1 00:00:03,000 --> 00:00:07,040 It takes 120 trillion cells to make a human. 2 00:00:08,000 --> 00:00:11,520 They are the fundamental units of life, 3 00:00:11,520 --> 00:00:17,000 making up our brain, muscles, organs - every part of us. 4 00:00:21,720 --> 00:00:23,200 In the last decade, 5 00:00:23,200 --> 00:00:27,000 scientists have been able to witness what once seemed impossible - 6 00:00:27,000 --> 00:00:30,000 the world inside a human cell. 7 00:00:35,400 --> 00:00:37,320 When I was a student, 8 00:00:37,320 --> 00:00:41,960 the idea that we could burrow deep inside a living cell was unthinkable. 9 00:00:44,440 --> 00:00:48,680 Recent advances have made it so scientists can see inside cells 10 00:00:48,680 --> 00:00:49,960 like never before. 11 00:00:49,960 --> 00:00:53,560 We can see the parts of single cells and how they work together. 12 00:00:55,560 --> 00:00:58,720 The more we learn about the universe, the simpler it seems. 13 00:00:58,720 --> 00:01:00,640 But the cell isn't like that. 14 00:01:00,640 --> 00:01:04,320 The more we find out, the more complicated things get. 15 00:01:06,160 --> 00:01:09,840 But these beautiful worlds are also on the front line of the longest war 16 00:01:09,840 --> 00:01:11,520 in history. 17 00:01:11,520 --> 00:01:15,720 This is a battle that goes back into the depths of time, 18 00:01:15,720 --> 00:01:20,000 to a time when the earth was dominated by single cells and viruses. 19 00:01:20,000 --> 00:01:23,880 Every day, our cells confront these ancient virus enemies, 20 00:01:23,880 --> 00:01:27,880 tiny, ruthless machines that kill to reproduce. 21 00:01:27,880 --> 00:01:31,760 There is this whole mechanism inside cells that are taking out viruses 22 00:01:31,760 --> 00:01:34,480 that previously we just didn't know was there. 23 00:01:34,480 --> 00:01:38,440 It is a four-billion-year-old struggle that has changed the course 24 00:01:38,440 --> 00:01:39,760 of our evolution. 25 00:01:41,000 --> 00:01:44,440 This battle of these viruses against your cells, 26 00:01:44,440 --> 00:01:47,560 this amazing, epic science fiction movie, 27 00:01:47,560 --> 00:01:50,520 it's going on inside your body all the time, 28 00:01:50,520 --> 00:01:52,160 and you don't even know it. 29 00:02:17,000 --> 00:02:22,280 Cells are the basic building blocks of living tissue, 30 00:02:22,280 --> 00:02:25,640 and the smallest units of what makes us human. 31 00:02:28,000 --> 00:02:29,400 And yet... 32 00:02:31,080 --> 00:02:36,760 ..beneath the surface of every one lies a world stranger 33 00:02:36,760 --> 00:02:38,240 than any in science fiction. 34 00:02:42,840 --> 00:02:46,280 A world in which a billion microscopic machines 35 00:02:46,280 --> 00:02:50,960 all play their part, working in concert through every second 36 00:02:50,960 --> 00:02:52,280 of our life. 37 00:02:53,600 --> 00:02:57,240 Every one of us in made of 120 trillion cells, 38 00:02:57,240 --> 00:02:59,280 and every one of those cells is different. 39 00:02:59,280 --> 00:03:01,960 But they contain the same instructions. 40 00:03:01,960 --> 00:03:05,200 Cells are a bit like babies. When they're born, they all look the same 41 00:03:05,200 --> 00:03:06,680 but they change very quickly. 42 00:03:06,680 --> 00:03:09,760 In different countries they learn to speak different languages, 43 00:03:09,760 --> 00:03:12,480 and our bodies are like that - some cells speak heart, 44 00:03:12,480 --> 00:03:14,160 and some cells speak liver. 45 00:03:17,360 --> 00:03:21,640 The workers of this incredible world are proteins, 46 00:03:21,640 --> 00:03:24,920 chains of complex chemicals 47 00:03:24,920 --> 00:03:29,120 that can lock together to transform into spectacular machines. 