment...
0:04:43 > 0:04:48..the cell swims around, powered by cohorts of cilia -
0:04:48 > 0:04:50tiny hairs embedded in the cell membrane.
0:04:55 > 0:04:57If it bumps into something,
0:04:57 > 0:05:01the cilia change direction and it reverses away...
0:05:04 > 0:05:08..they're clearly demonstrating a sense of touch.
0:05:12 > 0:05:15Even though they're single-celled organisms,
0:05:15 > 0:05:17they have no central nervous system,
0:05:17 > 0:05:21they can still do what all life does -
0:05:21 > 0:05:25they can sense their environment and they can react to it,
0:05:25 > 0:05:28and they do that using electricity.
0:05:30 > 0:05:33By manipulating the number of positive ions
0:05:33 > 0:05:35inside and outside its membrane,
0:05:35 > 0:05:40the paramecium creates a potential difference of 40 millivolts.
0:05:45 > 0:05:49When it bumps into something its cell membrane deforms,
0:05:49 > 0:05:53opening channels that allow positive ions to flood back
0:05:53 > 0:05:54across the membrane...
0:05:55 > 0:06:00..as the potential difference falls, it sets off an electrical pulse
0:06:00 > 0:06:04that triggers the cilia to start beating in the opposite direction.
0:06:06 > 0:06:10That electrical pulse spreads around the whole cell in a wave,
0:06:10 > 0:06:13called an action potential...
0:06:14 > 0:06:17..and the paramecium reverses out of trouble.
0:06:20 > 0:06:25Now, this ability to precisely control flows of electric charge
0:06:25 > 0:06:29across a membrane is not unique to the paramecium,
0:06:29 > 0:06:33it actually lies at the heart of all animal senses.
0:06:33 > 0:06:38In fact, every time I sense anything in the world -
0:06:38 > 0:06:40with my eyes, with my ears or with my fingers -
0:06:40 > 0:06:45at some point between that sensation and my brain
0:06:45 > 0:06:48something very similar to that will happen.
0:07:01 > 0:07:03Sight is our dominant sense...
0:07:05 > 0:07:07..almost all animals can see.
0:07:07 > 0:07:11In fact, 96% of animal species have eyes.
0:07:15 > 0:07:18And what's interesting is that, at the molecular level,
0:07:18 > 0:07:22every eye in the world works in the same way.
0:07:27 > 0:07:29In order to form an image of the world,
0:07:29 > 0:07:33then, obviously, the first thing you have to do is detect light...
0:07:33 > 0:07:39and...I have a sample, here, of the molecules that do that,
0:07:39 > 0:07:41that detect light in my eye.
0:07:42 > 0:07:44It's actually specifically the molecule
0:07:44 > 0:07:49that's in the black and white receptor cells in my eyes, the rods.
0:07:49 > 0:07:51It's called rhodopsin
0:07:51 > 0:07:53and the moment I expose this to light
0:07:53 > 0:07:57you'll see an immediate physical change.
0:08:01 > 0:08:02There you go!
0:08:02 > 0:08:04Did you see that? It was very quick.
0:08:04 > 0:08:08It came out very pink indeed and it immediately went yellow.
0:08:08 > 0:08:11This subtle shift in colour
0:08:11 > 0:08:14is caused by the rhodopsin molecule changing shape
0:08:14 > 0:08:16as it absorbs the light.
0:08:16 > 0:08:21In my eyes, what happens is, that change in structure
0:08:21 > 0:08:23triggers an electrical signal,
0:08:23 > 0:08:26which ultimately goes all the way to my brain,
0:08:26 > 0:08:29which forms an image of the world.
0:08:31 > 0:08:33It's this chemical reaction that's responsible
0:08:33 > 0:08:35for all vision on the planet.
0:08:38 > 0:08:42Closely related molecules lie at the heart of every animal eye...
0:08:44 > 0:08:48..and that tells us that this must be a very ancient mechanism.
0:08:52 > 0:08:57To find its origins, we must find a common ancestor
0:08:57 > 0:09:00that links every organism that uses rhodopsin today.
0:09:01 > 0:09:03We know that common ancestor must have lived
0:09:03 > 0:09:07before all animals' evolutionary lines diverged...
0:09:09 > 0:09:11..but it may have lived at any time before then.
