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00:00It takes 120 trillion cells to make a human.
00:07They are the fundamental units of life,
00:11making up our brain, muscles, organs, every part of us.
00:21In the last decade, scientists have been able to witness what once seemed impossible,
00:26the world inside a human cell.
00:35When I was a student, the idea that we could burrow deep inside a living cell was unthinkable.
00:43Recent advances have made it so scientists can see inside cells like never before.
00:49We can see the parts of single cells and how they work together.
00:55The more we learn about the universe, the simpler it seems.
00:59But the cell isn't like that.
01:01The more we find out, the more complicated things get.
01:06But these beautiful worlds are also on the front line of the longest war in history.
01:12This is a battle that goes back into the depths of time,
01:15to a time when the Earth was dominated by single cells and viruses.
01:20Every day, our cells confront these ancient virus enemies,
01:24tiny, ruthless machines that kill to reproduce.
01:28There is this whole mechanism inside cells that are taking out viruses
01:32that previously we just didn't know was there.
01:34It is a four billion year old struggle that has changed the course of our evolution.
01:41This battle of these viruses against your cells,
01:44this amazing, epic science fiction movie,
01:47it's going on inside your body all the time.
01:50And you don't even know it.
02:17Our cells are the basic building blocks of living tissue
02:22and the smallest units of what makes us human.
02:28And yet, beneath the surface of every one
02:35lies a world stranger than any in science fiction.
02:40A world in which a billion microscopic machines all play their part,
02:45working in concert through every second of our life.
02:51Every one of us is made of 120 trillion cells,
02:54and every one of those cells is different,
02:56but they contain the same instructions.
02:59Cells are a bit like babies.
03:00When they're born, they all look the same, but they change very quickly.
03:03In different countries, they learn to speak different languages.
03:06And our bodies are like that.
03:07Some cells speak heart, and some cells speak liver.
03:14The workers of this incredible world are proteins,
03:18chains of complex chemicals
03:21that can lock together to transform into spectacular machines.
03:28Others work to create incredible structures,
03:31like the internal skeleton that makes up our brains.
03:34Like the internal skeleton that holds the cell together.
03:38These great trusses are constantly adjusting to stresses and strains,
03:43building and rebuilding to give the cell its shape and strength.
03:51Then there are the motor proteins, haulage workers,
03:54that use the cell's skeleton as highways to deliver food, chemicals
03:58and the essential building materials of life to wherever they are needed.
04:05They are just one of the astonishing micromachines
04:09that keep this bustling community healthy.
04:12Scientists are asked all the time,
04:14how do things in a cell know how to get where they're supposed to go to do their job?
04:18And for sure, cells are very chaotic,
04:20and things are bumping into each other,
04:22and most of that's just random.
04:24But enough things get where they're supposed to go
04:27that the entire system works.
04:31And powering all this activity are the cell's power stations.
04:37Inside these free-floating structures, called mitochondria,
04:41turbines spin at over 1,000 times per minute...
04:48..recharging billions of tiny chemical batteries.
05:00Everything we do,
05:02every heartbeat,
05:04every movement,
05:06every thought,
05:08is powered by the batteries charged by these cellular power stations.
05:14MUSIC
05:27And everything in this world works to a master plan.
05:34And the plan is protected deep in the heart of every cell.
05:39The nucleus is the vault containing the instruction manual for life.
05:46DNA.
05:52DNA is a chain of chemicals organised into genes.
05:59Each gene holds the instructions to build a specific protein.
06:04The double helix contains over 20,000 instructions
06:08that tell our cells what to make and when.
06:12How to organise not just our cells, but our entire bodies.
06:19The double helix has become the icon of the 21st century,
06:23and it's pretty amazing stuff.
06:25There's six feet of DNA in every cell of the body.
06:28And if all those bits were set out in a straight line,
06:31they'd reach to the moon and back thousands of times.
06:45But this crucial chain of chemicals would be useless
06:48without an army of microscopic machines that endlessly travel its length,
06:53repairing it and transcribing it...