48 00:03:31,560 --> 00:03:34,560 Others work to create incredible structures, 49 00:03:34,560 --> 00:03:38,400 like the internal skeleton that holds the cell together. 50 00:03:38,400 --> 00:03:44,000 These great trusses are constantly adjusting to stresses and strains, 51 00:03:44,000 --> 00:03:48,360 building and rebuilding to give the cell its shape and strength. 52 00:03:51,680 --> 00:03:55,120 Then there are the motor-proteins, haulage workers that use 53 00:03:55,120 --> 00:03:58,800 the cell's skeleton as highways to deliver food, chemicals 54 00:03:58,800 --> 00:04:02,920 and the essential building materials of life to wherever they are needed. 55 00:04:06,960 --> 00:04:10,760 They are just one of the astonishing micro machines that keep 56 00:04:10,760 --> 00:04:13,320 this bustling community healthy. 57 00:04:13,320 --> 00:04:16,760 Scientists are asked all the time, how do things in a cell know 58 00:04:16,760 --> 00:04:19,520 how to get where they're supposed to go to do their job? 59 00:04:19,520 --> 00:04:22,360 And for sure cells are very chaotic and things are bumping 60 00:04:22,360 --> 00:04:25,000 into each other and most of that's just random. 61 00:04:25,000 --> 00:04:28,240 But enough things get where they're supposed to go 62 00:04:28,240 --> 00:04:29,880 that the entire system works. 63 00:04:32,360 --> 00:04:36,160 And powering all this activity are the cell's power stations. 64 00:04:38,760 --> 00:04:42,520 Inside these free-floating structures called mitochondria, 65 00:04:42,520 --> 00:04:45,920 turbines spin at over 1,000 times per minute... 66 00:04:49,160 --> 00:04:52,720 ..recharging billions of tiny chemical batteries. 67 00:05:00,960 --> 00:05:04,800 Everything we do, every heartbeat, 68 00:05:04,800 --> 00:05:08,440 every movement, every thought, 69 00:05:08,440 --> 00:05:12,640 is powered by the batteries charged by these cellular power stations. 70 00:05:27,360 --> 00:05:30,760 And everything in this world works to a master plan. 71 00:05:34,680 --> 00:05:37,920 And the plan is protected deep in the heart of every cell. 72 00:05:39,720 --> 00:05:43,800 The nucleus is the vault containing the instruction manual for life. 73 00:05:47,000 --> 00:05:48,680 DNA. 74 00:05:52,360 --> 00:05:56,600 DNA is a chain of chemicals, organised into genes. 75 00:05:59,280 --> 00:06:03,120 Each gene holds the instructions to build a specific protein. 76 00:06:05,320 --> 00:06:09,160 The double helix contains over 20,000 instructions 77 00:06:09,160 --> 00:06:13,240 that tell our cells what to make and when, 78 00:06:13,240 --> 00:06:17,920 how to organise not just our cells, but our entire bodies. 79 00:06:20,200 --> 00:06:23,800 The double helix has become the icon of the 21st century, 80 00:06:23,800 --> 00:06:25,760 and it's pretty amazing stuff. 81 00:06:25,760 --> 00:06:29,080 There's six feet of DNA in every cell of the body. 82 00:06:29,080 --> 00:06:32,360 And if all of those bits were set out in a straight line, 83 00:06:32,360 --> 00:06:35,800 they'd reach to the moon and back thousands of times. 84 00:06:46,160 --> 00:06:49,160 But this crucial chain of chemicals would be useless 85 00:06:49,160 --> 00:06:54,080 without an army of microscopic machines that endlessly travel its length, 86 00:06:54,080 --> 00:06:56,920 repairing it and transcribing it, 87 00:06:56,920 --> 00:07:01,640 turning the DNA into instructions that the cell can understand. 