0:09:14 > 0:09:16So what is that common ancestor?
0:09:16 > 0:09:20Well, here's where we approach the cutting edge of scientific research.
0:09:20 > 0:09:24The answer is that we don't know for sure
0:09:24 > 0:09:27but a clue might be found...here...
0:09:28 > 0:09:31..in these little green blobs,
0:09:31 > 0:09:36which are actually colonies of algae, algae called volvox.
0:09:39 > 0:09:42We have very little in common with algae -
0:09:42 > 0:09:44we've been separated, in evolutionary terms,
0:09:44 > 0:09:46for over a billion years...
0:09:47 > 0:09:51..but we do share one surprising similarity.
0:09:51 > 0:09:56These volvox have light-sensitive cells that control their movement...
0:09:57 > 0:10:02..and the active ingredient in those cells is a form of rhodopsin,
0:10:02 > 0:10:03so similar to our own
0:10:03 > 0:10:06that it's thought they may share a common origin.
0:10:11 > 0:10:12'What does that mean?'
0:10:12 > 0:10:17Does it mean that we share a common ancestor with the algae
0:10:17 > 0:10:21and in that common ancestor the seeds of vision can be found?
0:10:24 > 0:10:28To find a source that may have passed this ability to detect light
0:10:28 > 0:10:30to both us and the algae,
0:10:30 > 0:10:33we need to go much further back down the evolutionary tree...
0:10:38 > 0:10:40..to organisms like cyanobacteria...
0:10:41 > 0:10:45..they were among the first living things to evolve on the planet
0:10:45 > 0:10:48and it's thought that the original rhodopsins
0:10:48 > 0:10:52may have developed in these ancient photosynthetic cells.
0:10:55 > 0:10:59So, the origin of my ability to see
0:10:59 > 0:11:03may have been well over a billion years ago,
0:11:03 > 0:11:10in an organism as seemingly simple as a cyanobacterium.
0:11:18 > 0:11:20The basic chemistry of vision
0:11:20 > 0:11:23may have been established for a long time
0:11:23 > 0:11:25but it's a long way from that chemical reaction
0:11:25 > 0:11:29to a fully functioning eye that can create an image of the world.
0:11:33 > 0:11:36'The eye is a tremendously complex piece of machinery'
0:11:36 > 0:11:39built from lots of interdependent parts
0:11:39 > 0:11:43and it seems very difficult to imagine how that could have evolved
0:11:43 > 0:11:46in a series of small steps
0:11:46 > 0:11:50but, actually, we understand that process very well indeed.
0:11:50 > 0:11:53I can show you by building an eye.
0:12:04 > 0:12:06The first step in building an eye
0:12:06 > 0:12:09would be to take some kind of light-sensitive pigment -
0:12:09 > 0:12:11rhodopsin, for example -
0:12:11 > 0:12:14and build it onto a membrane.
0:12:14 > 0:12:18So, imagine this is such a membrane with the pigment cells attached.
0:12:18 > 0:12:22Then immediately you have something that can detect the difference
0:12:22 > 0:12:25between dark and light.
0:12:25 > 0:12:28But the disadvantage, as you can see,
0:12:28 > 0:12:31is that there's no image formed, at all -
0:12:31 > 0:12:35it just allows you to tell the difference between light and dark.
0:12:35 > 0:12:40But you can improve that a lot by adding...
0:12:40 > 0:12:45an aperture, a small hole in front of the retina.
0:12:45 > 0:12:47So this is a movable aperture,
0:12:47 > 0:12:51just like the type of thing you've got in your camera.
0:12:51 > 0:12:56Now, you'll see that the image gets sharper...
0:12:58 > 0:13:01..but the problem is that, in order to make it sharper,
0:13:01 > 0:13:04you have to narrow down the aperture,
0:13:04 > 0:13:07and that means that you get less and less light.
0:13:07 > 0:13:10So this eye becomes less and less sensitive.
0:13:10 > 0:13:14So there's one more improvement that nature made,
0:13:14 > 0:13:19which is to replace the pinhole, the simple aperture...
0:13:21 > 0:13:23..with a lens.
0:13:29 > 0:13:30Look at that!
0:13:31 > 0:13:34A beautifully sharp image.