06:56..turning the DNA into instructions
06:59that the cell can understand.
07:09Once a gene has been copied,
07:11the instructions are carried outside the nucleus.
07:16Here, mobile factories read them...
07:21..and turn them into proteins.
07:29Up to two million different kinds...
07:33..each with its own specific shape and purpose.
07:45And little goes to waste in the cell.
07:48Used and faulty proteins are tagged for recycling...
07:53..then chewed apart by powerful roving shredders called proteasomes,
07:59reducing them to tiny building blocks for new proteins.
08:05But each cell is also part of a wider neighbourhood of cells,
08:10all continually communicating with each other.
08:13Fragments of shredded proteins are constantly transported to the surface.
08:23Here, they are presented for inspection...
08:29..to be monitored by the guardians of our body's immune system...
08:36..our white blood cells.
08:43These roving soldiers check the protein fragments
08:47for signs of damage or infection.
08:52And for the moment, everything is in order.
09:23Every single human cell contains this world of breathtaking complexity,
09:29organised by the nuclear machines at its heart,
09:32ceaselessly working from instructions written down in our DNA.
09:38But our cells are under constant attack.
09:42And this cell is about to face an ancient enemy...
09:49..in an encounter that starts with an event so commonplace...
09:56..that it's almost impossible to predict.
10:00Every day, our bodies are constantly bombarded by these invisible critters,
10:05bacteria and viruses, but we have our skin.
10:08It's our first line of defence that keeps them out.
10:11But we have Achilles' heels.
10:13We have openings to the outside world,
10:15our mouths, our noses, our eyes, our ears,
10:18our eyes, our nose, our ears,
10:20our eyes, our nose, our ears,
10:22our nose, our nose, our nose,
10:24our nose, our nose, our nose,
10:26our nose, our nose, our nose,
10:28our mouths, our noses,
10:30we touch things, we rub our lips,
10:32we rub our eyes or wipe our nose.
10:34They can get in, and once they're in, they're in.
10:50Inhaled from a sneeze, an alien army is being carried into our body.
10:58A million invaders, hell-bent on destruction.
11:16This is one of our most common enemies.
11:20The Adenovirus.
11:23It's a masterpiece of design.
11:27And each one has a single aim.
11:37To breach a cell's defences
11:41and reach the nucleus.
11:45Once inside, any one of these viruses can take a life.
11:52Take control of the cell
11:56and reproduce 10,000 times over.
12:01The result could be anything.
12:04From the common cold, to pneumonia, even death.
12:15But our bodies are prepared.
12:23As the viruses approach the cell,
12:26they are met by a cloud of resistance.
12:34Antibodies, Y-shaped proteins that identify alien intruders,
12:41patrol the space between our cells,
12:45looking for viruses.
12:47Recognising the invader, they lock to the virus's armour-plating.
12:52Shackling them together, making the viruses easy meat
12:56for the white blood cells that feed on alien invaders like these.
13:00Antibodies and white blood cells form the front line of our immune system.
13:06The immune system is certainly amazing,
13:09and it actually is a very powerful weapon.
13:11But that's just one part of your body's defences.
13:14Our DNA encodes all these other features
13:17that help us to fight against virus at every single step.
13:31Despite the body's alien nature,
13:34the immune system is a very powerful weapon.
13:38Despite the body's early immune response,
13:42hundreds of thousands of viruses make it through to our cell.
13:50But at the surface, they face their next obstacle.
14:08The cell's membrane, or skin.
14:13The surface of the cell is an amazingly complicated place.
14:17There are hundreds, maybe thousands of receptor proteins
14:20sticking out of the cell, and they all have a unique function to play.
14:24Some of them would be just transporting information
14:27from outside the cell into the cell.
14:29Other receptors can bring whole cargos in.
14:32The surface of each cell is a living barrier,
14:36teeming with security proteins
14:39that constantly monitor molecules as they enter and leave.