88 00:07:09,120 --> 00:07:11,800 Once a gene has been copied, 89 00:07:11,800 --> 00:07:14,600 the instructions are carried outside the nucleus. 90 00:07:17,320 --> 00:07:23,120 Here, mobile factories read them and turn them into proteins. 91 00:07:29,240 --> 00:07:33,080 Up to two million different kinds, 92 00:07:33,080 --> 00:07:36,520 each with its own specific shape and purpose. 93 00:07:44,880 --> 00:07:48,480 And little goes to waste in the cell. 94 00:07:48,480 --> 00:07:51,800 Used and faulty proteins are tagged for recycling... 95 00:07:55,880 --> 00:08:00,960 ..then chewed apart by powerful roving shredders called proteasomes, 96 00:08:00,960 --> 00:08:04,360 reducing them to tiny building blocks for new proteins. 97 00:08:07,280 --> 00:08:12,120 But each cell is also part of a wider neighbourhood of cells, 98 00:08:12,120 --> 00:08:15,640 all continually communicating with each other. 99 00:08:15,640 --> 00:08:19,880 Fragments of shredded proteins are constantly transported to the surface. 100 00:08:23,080 --> 00:08:25,880 Here, they are presented for inspection... 101 00:08:29,400 --> 00:08:33,000 ..to be monitored by the guardians of our body's immune system... 102 00:08:36,320 --> 00:08:38,480 ..our white blood cells. 103 00:08:43,640 --> 00:08:47,560 These roving soldiers check the protein fragments for signs 104 00:08:47,560 --> 00:08:49,080 of damage or infection. 105 00:08:57,240 --> 00:09:00,640 And for the moment, everything is in order. 106 00:09:24,040 --> 00:09:26,640 Every single human cell contains 107 00:09:26,640 --> 00:09:29,680 this world of breathtaking complexity, 108 00:09:29,680 --> 00:09:33,080 organised by the nuclear machines at its heart, 109 00:09:33,080 --> 00:09:37,200 ceaselessly working from instructions written down in our DNA. 110 00:09:50,640 --> 00:09:54,280 But our cells are under constant attack, 111 00:09:54,280 --> 00:09:59,360 and this cell is about to face an ancient enemy... 112 00:10:00,560 --> 00:10:04,200 ..in an encounter that starts with an event so commonplace... 113 00:10:07,880 --> 00:10:09,760 ..you seldom even notice it. 114 00:10:14,000 --> 00:10:16,880 Every day our bodies are constantly bombarded 115 00:10:16,880 --> 00:10:18,600 by these invisible critters, 116 00:10:18,600 --> 00:10:21,240 bacteria and viruses. But we have our skin, 117 00:10:21,240 --> 00:10:24,640 it's our first line of defence that keeps them out. 118 00:10:24,640 --> 00:10:28,560 But we have Achilles heels - we have openings to the outside world, 119 00:10:28,560 --> 00:10:31,640 our mouths, our noses, we touch things, we rub our lips, 120 00:10:31,640 --> 00:10:35,320 we rub our eyes or wipe our nose. They can get in. 121 00:10:35,320 --> 00:10:37,720 And once they're in, they're in. 122 00:10:50,320 --> 00:10:52,320 Inhaled from a sneeze, 123 00:10:52,320 --> 00:10:55,160 an alien army is being carried into our body. 124 00:10:58,440 --> 00:11:01,880 A million invaders, hellbent on destruction. 125 00:11:16,240 --> 00:11:21,360 This is one of our most common enemies - the adenovirus. 126 00:11:26,800 --> 00:11:30,560 It's a masterpiece of design, 127 00:11:30,560 --> 00:11:33,320 and each one has a single aim... 128 00:11:40,880 --> 00:11:42,720 ..to breach a cell's defences... 129 00:11:44,760 --> 00:11:46,720 ..and reach the nucleus. 130 00:11:49,040 --> 00:11:53,360 Once inside, any one of these viruses can take control of the cell... 131 00:11:56,320 --> 00:11:59,800 ..and reproduce 10,000 times over. 132 00:12:01,400 --> 00:12:03,920 The result could be anything, 133 00:12:03,920 --> 00:12:07,800 from the common cold to pneumonia - even death. 