0:13:37 > 0:13:41The lens is the crowning glory of the evolution of the eye...
0:13:42 > 0:13:47..by bending light onto the retina, it allows the aperture to be opened,
0:13:47 > 0:13:49letting more light into the eye,
0:13:49 > 0:13:52and a bright, detailed image is formed.
0:13:59 > 0:14:03Our brain's ability to process the information in that image
0:14:03 > 0:14:05completes our visual system...
0:14:08 > 0:14:11..and allows us to respond to the world around us.
0:14:13 > 0:14:17As well as sight, we use another sense - hearing -
0:14:17 > 0:14:19to build up a picture of the world,
0:14:19 > 0:14:22and it too has an ancient evolutionary past.
0:14:29 > 0:14:32The story of how we developed our ability to hear
0:14:32 > 0:14:36is one of the great examples of evolution in action...
0:14:38 > 0:14:42..because the first animals to crawl out of the water onto the land
0:14:42 > 0:14:45would have had great difficulty hearing anything
0:14:45 > 0:14:47in their new environment.
0:14:53 > 0:14:56These are the Everglades...
0:14:56 > 0:14:59a vast area of swamps and wetlands
0:14:59 > 0:15:03that has covered the southern tip of Florida for over 4,000 years.
0:15:17 > 0:15:20Through the creatures we find here,
0:15:20 > 0:15:24like the American alligator, a member of the crocodile family,
0:15:24 > 0:15:28we can trace the story of how our hearing developed
0:15:28 > 0:15:29as we emerged onto the land.
0:15:35 > 0:15:39These are the smallest three bones in the human body.
0:15:39 > 0:15:43They're called the malleus, the incus and the stapes,
0:15:43 > 0:15:49and they sit between the eardrum and the entrance to your inner ear,
0:15:49 > 0:15:52so the place where the fluid sits.
0:15:53 > 0:15:57The bones help to channel sound into the ear through two mechanisms...
0:16:00 > 0:16:03..first, they act as a series of levers,
0:16:03 > 0:16:06magnifying the movement of the eardrum...
0:16:09 > 0:16:13..and second, because the surface area of the eardrum
0:16:13 > 0:16:17is 17 times greater than the footprint of the stapes,
0:16:17 > 0:16:21the vibrations are passed into the inner ear with much greater force.
0:16:23 > 0:16:26And that has a dramatic effect.
0:16:26 > 0:16:32Rather than 99.9% of the sound energy being reflected away,
0:16:32 > 0:16:35it turns out that with this arrangement, 60% of the sound energy
0:16:35 > 0:16:40is passed from the eardrum into the inner ear.
0:16:42 > 0:16:45Now, this set up is so intricate and so efficient
0:16:45 > 0:16:47that it almost looks as if these bones
0:16:47 > 0:16:51could only ever have been for this purpose
0:16:51 > 0:16:55but, in fact, you can see their origin
0:16:55 > 0:16:58if you look way back in our evolutionary history.
0:16:59 > 0:17:02Back around 530 million years,
0:17:02 > 0:17:07to when the oceans were populated with jawless fish called agnathans -
0:17:07 > 0:17:09they're similar to the modern lamprey.
0:17:09 > 0:17:12Now, they didn't have a jaw
0:17:12 > 0:17:16but they had gills, supported by gill arches.
0:17:16 > 0:17:20Now, over a period of around 50 million years
0:17:20 > 0:17:23the most forward of those gill arches
0:17:23 > 0:17:26migrated forward in the head...
0:17:28 > 0:17:31..to form jaws.
0:17:31 > 0:17:35And you see fish like these, the first jawed fish,
0:17:35 > 0:17:38in the fossil record around 460 million years ago.
0:17:38 > 0:17:43And there, at the back of the jaw, there is that bone,
0:17:43 > 0:17:48the hyomandibula, supporting the rear of the jaw.
0:17:48 > 0:17:50Then, around 400 million years ago,
0:17:50 > 0:17:55the first vertebrates made the journey from the sea to the land.
0:17:55 > 0:17:57Their fins became legs.