14:50Small molecules like water and oxygen
14:53can simply seep through the membrane.
15:02Larger molecules like sugar
15:05are allowed entry through specialised pumps.
15:09But the largest deliveries require a special key
15:13before they are allowed into the cell.
15:17These protein keys are recognised by teams of mobile sentries
15:22that continually roam the surface.
15:25This sophisticated system is designed
15:28to keep harmful molecules out of the cell.
15:32But over billions of years of evolution,
15:35the adenovirus has evolved its very own key,
15:39etched into the end of these projecting fibres.
15:47Antibodies still cling to some of these fibres,
15:50but they are no longer necessary.
15:52Antibodies still cling to some of these fibres,
15:55blocking many of the counterfeit keys,
15:58but not all.
16:00One by one, sentries all over the cell's surface are fooled.
16:11And the virus army quietly slips inside.
16:17In this ancient battle for the cell,
16:19it's round two to the virus.
16:30So how far back does it go, this cat-and-mouse game,
16:34this battle between cells and viruses?
16:37Every indication suggests it goes right back
16:40to the origins of life on Earth.
16:42Wherever life started, very early on,
16:45there was a divergence.
16:46Two different strategies that life followed.
16:49One of them was to become more complex, to become cells,
16:52to become ultimately organisms like ourselves.
16:55The other way was to remain simple, to become viruses,
16:59and to exploit those cells to their own ends,
17:02to replicate themselves.
17:11Beneath the surface,
17:13the cell prepares to receive the deadly invaders.
17:17Fooled into thinking that the virus is an important nutrient,
17:21special proteins slot together to form a spherical mould.
17:30They pinch out a bubble of cellular membrane,
17:34wrapping the virus inside.
17:39Finally, a separate protein pinches the bubble free,
17:43delivering the virus into the cell's interior.
17:47Unwittingly, the cell has just taken a large step
17:52towards its own downfall.
18:04Every single member of this invading virus army
18:07has the weaponry to ultimately destroy this cell.
18:12Its protein shell is a multi-layered,
18:14cloak of deception,
18:16which is still more surprises in store.
18:21And at its heart, it carries a tiny string of DNA.
18:25Its ultimate weapon.
18:27It's a masterpiece of evolution and design.
18:30And yet scientists still can't decide
18:33if it's actually alive or dead.
18:36At the level of large animals like ourselves,
18:39the difference between living things and non-living things
18:41is very obvious.
18:43Come down a level, though, to cells,
18:45and it becomes a bit more ambiguous.
18:48For our own cells, of course,
18:50you can still tell immediately that they're alive.
18:53Come down another level, though, to the virus,
18:56and it's no longer obviously alive.
18:58They don't look alive,
19:00yet they behave perhaps as if they are.
19:03They behave with a sense of purpose.
19:07A virus isn't strictly alive.
19:09It can't make more of itself on its own.
19:12It only can replicate
19:14if it uses parts that it hijacks from a cell.
19:20But the cell still has a formidable array of defences
19:24to keep these killing machines at bay.
19:31Every delivery that the cell receives
19:34is taken to a sorting station,
19:36called an endosome.
19:39Endosomes process incoming supplies
19:42and decide where inside the cell they will be delivered.
19:48The first step of the process
19:51is to break them down.
19:55The virus army is about to be digested.
20:01The walls of the sorting stations
20:03are fitted with specialised protein pumps.
20:10The pumps draw in special atoms,
20:13turning the inside of the endosome
20:16into an acid bath.
20:24The acid breaks down large nutrients
20:27into smaller molecules
20:29that are easier for the cell to transport and use.
20:34And as the acid eats away at the virus's outer shell,
20:38it begins to break apart.
20:43This should spell disaster for the adenovirus.
20:48But the acid is part of its escape plan.
20:53The virus fibres are the first to break away.
20:59But their disintegration
21:00releases a special protein hidden inside the virus
21:05that targets the wall of the sorting station.