134 00:12:15,520 --> 00:12:17,440 But our bodies are prepared. 135 00:12:28,960 --> 00:12:31,920 As the viruses approach the cell, 136 00:12:31,920 --> 00:12:34,280 they are met by a cloud of resistance. 137 00:12:39,640 --> 00:12:45,640 Antibodies, Y-shaped proteins that identify alien intruders 138 00:12:45,640 --> 00:12:49,800 patrol the space between our cells, 139 00:12:49,800 --> 00:12:51,200 looking for viruses. 140 00:12:54,640 --> 00:13:00,320 Recognising the invader, they lock to the virus's armour plating, 141 00:13:00,320 --> 00:13:03,680 shackling them together, making the viruses easy meat 142 00:13:03,680 --> 00:13:07,920 for the white blood cells that feed on alien invaders like these. 143 00:13:07,920 --> 00:13:11,280 Antibodies and white blood cells form the front line 144 00:13:11,280 --> 00:13:12,920 of our immune system. 145 00:13:12,920 --> 00:13:16,240 The immune system is certainly amazing, 146 00:13:16,240 --> 00:13:19,400 and it actually evolved to see invading microbes 147 00:13:19,400 --> 00:13:23,160 and get rid of them. But that's just one part of your body's defences. 148 00:13:23,160 --> 00:13:26,560 Our DNA encodes all these other features that help us 149 00:13:26,560 --> 00:13:29,760 to fight against virus at every single step. 150 00:13:39,720 --> 00:13:42,120 Despite the body's early immune response... 151 00:13:43,920 --> 00:13:47,240 ..hundreds of thousands of viruses make it through to our cell. 152 00:13:51,120 --> 00:13:54,560 But at the surface, they face their next obstacle... 153 00:14:08,960 --> 00:14:11,800 ..the cell's membrane, or skin. 154 00:14:14,040 --> 00:14:17,240 The surface of the cell is an amazingly complicated place. 155 00:14:17,240 --> 00:14:20,360 There are hundreds, maybe thousands of receptor proteins 156 00:14:20,360 --> 00:14:23,800 sticking out of the cell and they all have a unique function to play. 157 00:14:23,800 --> 00:14:26,680 Some of them will be just transporting information 158 00:14:26,680 --> 00:14:28,320 from outside of the cell into the cell. 159 00:14:28,320 --> 00:14:30,800 Other receptors can bring whole cargoes in. 160 00:14:35,200 --> 00:14:38,240 The surface of each cell is a living barrier, 161 00:14:38,240 --> 00:14:42,120 teeming with security proteins that constantly monitor molecules 162 00:14:42,120 --> 00:14:43,840 as they enter and leave. 163 00:14:52,000 --> 00:14:54,080 Small molecules like water 164 00:14:54,080 --> 00:14:57,120 and oxygen can simply seep through the membrane. 165 00:15:04,080 --> 00:15:06,440 Larger molecules, like sugar, 166 00:15:06,440 --> 00:15:09,240 are allowed entry through specialised pumps. 167 00:15:11,160 --> 00:15:14,200 But the largest deliveries require a special key 168 00:15:14,200 --> 00:15:16,360 before they are allowed into the cell. 169 00:15:19,040 --> 00:15:23,600 These protein keys are recognised by teams of mobile sentries 170 00:15:23,600 --> 00:15:25,640 that continually roam the surface. 171 00:15:29,840 --> 00:15:33,960 This sophisticated system is designed to keep harmful molecules 172 00:15:33,960 --> 00:15:35,240 out of the cell. 173 00:15:36,320 --> 00:15:39,160 But over billions of years of evolution, 174 00:15:39,160 --> 00:15:43,240 the adenovirus has evolved its very own key, 175 00:15:43,240 --> 00:15:46,480 etched into the end of these projecting fibres. 176 00:15:51,120 --> 00:15:54,160 Antibodies still cling to the some of these fibres, 177 00:15:54,160 --> 00:15:58,680 blocking many of the counterfeit keys - but not all. 178 00:15:58,680 --> 00:16:03,120 One by one, sentries all over the cell's surface are fooled. 