0:17:57 > 0:18:01But in their skull and throat other changes were happening -
0:18:01 > 0:18:04the gills were no longer needed
0:18:04 > 0:18:08to breathe the oxygen in the atmosphere and so they faded away,
0:18:08 > 0:18:13and became different structures in the head and throat,
0:18:13 > 0:18:16and that bone, the hyomandibular,
0:18:16 > 0:18:19became smaller and smaller...
0:18:19 > 0:18:23until its function changed.
0:18:23 > 0:18:28It now was responsible for picking up vibrations in the jaw
0:18:28 > 0:18:32and transmitting them to the inner ear of the reptiles.
0:18:32 > 0:18:38And that is still true today, of our friends...over there.
0:18:41 > 0:18:44But, even then, the process continued.
0:18:46 > 0:18:51Around 210 million years ago, the first mammals evolved,
0:18:51 > 0:18:55and, unlike our friends, the reptiles, here,
0:18:55 > 0:18:59mammals have a jaw that's made of only one bone.
0:18:59 > 0:19:04A reptile's jaw is made of several bones fused together.
0:19:04 > 0:19:08So that freed up two bones...
0:19:08 > 0:19:12which moved...and shrank...
0:19:13 > 0:19:15..and eventually...
0:19:15 > 0:19:21became the malleus, the incus and the stapes.
0:19:21 > 0:19:24So, this is the origin of those three tiny bones
0:19:24 > 0:19:28that are so important to mammalian hearing.
0:19:33 > 0:19:34He's quite big, isn't he?
0:19:41 > 0:19:43So, the evolution of our senses
0:19:43 > 0:19:46can be closely linked to changing environments.
0:19:49 > 0:19:52However, life is not only shaped by its surroundings...
0:19:54 > 0:19:57..there are limitations to form and function..
0:19:57 > 0:20:00imposed by the fundamental forces of nature.
0:20:03 > 0:20:06..and you can clearly see them at work
0:20:06 > 0:20:08when you examine the size of life.
0:20:22 > 0:20:25Our world is covered in giants...
0:20:29 > 0:20:32..the largest things that ever lived on this planet
0:20:32 > 0:20:33weren't the dinosaurs.
0:20:33 > 0:20:37They're not even blue whales - they're trees.
0:20:37 > 0:20:38These are mountain ash,
0:20:38 > 0:20:41they're the largest flowering plant in the world.
0:20:41 > 0:20:46They grow about a metre a year, and these trees are 60, 70,
0:20:46 > 0:20:47even 80 metres high.
0:20:47 > 0:20:51But to get this big you need to face some very significant
0:20:51 > 0:20:52physical challenges.
0:21:02 > 0:21:05These giants can live to well over 300 years old...
0:21:06 > 0:21:08..but they don't keep growing for ever...
0:21:10 > 0:21:14..there are limits to how big each tree can get.
0:21:14 > 0:21:16As with all living things,
0:21:16 > 0:21:20the structure, form and function of these trees has been shaped
0:21:20 > 0:21:23by the process of evolution, through natural selection
0:21:23 > 0:21:28but evolution doesn't have a free hand.
0:21:28 > 0:21:31It is constrained by the universal laws of physics.
0:21:38 > 0:21:40Each tree has to support its mass
0:21:40 > 0:21:43against the downward force of Earth's gravity...
0:21:45 > 0:21:48..at the same time, the trees rely on the strength
0:21:48 > 0:21:51of the interactions between molecules
0:21:51 > 0:21:54to raise a column of water from the ground
0:21:54 > 0:21:56up to the leaves in the canopy.
0:22:01 > 0:22:04And it's these fundamental properties of nature
0:22:04 > 0:22:09that act together to limit the maximum height of a tree,
0:22:09 > 0:22:14which, theoretically, lies somewhere in the region of 130 metres.
0:22:21 > 0:22:25Gravity doesn't just influence how tall a plant can grow...
0:22:26 > 0:22:28..it also affects how big animals can get.
0:22:38 > 0:22:41To show you how, I've come to track down
0:22:41 > 0:22:46one of Australia's most iconic animals - the red kangaroo.
0:22:50 > 0:22:53Red kangaroos are Australia's largest native land mammal.
0:22:56 > 0:22:59The evolution of the ability to hop
0:22:59 > 0:23:04gives kangaroos a cheap and efficient way to get around
0:23:04 > 0:23:06but not everything can move like a kangaroo.