21:11Tearing the membrane apart
21:14and setting the virus free.
21:23But not every virus escapes.
21:25Many still carry antibodies locked to their surface.
21:29Their primary job was to alert the immune system
21:32to intruders,
21:34but their firm grip now ties the shell together.
21:38The fibres cannot break free
21:41and the escape protein stays trapped inside the shell.
21:49Countless viruses are eaten away by the virus.
21:52Countless viruses are eaten away before they can escape.
22:04But enough are released.
22:07Now there is nothing between these viruses
22:10and the nucleus of the cell, their ultimate goal.
22:15Yet although they are just five micrometres from their target,
22:22most might as well be a million miles away.
22:36For 90% of the army,
22:39the invasion will end here,
22:41floating helplessly beneath the surface.
22:52Although they are surrounded by the constant bustle of cellular activity,
22:57the inert invaders have no way of moving themselves.
23:05And they have no way of utilising the energy
23:08generated by the cell's floating power stations,
23:13the mitochondria.
23:22Inside each mitochondrion,
23:25the food we eat and the air we breathe
23:28drives thousands of turbines
23:31that continually recharge billions of tiny batteries.
23:36But what is even more extraordinary
23:39is that scientists believe that mitochondria were once
23:42the backbone of the human body.
23:45But what is even more extraordinary
23:48is that scientists believe that mitochondria were once
23:51simple cells themselves.
23:54Then one was swallowed by another cell,
23:57firing one of the greatest leaps in evolution,
24:00complex life.
24:04To be complex at all,
24:06you must have all this machinery, all these proteins
24:09encoded by genes, and to support all of that
24:12requires a tremendous amount of energy.
24:15All complex life share a single common ancestor
24:17and that ancestor arose just once
24:20in four billion years of life on Earth.
24:23For two to three billion years, it was bacteria and nothing else,
24:26and then this complex cell arose.
24:29One simple cell got inside another simple cell.
24:32It's a very rare event in itself, and once it happened,
24:35it transforms the energetic possibilities of life.
24:39Without that energy, evolution could never have produced
24:42the astonishing diversity of life that we see around us.
24:45Without that energy, we wouldn't see plants and animals,
24:48we wouldn't see ourselves.
24:51The world would be an almost sterile desert.
25:07Throughout each cell, hundreds of mitochondria
25:10feed energy to power the network of proteins
25:12that make us the complex creatures that we are.
25:21The virus has evolved into a model of efficiency,
25:24but the simplicity of its design
25:27makes it useless without the machinery of complex life.
25:34But just beneath the surface,
25:37large numbers of motor proteins, molecular haulage workers,
25:39await nutrients processed for delivery by the endosomes.
26:00And in this billion-year arms race,
26:04the virus has evolved the precise mechanism
26:06to attach to the cell's motor proteins.
26:21Now it can use the energy of the mitochondria.
26:29The virus is on its way.
26:37It has hijacked the cell's own transport system
26:40and is being carried towards the nucleus
26:43and its ultimate prize,
26:46the DNA machines it needs to take control of the cell.
26:57These microscopic motorised legs
27:00are a wonder of the natural world.
27:03Slowed down to one-thirty,
27:06at one-fortieth of their normal speed,
27:09their movement is clearly visible.
27:12But at their actual speed, over 100 steps a second,
27:15they would appear a blur.
27:30But speed isn't everything.
27:33Cells are densely packed
27:36and internal highways are littered with obstacles.
27:42And these motor proteins can only move in one direction.
27:44For this virus, it seems to be the end of the road.
28:15But scientists have recently discovered
28:18the virus locks on to a second motor protein.
28:31And this one is built to move in the opposite direction.
28:45Together, the two motor proteins
28:48can navigate around almost any obstacle.
28:54And once again, the invader benefits.
29:15The virus is on the move again.
29:24And it leads an army of hundreds.
29:44It's been almost an hour since the adenovirus first attacked the cell.