179 00:16:10,360 --> 00:16:13,720 And the virus army quietly slips inside. 180 00:16:16,280 --> 00:16:21,800 In this ancient battle for the cell, it's round two to the virus. 181 00:16:30,320 --> 00:16:34,240 So, how far back does it go, this cat and mouse game, 182 00:16:34,240 --> 00:16:37,240 this battle between cells and viruses? 183 00:16:37,240 --> 00:16:39,480 Every indication suggests it goes right back 184 00:16:39,480 --> 00:16:41,320 to the origins of life on Earth. 185 00:16:42,600 --> 00:16:46,440 Wherever life started, very early on there was a divergence, 186 00:16:46,440 --> 00:16:48,840 two different strategies that life followed. 187 00:16:48,840 --> 00:16:51,680 One of them was to become more complex, to become cells, 188 00:16:51,680 --> 00:16:54,920 to become, ultimately, organisms like ourselves. 189 00:16:54,920 --> 00:16:58,800 The other way was to remain simple - to become viruses, 190 00:16:58,800 --> 00:17:02,600 and to exploit those cells to their own ends, to replicate themselves. 191 00:17:10,760 --> 00:17:12,560 Beneath the surface, 192 00:17:12,560 --> 00:17:15,840 the cell prepares to receive the deadly invaders. 193 00:17:17,080 --> 00:17:20,840 Fooled into thinking that the virus is an important nutrient, 194 00:17:20,840 --> 00:17:24,640 special proteins slot together to form a spherical mould. 195 00:17:30,640 --> 00:17:33,600 They pinch out a bubble of cellular membrane, 196 00:17:33,600 --> 00:17:35,360 wrapping the virus inside. 197 00:17:38,840 --> 00:17:43,360 Finally, a separate protein pinches the bubble free, 198 00:17:43,360 --> 00:17:46,320 delivering the virus into the cell's interior. 199 00:17:50,240 --> 00:17:54,320 Unwittingly, the cell has just taken a large step 200 00:17:54,320 --> 00:17:55,960 towards to its own downfall. 201 00:18:06,240 --> 00:18:09,480 Every single member of this invading virus army 202 00:18:09,480 --> 00:18:12,320 has the weaponry to ultimately destroy this cell. 203 00:18:14,080 --> 00:18:17,800 Its protein shell is a multi-layered cloak of deception, 204 00:18:17,800 --> 00:18:20,240 which has still more surprises in store. 205 00:18:22,880 --> 00:18:27,360 And at its heart, it carries a tiny string of DNA, 206 00:18:27,360 --> 00:18:29,400 its ultimate weapon. 207 00:18:29,400 --> 00:18:32,680 It's a masterpiece of evolution and design. 208 00:18:32,680 --> 00:18:35,080 And yet scientists still can't decide 209 00:18:35,080 --> 00:18:37,000 if it's actually alive or dead. 210 00:18:37,000 --> 00:18:40,320 At the level of large animals like ourselves, 211 00:18:40,320 --> 00:18:43,520 the difference between living things and non-living things 212 00:18:43,520 --> 00:18:45,000 is very obvious. 213 00:18:45,000 --> 00:18:49,720 Come down a level, though, to cells, and it becomes a bit more ambiguous. 214 00:18:49,720 --> 00:18:53,160 For our own cells, of course, you can still tell immediately 215 00:18:53,160 --> 00:18:56,920 that they are alive. Come down another level, though, to the virus, 216 00:18:56,920 --> 00:19:01,000 and it's no longer obviously alive. They don't look alive. 217 00:19:01,000 --> 00:19:04,120 Yet they behave perhaps as if they are. 218 00:19:04,120 --> 00:19:06,440 They behave with a sense of purpose. 