0:23:09 > 0:23:13'The red kangaroo is the largest animal in the world
0:23:13 > 0:23:14'that moves in this unique way -'
0:23:14 > 0:23:18hopping across the landscape at high speed -
0:23:18 > 0:23:20and there are reasons why there aren't, you know,
0:23:20 > 0:23:23giant hopping elephants or dinosaurs,
0:23:23 > 0:23:27and they're not really biological.
0:23:27 > 0:23:30It's not down to the details of evolution by natural selection
0:23:30 > 0:23:32or environmental pressures.
0:23:32 > 0:23:35The larger an animal gets,
0:23:35 > 0:23:40the more severe the restrictions on its body shape and its movement.
0:23:42 > 0:23:46And it's gravity that imposes these restrictions.
0:23:50 > 0:23:52To understand why this is the case,
0:23:52 > 0:23:56I want to explore what happens to the mass of a body
0:23:56 > 0:23:58when that body increases in size.
0:24:02 > 0:24:04Take a look at this block.
0:24:04 > 0:24:06Let's say it has width - one,
0:24:06 > 0:24:08length - one and height - one,
0:24:08 > 0:24:10and its volume is one.
0:24:10 > 0:24:12Multiplied by one, multiplied by one,
0:24:12 > 0:24:16which is one cubic...things,
0:24:16 > 0:24:17whatever the measurement is.
0:24:17 > 0:24:20Now, its mass is proportional to the volume,
0:24:20 > 0:24:24so we could say that the mass of this block is one unit as well.
0:24:24 > 0:24:28Let's say that we're going to double the size of this thing,
0:24:28 > 0:24:31in the sense that we want to double its width,
0:24:31 > 0:24:32double its length...
0:24:33 > 0:24:36..and double its height.
0:24:36 > 0:24:39Its volume is two, multiplied by two, multiplied by two,
0:24:39 > 0:24:42equals eight cubic things.
0:24:42 > 0:24:44Its volume is increased by a factor of eight
0:24:44 > 0:24:48and so its mass is increased by a factor of eight as well.
0:24:50 > 0:24:54So although I've only doubled the size of the blocks,
0:24:54 > 0:24:56I've increased the total mass by eight.
0:24:58 > 0:24:59As things get bigger,
0:24:59 > 0:25:04the mass of the body goes up by the cube of the increase in size.
0:25:07 > 0:25:10Because of this scaling relationship,
0:25:10 > 0:25:14the larger you get, the greater the effect of gravity.
0:25:16 > 0:25:17As things get bigger,
0:25:17 > 0:25:22the huge increase in mass has a significant impact on the way large
0:25:22 > 0:25:28animals support themselves against gravity and how they move about.
0:25:33 > 0:25:35No matter how energy efficient
0:25:35 > 0:25:38and advantageous it is to hop like a kangaroo,
0:25:38 > 0:25:41as you get bigger it's just not physically possible.
0:25:49 > 0:25:53So gravity limits how big life can get.
0:25:58 > 0:26:00But it's not the physical force
0:26:00 > 0:26:03that controls how small an animal can get.
0:26:04 > 0:26:09And, in fact, the smaller you are, the less gravity affects your life.
0:26:11 > 0:26:16This is the rhinoceros beetle, named for obvious reasons,
0:26:16 > 0:26:19but, actually, it's only the males that have the distinctive
0:26:19 > 0:26:20horns on their heads.
0:26:23 > 0:26:27Gram for gram, these insects are among the strongest animals alive.
0:26:30 > 0:26:34I can demonstrate that by just getting hold of the top of his head.
0:26:34 > 0:26:36It doesn't hurt him at all...
0:26:36 > 0:26:41but watch what he is able to do.
0:26:47 > 0:26:48Look at that.
0:26:48 > 0:26:50So he's hanging onto this branch,
0:26:50 > 0:26:53which is many times his own bodyweight.
0:26:54 > 0:26:57Absolutely no distress at all.
0:26:59 > 0:27:01As things get smaller,
0:27:01 > 0:27:05it's a rule of nature that they inevitably get stronger.
0:27:07 > 0:27:08The reason is quite simple -
0:27:08 > 0:27:12small things have relatively large muscles
0:27:12 > 0:27:14compared to their tiny body mass...
0:27:14 > 0:27:16and this makes them very powerful.