30:07The nucleus is just one more hour away.
30:10Until recently, scientists thought
30:13that once the viral army was on the march,
30:16nothing could stop it.
30:19But then they found that the cell
30:22has its own internal immune system.
30:25There is this whole mechanism inside cells
30:28that are taking out viruses that previously we just didn't know was there.
30:31And I remember the day on which we published the paper about it.
30:34I woke up to hear it being announced on the national radio
30:36and then went into a shop to pick up the newspapers
30:39to discover it was on the front page.
31:06Dotted along the cell's highway system,
31:09a special protein searches for anything
31:12carrying antibodies from the surface.
31:15The clever thing about this protein
31:18is that it uses systems that the cell already has in place.
31:21Once it's stuck to the antibody,
31:24it sends signals to the cell
31:27and then it sends signals back to the cell
31:30and then it sends signals back to the cell
31:33and then it sends signals back to the cell
31:36and then it sends signals to a cellular machine called the proteasome.
31:39And the proteasome plays a role of recycling proteins in the cell.
31:42So it gets brought along to the virus and it destroys the virus,
31:45breaking down all its parts into tiny fragments.
31:48Once attached,
31:51the defence protein initiates a chain reaction,
31:54attracting specialised tagging proteins.
31:57Together, they mark the virus for destruction.
32:07Then it's only a matter of time
32:13before the recyclers arrive.
32:26They rip the virus to shreds
32:36Somewhere inside your body, this battle is reaching.
33:05Right now.
33:15The discovery of TRN21 provides potentially new ways of making therapeutics to fight viruses.
33:21And one way this could work is if we could find ways of encouraging the immune system to make more TRN21.
33:26So as soon as that virus enters into the cell, the TRN21 is ready to recognize the antibodies and destroy the virus.
33:43By working together, the defense proteins and recycling shredders can destroy an army of viruses in just a few hours.
34:11But it only takes a single virus to take control of an entire cell, spreading infection throughout the body.
34:24With no antibodies attached, this virus has evaded the cell's shredders.
34:39Nothing now stands between it and its target.
34:56The virus is now just one thousandth of a millimeter from the nucleus.
35:05But if it is to achieve its ultimate goal, it first has to get inside.
35:13Compared to the cell, the virus is tiny. But really, they're just different versions of the same machine.
35:19And its only job is to copy itself. But the virus needs to take advantage of our cell mechanism for its own selfish ends.
35:30At the heart of every cell lies the nucleus. And it is a world all of its own.
35:42Its surface is made of the same molecules as the cell membrane.
35:48But entry into this world is governed by completely different gateways.
35:55Across the surface, protein arms search for molecules to draw inside nuclear pores.
36:05Through these gateways, billions of chemical messages and instructions pass between the DNA and the cell.
36:15But only if they are recognized by the protein arms.
36:24But once again, the viral shell carries a counterfeit pass.
36:32The arms lock on, but the virus is too large to be ferried inside.
36:43Thinking that they have hit an obstruction, the motor proteins shunt the virus into reverse.
37:14Pulled in two directions, the virus is ripped apart.
37:31But what looks like a catastrophe for the virus is, in fact, its master stroke.
37:43Now, the single strand of DNA it held inside is carried through the pore and into the cell's control center.
38:14Inside the human cell nucleus, there are about 23,000 genes.
38:22They code for thousands and thousands of biochemical pathways.
38:30The virus has just got 40. But with those 40, it can do remarkable things.
38:39It's so tiny, just a piece of DNA, a couple of proteins to make its shell,
38:47and yet it can take over and wreak havoc in a huge human cell. It's brilliant.
38:55The adenovirus has proven itself a master of deception,
39:03continually exploiting the cell's processes to further its own deadly aims.
39:11But its greatest trick is yet to come.
39:20The cell's DNA machines have no way of telling the difference between its own DNA and the DNA of the virus.
39:33Blindly, they set about converting its deadly code into thousands of instructions for the cell to act upon.