219 00:19:08,680 --> 00:19:11,040 A virus isn't strictly alive, 220 00:19:11,040 --> 00:19:13,400 it can't make more of itself on its own. 221 00:19:13,400 --> 00:19:18,400 It only can replicate if it uses parts that it hijacks from a cell. 222 00:19:22,000 --> 00:19:25,280 But the cell still has a formidable array of defences 223 00:19:25,280 --> 00:19:27,360 to keep these killing machines at bay. 224 00:19:33,000 --> 00:19:36,800 Every delivery that the cell receives is taken 225 00:19:36,800 --> 00:19:39,960 to a sorting station, called an endosome. 226 00:19:41,680 --> 00:19:45,400 Endosomes process incoming supplies and decide where 227 00:19:45,400 --> 00:19:47,480 inside the cell they will be delivered. 228 00:19:50,440 --> 00:19:54,520 The first step of the process is to break them down. 229 00:19:57,600 --> 00:20:00,640 The virus army is about to be digested. 230 00:20:03,360 --> 00:20:06,080 The walls of the sorting stations are fitted 231 00:20:06,080 --> 00:20:07,880 with specialised protein pumps. 232 00:20:12,440 --> 00:20:15,240 The pumps draw in special atoms, 233 00:20:15,240 --> 00:20:18,720 turning the inside of the endosome into an acid bath. 234 00:20:26,080 --> 00:20:30,240 The acid breaks down large nutrients into smaller molecules 235 00:20:30,240 --> 00:20:33,240 that are easier for the cell to transport and use. 236 00:20:36,320 --> 00:20:40,640 And as the acid eats away at the virus's outer shell, 237 00:20:40,640 --> 00:20:42,000 it begins to break apart. 238 00:20:45,080 --> 00:20:47,880 This should spell disaster for the adenovirus. 239 00:20:50,760 --> 00:20:53,440 But the acid is part of its escape plan. 240 00:20:55,640 --> 00:20:58,280 The virus fibres are the first to break away. 241 00:21:01,720 --> 00:21:04,840 But their disintegration releases a special protein 242 00:21:04,840 --> 00:21:06,480 hidden inside the virus... 243 00:21:08,640 --> 00:21:11,480 ..that targets the wall of the sorting station... 244 00:21:14,840 --> 00:21:19,560 ..tearing the membrane apart and setting the virus free. 245 00:21:26,960 --> 00:21:28,520 But not every virus escapes. 246 00:21:30,280 --> 00:21:34,360 Many still carry antibodies locked to their surface. 247 00:21:34,360 --> 00:21:38,000 Their primary job was to alert the immune system to intruders, 248 00:21:38,000 --> 00:21:41,120 but their firm grip now ties the shell together. 249 00:21:42,760 --> 00:21:45,000 The fibres cannot break free, 250 00:21:45,000 --> 00:21:48,640 and the escape protein stays trapped inside the shell. 251 00:21:53,440 --> 00:21:56,800 Countless viruses are eaten away before they can escape. 252 00:22:05,280 --> 00:22:07,520 But enough are released. 253 00:22:07,520 --> 00:22:10,200 Now there is nothing between these viruses 254 00:22:10,200 --> 00:22:13,040 and the nucleus of the cell - their ultimate goal. 255 00:22:15,880 --> 00:22:19,320 Yet although they are just five micrometres from their target... 256 00:22:22,160 --> 00:22:25,400 ..most might as well be a million miles away. 257 00:22:35,960 --> 00:22:41,200 For 90% of the army, the invasion will end here, 258 00:22:41,200 --> 00:22:43,800 floating helplessly beneath the surface. 259 00:22:52,680 --> 00:22:56,960 Although they are surrounded by the constant bustle of cellular activity, 260 00:22:56,960 --> 00:23:00,120 the inert invaders have no way of moving themselves. 261 00:23:05,640 --> 00:23:08,840 And they have no way of utilising the energy generated 262 00:23:08,840 --> 00:23:11,120 by the cells' floating power stations... 263 00:23:13,280 --> 00:23:14,960 ..the mitochondria. 264 00:23:33,480 --> 00:23:38,360 Inside each mitochondrion, the food we eat and the air we breathe 265 00:23:38,360 --> 00:23:43,000 drives thousands of turbines that continually recharge billions 266 00:23:43,000 --> 00:23:44,000 of tiny batteries. 