0:27:24 > 0:27:27The beetles also appear to have a cavalier attitude
0:27:27 > 0:27:29to the effects of gravity.
0:27:32 > 0:27:33If they should fall...
0:27:36 > 0:27:38..they just bounce and walk off.
0:27:42 > 0:27:47If I fell a similar distance, relative to my size, I'd break.
0:27:49 > 0:27:52So why does size make such a difference?
0:27:59 > 0:28:02Time for a bit of fundamental physics.
0:28:02 > 0:28:06All things fall at the same rate under gravity.
0:28:06 > 0:28:09That's because they're following geodesics through curved space time
0:28:09 > 0:28:11but that's not important.
0:28:11 > 0:28:13The important thing for biology
0:28:13 > 0:28:17is that although everything falls at the same rate,
0:28:17 > 0:28:20it doesn't meet the same fate when it hits the ground.
0:28:24 > 0:28:27A grape...bounces.
0:28:33 > 0:28:34A melon...
0:28:39 > 0:28:40..doesn't bounce.
0:28:44 > 0:28:49Now the reasons for that are quite complex, actually.
0:28:49 > 0:28:54First of all, the grape has a larger surface area,
0:28:54 > 0:28:56in relation to its volume and therefore its mass,
0:28:56 > 0:28:57than the melon.
0:28:57 > 0:29:01And so, although in a vacuum, if you took away the air,
0:29:01 > 0:29:03they would both fall at the same rate,
0:29:03 > 0:29:07actually, in reality, the grape falls a bit slower than the melon.
0:29:07 > 0:29:09Also, the melon is more massive
0:29:09 > 0:29:13and so it has more kinetic energy when it hits the ground.
0:29:13 > 0:29:18Remember, from physics class, kinetic energy is a half MV squared,
0:29:18 > 0:29:20so you reduce M, you reduce the energy.
0:29:20 > 0:29:23The upshot of that is that the melon has a lot more energy -
0:29:23 > 0:29:27when it hits the ground it has to dissipate it in some way,
0:29:27 > 0:29:30and it dissipates it by exploding.
0:29:35 > 0:29:37The influence of Earth's gravity on your life
0:29:37 > 0:29:41becomes progressively diminished the smaller you get.
0:29:48 > 0:29:53However, having a larger surface area in relation to mass
0:29:53 > 0:29:57doesn't mean that life is always easy for small organisms.
0:29:57 > 0:30:00In fact, it can pose a brand new challenge...
0:30:02 > 0:30:03..keeping warm.
0:30:14 > 0:30:18These are southern bent-wing bats...
0:30:19 > 0:30:21..one of the rarest bat species in Australia.
0:30:24 > 0:30:28Every evening they emerge in their thousands from this cave
0:30:28 > 0:30:30in order to feed.
0:30:32 > 0:30:33When fully grown,
0:30:33 > 0:30:37these bats are just five and a half centimetres long,
0:30:37 > 0:30:40and weigh around 18 grams.
0:30:40 > 0:30:45Because of their size, they face a constant struggle to stay alive.
0:30:53 > 0:30:57We're using a thermal camera here to look at the bats
0:30:57 > 0:30:59and you can see that they appear as streaks across the sky.
0:30:59 > 0:31:01They appear as brightly as me,
0:31:01 > 0:31:04that's because they're roughly the same temperature as me.
0:31:04 > 0:31:06They're known as endotherms,
0:31:06 > 0:31:10they're animals that maintain their body temperature,
0:31:10 > 0:31:12and that takes a lot of effort.
0:31:12 > 0:31:14I mean, these bats have to eat something like
0:31:14 > 0:31:17three-quarters of their own bodyweight every night,
0:31:17 > 0:31:22and a lot of that energy goes into maintaining their temperature.
0:31:24 > 0:31:26As with all living things,
0:31:26 > 0:31:30the bats eat to provide energy to power their metabolism.
0:31:30 > 0:31:31Although, like us,
0:31:31 > 0:31:34they have a high body temperature when they're active,
0:31:34 > 0:31:39keeping warm is a considerable challenge on account of their size.
0:31:43 > 0:31:46The bats lose heat mostly through the surface of their bodies...