39:41Blueprints for the cell's own destruction.
40:12But the machines that turn the blueprints into proteins lie outside the nucleus.
40:24Out in the main body of the cell, the instructions are met by a squadron of mobile proteins.
40:33The ribosomes precisely follow the instruction and start to construct viral proteins.
40:39Each is carefully folded into a specific shape, with a unique job to do.
40:45These large celling machines, ribosomes, are absolutely fundamental to life.
40:51They are the backbone of the cell.
40:58These large celling machines, ribosomes, are absolutely fundamental to life.
41:04And very similar forms of them are found in every type of living cell on the planet.
41:10They read the genetic information and they decode it, bringing in the building blocks that make up proteins
41:16and sticking them together to make these functional molecules that are going to work inside the living cell.
41:27Only these functional molecules are the kit of parts needed to build an enemy army.
41:35But the army will not be built out here.
42:06The raw material for the new army is drawn back inside the nucleus, ready for construction.
42:17With its mission reaching its climax, the virus turns its attention to the cell's DNA,
42:23halting any process it doesn't need.
42:29The virus has taken control of the cell.
42:35The virus has taken control of the cell.
42:42The virus has taken complete control.
42:53And yet the cell still has a small window of opportunity.
42:59Before all normal activity stops, it has just enough time to send a message to the outside world.
43:12This parcel contains fragments of the viral army.
43:19The parcel merges with the cell membrane, and the enemy fragments are pushed to the surface.
43:25Flags, warning of the invasion that has taken place.
43:31The virus has taken control of the cell.
43:37The virus has taken control of the cell.
43:43Flags, warning of the invasion that has taken place.
43:49If patrolling white blood cells spot the distress signal,
43:55they will destroy the cell along with the entire alien army inside.
44:01If not, the infection will spread from cell to cell to cell.
44:14After just one day of occupation, the virus has complete control over the cell.
44:22With routine maintenance halted, the cell has started to decay.
44:28And all activity is now focused on building the brand new viral army inside the nucleus.
44:44The new army self-assembles.
44:50How do viruses know how to invade our cells, how to break and enter the nucleus itself?
44:56We know that viruses and cells co-evolved together over long periods of time, but it's more than that.
45:02We're actually surprisingly closely related.
45:05It turns out that the viruses that attack us are actually made from bits and pieces of our own cells.
45:11As our cells were evolving, as the nucleus itself was first coming to be,
45:16so these viruses were cobbled together from bits and pieces,
45:19and they can attack our nucleus because they're made of the same stuff.
45:25Already built into its surface are the binding sites for the cell's motorised legs.
45:36Fibres snap into place, arming each virus with the keys to enter other cells.
45:46But these shells are harmless without its instructions.
45:54The final component is loaded.
45:58Identical copies of the virus's deadly DNA.
46:06Carried by powerful motors, long strands of DNA are fed into every single virus.
46:17All this is the result of one single virus getting into the nucleus of another virus.
46:28All this is the result of one single virus getting through our cell's defences.
46:42It's been two days since the virus entered our cells.
46:49It's been two days since the virus entered the body,
46:53and the nucleus, once the centre of cellular organisation,
46:57now harbours an army of 10,000 deadly viruses.
47:19But before it can begin its conquest, it has to overcome two barriers.
47:25The army is trapped inside the tough nuclear membrane held at the centre of the cell itself.
47:33And then there is the skin of the cell itself.
47:40The protein factories outside the nucleus are instructed to build viral saboteurs.
47:54The first are released into the decaying cell and target its cytoskeleton.
48:03The effects are cataclysmic.
48:10Without support, the cell starts to collapse.
48:23Now the virus turns its attention to the nuclear membrane.
48:33A second protein is released,
48:39called the adenovirus death protein.
48:43It burrows into the membrane and weakens it.
48:49The nucleus can no longer contain the bulging army.
49:19Beyond the nucleus, the cell is a wasteland.