267 00:23:46,000 --> 00:23:48,280 But what is even more extraordinary 268 00:23:48,280 --> 00:23:52,840 is that scientists believe that mitochondria were once simple cells themselves. 269 00:23:54,800 --> 00:23:57,040 Then they one was swallowed by another cell, 270 00:23:57,040 --> 00:24:00,280 firing one of the greatest leaps in evolution - 271 00:24:00,280 --> 00:24:02,080 complex life. 272 00:24:04,160 --> 00:24:07,760 To be complex at all, you must have all this machinery, 273 00:24:07,760 --> 00:24:10,120 all these proteins encoded by genes. 274 00:24:10,120 --> 00:24:14,480 And to support all of that requires a tremendous amount of energy. 275 00:24:14,480 --> 00:24:17,960 All complex life share a single common ancestor, 276 00:24:17,960 --> 00:24:22,000 and that ancestor arose just once in four billion years 277 00:24:22,000 --> 00:24:23,360 of life on Earth. 278 00:24:23,360 --> 00:24:26,640 For two to three billion years it was bacteria and nothing else, 279 00:24:26,640 --> 00:24:28,960 and then this complex cell arose. 280 00:24:28,960 --> 00:24:32,320 One simple cell got inside another simple cell, 281 00:24:32,320 --> 00:24:34,200 it's a very rare event in itself. 282 00:24:34,200 --> 00:24:37,840 And once this happened, it transforms the energetic possibilities of life. 283 00:24:39,240 --> 00:24:42,600 Without that energy, evolution could never have produced 284 00:24:42,600 --> 00:24:45,960 the astonishing diversity of life that we see around us. 285 00:24:45,960 --> 00:24:48,400 Without that energy, we wouldn't see plants and animals, 286 00:24:48,600 --> 00:24:49,520 Without that energy, we wouldn't see plants and animals, 287 00:24:49,520 --> 00:24:54,240 we wouldn't see ourselves. The world would be an almost sterile desert. 288 00:25:08,520 --> 00:25:13,000 Throughout each cell, hundreds of mitochondria feed energy to power 289 00:25:13,000 --> 00:25:16,960 the network of protein that make us the complex creatures that we are. 290 00:25:23,200 --> 00:25:27,240 The virus has evolved into a model of efficiency. 291 00:25:27,240 --> 00:25:30,440 But the simplicity of its design makes it useless 292 00:25:30,440 --> 00:25:32,480 without the machinery of complex life. 293 00:25:36,120 --> 00:25:40,360 But just beneath the surface, large numbers of motor proteins, 294 00:25:40,360 --> 00:25:42,880 molecular haulage workers, 295 00:25:42,880 --> 00:25:46,560 await nutrients processed for delivery by the endosomes. 296 00:26:01,720 --> 00:26:04,640 And in this billion-year arms race... 297 00:26:06,640 --> 00:26:09,920 ..the virus has evolved the precise mechanism 298 00:26:09,920 --> 00:26:12,280 to attach to the cell's motor proteins. 299 00:26:23,280 --> 00:26:26,720 Now it can use the energy of the mitochondria. 300 00:26:31,600 --> 00:26:33,840 The virus is on its way. 301 00:26:39,240 --> 00:26:43,160 It has hijacked the cell's own transport system, 302 00:26:43,160 --> 00:26:47,000 and is being carried towards the nucleus and its ultimate prize, 303 00:26:47,000 --> 00:26:50,680 the DNA machines it needs to take control of the cell. 304 00:26:58,840 --> 00:27:04,560 These microscopic motorised legs are a wonder of the natural world. 305 00:27:04,560 --> 00:27:07,960 Slowed down to one-thirtieth of their normal speed, 306 00:27:07,960 --> 00:27:10,200 their movement is clearly visible. 307 00:27:10,200 --> 00:27:14,280 But at their actual speed, over 100 steps a second, 308 00:27:14,280 --> 00:27:15,760 they would appear a blur. 309 00:27:30,080 --> 00:27:32,720 But speed isn't everything. 