0:31:48 > 0:31:51..but because of simple laws governing the relationship
0:31:51 > 0:31:54between the surface area of a body and its volume,
0:31:54 > 0:31:57being small creates a problem.
0:32:00 > 0:32:01So, let's look at our blocks again
0:32:01 > 0:32:05but this time for surface area to volume.
0:32:05 > 0:32:07Here's a big thing, it's made of eight blocks,
0:32:07 > 0:32:08so its volume is eight units,
0:32:08 > 0:32:12and its surface area is two by two on each side,
0:32:12 > 0:32:16so that's four, multiplied by the six faces is 24.
0:32:16 > 0:32:20So the surface area to volume ratio is 24 to eight,
0:32:20 > 0:32:22which is three to one.
0:32:24 > 0:32:25Now, look at a smaller thing.
0:32:25 > 0:32:28This is one block, so its volume is one unit.
0:32:28 > 0:32:33The surface area is one by one, by one, six times, so it's six.
0:32:33 > 0:32:38So this has a surface area to volume ratio of six to one.
0:32:38 > 0:32:42So, as you go from big to small,
0:32:42 > 0:32:46your surface area to volume ratio increases.
0:32:46 > 0:32:50Small animals, like bats, have a huge surface area
0:32:50 > 0:32:52compared to their volume.
0:32:52 > 0:32:57As a result, they naturally lose heat at a very high rate.
0:32:58 > 0:33:03To help offset the cost of losing so much energy in the form of heat,
0:33:03 > 0:33:07the bats are forced to maintain a high rate of metabolism.
0:33:07 > 0:33:11They breathe rapidly, their little heart races,
0:33:11 > 0:33:13and they have to eat a huge amount.
0:33:13 > 0:33:19So a bat's size clearly affects the speed at which it lives its life.
0:33:28 > 0:33:31Right across the natural world, the size you are
0:33:31 > 0:33:34has a profound effect on your metabolic rate,
0:33:34 > 0:33:36or your speed of life.
0:33:40 > 0:33:45'Big animals have a much smaller surface area to volume ratio'
0:33:45 > 0:33:46than small animals,
0:33:46 > 0:33:50and that means that their rate of heat loss is much smaller.
0:33:50 > 0:33:54And that means that there's an opportunity there for large animals.
0:33:54 > 0:33:56They don't have to eat as much food to stay arm
0:33:56 > 0:34:00and therefore they can afford a lower metabolic rate.
0:34:05 > 0:34:08And there's one last consequence of all these scaling laws
0:34:08 > 0:34:12that I suspect you'll care about more than anything else
0:34:12 > 0:34:16and it's this - there's a strong correlation
0:34:16 > 0:34:20between the effective cellular metabolic rate of an animal
0:34:20 > 0:34:21and its lifespan.
0:34:21 > 0:34:26In other words, as things get bigger they tend to live longer.
0:34:33 > 0:34:38So, the physical forces of nature control life in all its sizes.
0:34:42 > 0:34:47Size, in turn, can determine how much energy a body needs
0:34:47 > 0:34:50and how long that body will last.
0:34:55 > 0:34:58Every single organism on the planet has something in common...
0:34:59 > 0:35:02..they've all been influenced by the environment
0:35:02 > 0:35:05and shaped by the laws of physics...
0:35:06 > 0:35:08..but I think the beauty of life
0:35:08 > 0:35:12is that although the same processes apply to all organisms,
0:35:12 > 0:35:15no two living things are the same...
0:35:17 > 0:35:20..and the tree of life, with its myriad branches,
0:35:20 > 0:35:24has spread to just about every corner of the planet.
0:35:26 > 0:35:28So, when you go outside tomorrow,
0:35:28 > 0:35:31just take a look at a little piece of your world
0:35:31 > 0:35:34in a corner of your garden, or a park,
0:35:34 > 0:35:39or even the grass that's growing in a crack in the pavement
0:35:39 > 0:35:43because there will be life there and it will be unique.
0:35:43 > 0:35:47There'll be nowhere like that anywhere else in the universe
0:35:47 > 0:35:49and that makes our tree,
0:35:49 > 0:35:53from the sturdiest branch to the most fragile twig,
0:35:53 > 0:35:54indescribably valuable.
0:36:20 > 0:36:23Subtitles by Red Bee Media Ltd