49:33Unrecognisable from the harmonious, buzzing community of just 48 hours ago.
49:40The cell is now completely helpless to stop the virus army from flooding into surrounding tissue.
49:51Attacking neighbouring cells,
49:55the virus army is forced to retreat.
49:59The battle for this cell is over.
50:30But the war has only just begun.
50:49While the virus has been busy inside the cell,
50:55our antibodies have adapted and now come back in force,
51:01carrying new receptors, tailor-made to lock on to the escaping army.
51:09Yet even in these numbers, they cannot stop every virus.
51:16But they are not alone.
51:20The cell's dying message to the outside world was not in vain.
51:24Giant white blood cells flock to the stricken cell to devour the escaping hordes.
51:30They too are learning how to tackle this particular invader.
51:35Once the virus has been detected by the immune system,
51:38there's a heightened level of security inside your body.
51:41And one of the results of this is that the cells that make antibodies,
51:44and make the right antibody for that virus,
51:46will make lots of copies of themselves.
51:48And then they will start pumping out up to 5,000 antibodies per second
51:52to flood your bloodstream, the spaces between your cells.
51:55So as viruses emerge from dying cells, they can get tagged by antibodies
51:58and then destroyed by white blood cells.
52:02Taking no chances, white blood cells engulf nearby cells that may have been infected.
52:12Meanwhile, surrounding healthy cells make the ultimate sacrifice,
52:18destroying themselves to stop the spread of the virus.
52:25It is only at this stage that we become aware of the battle taking place inside us.
52:30Increasing blood flow brings more white blood cells to the battleground,
52:34causing our nasal tissue to become inflamed.
52:37What we feel as a blocked nose is in fact the clearest sign of a viral onslaught.
52:45Once you've had an infection, one cell that makes the right antibody for that infection
52:50will be kept inside your bone marrow for the rest of your life,
52:53so that if you ever get another infection with the same virus,
52:56the immune system already knows how to respond,
52:58it knows what antibody to make, and it can respond very quickly and stop you getting sick.
53:06Working together, the body's immune system finally prevents the viral infection from spreading.
53:13It's one more battle in an unending war.
53:19The struggle between viruses and ourselves is evolution, but it's co-evolution.
53:25Both sides have to change.
53:27It's a bit like an arms race.
53:29One party gets better weapons, the other party has to match them.
53:36Even though the individual cells are fighting this epic battle against the viruses,
53:41you have trillions of cells, and so even if one cell loses its war,
53:45most of the time the organism wins and we get better.
54:03The war is over.
54:07For now.
54:11For now.
54:17Although many cells have been lost, there are many more healthy cells waiting to replace them.
54:30And at the heart of each one lies an identical copy of our DNA.
54:35Our DNA.
54:40Inherited from our parents and their parents over countless generations,
54:45our DNA connects us to a family tree that stretches back over 3 billion years,
54:51to the very first cell.
54:55A cell that existed long before humans, long before mammals, long before the dinosaurs.
55:02It's a lineage that connects us to every living creature and plant on Earth.
55:08We are all descended from a single prehistoric ancestor,
55:12a cell containing the single strand of DNA that started it all.
55:21But the virus is as old as we are.
55:24It has evolved alongside us, forcing us to adapt, to change or die in a deadly game of cat and mouse.
55:36This eternal arms race has driven our evolution and made us both stronger.
55:46We wouldn't be what we are today were it not for this battle with our ancient enemy.
55:55The story of the cell is a story of innovation and change.
55:58And because viruses continuously force cells to change,
56:02they actually aid their adaptation to different environments.
56:07And for that reason, they've also helped shape us, they've made us who we are.
56:13Every minute of every day, this battle with the virus rages within 7 billion of us.
56:24Though we are rarely aware of it, we fight each other, change each other, improve each other.
56:34Uncovering the truth about our ancestors here on BBC HD this week,
56:38a groundbreaking prehistoric autopsy gets underway tomorrow at 9.
57:03www.bbc.co.uk