310 00:27:32,720 --> 00:27:35,400 Cells are densely packed, 311 00:27:35,400 --> 00:27:38,760 and their internal highways are littered with obstacles. 312 00:27:42,240 --> 00:27:45,720 And these motor proteins can only move in one direction. 313 00:28:05,440 --> 00:28:08,680 For this virus, it seems to be the end of the road. 314 00:28:16,920 --> 00:28:20,960 But scientists have recently discovered the virus locks on 315 00:28:20,960 --> 00:28:22,600 to a second motor protein. 316 00:28:32,880 --> 00:28:36,520 And this one is built to move in the opposite direction. 317 00:28:52,760 --> 00:28:55,840 Together, the two motor proteins can navigate 318 00:28:55,840 --> 00:28:57,480 around almost any obstacles. 319 00:29:01,360 --> 00:29:04,400 And once again, the invader benefits. 320 00:29:23,040 --> 00:29:25,560 The virus is on the move again. 321 00:29:30,240 --> 00:29:34,000 And it leads an army of hundreds. 322 00:29:59,800 --> 00:30:03,720 It's been almost an hour since the adenovirus first attacked the cell. 323 00:30:06,880 --> 00:30:09,360 The nucleus is just one more hour away. 324 00:30:11,560 --> 00:30:15,600 Until recently, scientists thought that once the viral army 325 00:30:15,600 --> 00:30:19,240 was on the march, nothing could stop it. 326 00:30:19,240 --> 00:30:23,640 But then they found that the cell has its own internal immune system. 327 00:30:25,480 --> 00:30:29,680 There is this whole mechanism inside cells that are taking out viruses 328 00:30:29,680 --> 00:30:31,880 that previously we just didn't know was there. 329 00:30:31,880 --> 00:30:34,560 And I remember the day we published the paper about it, 330 00:30:34,560 --> 00:30:38,200 I woke up to hear it being announced on the national radio 331 00:30:38,200 --> 00:30:40,680 and then went into a shop to pick up the newspapers 332 00:30:40,680 --> 00:30:42,760 to discover it was on the front page. 333 00:31:18,800 --> 00:31:21,840 Dotted along the cell's highway system, 334 00:31:21,840 --> 00:31:25,760 a special protein searches for anything carrying antibodies 335 00:31:25,760 --> 00:31:28,000 from the surface. 336 00:31:28,000 --> 00:31:30,800 The clever thing about this protein is it uses systems 337 00:31:30,800 --> 00:31:33,600 that the cell already has in place. 338 00:31:33,600 --> 00:31:35,400 Once it's stuck to the antibody, 339 00:31:35,400 --> 00:31:39,040 it sends signals to a cellular machine called the proteasome. 340 00:31:39,040 --> 00:31:42,400 And the proteasome plays the role of recycling proteins in the cell. 341 00:31:42,400 --> 00:31:45,640 It gets brought along to the virus and it destroys the virus, 342 00:31:45,640 --> 00:31:49,520 breaking down all its parts into tiny fragments. 343 00:31:49,520 --> 00:31:54,360 Once attached, the defence protein initiates a chain reaction, 344 00:31:54,360 --> 00:31:57,000 attracting specialised tagging proteins. 345 00:31:58,560 --> 00:32:01,720 Together, they mark the virus for destruction. 346 00:32:12,040 --> 00:32:13,920 Then it's only a matter of time... 347 00:32:17,160 --> 00:32:19,360 ..before the recyclers arrive. 348 00:32:28,080 --> 00:32:31,120 They rip the virus to shreds. 349 00:33:02,000 --> 00:33:06,880 Somewhere inside your body, this battle is raging right now. 350 00:33:15,840 --> 00:33:19,240 The discovery of TRIM21 provides potentially new ways 351 00:33:19,240 --> 00:33:21,640 of making therapeutics to fight viruses, 352 00:33:21,640 --> 00:33:26,960 and one way this could work is if we find ways of encouraging the immune system to make more TRIM21. 353 00:33:26,960 --> 00:33:30,640 So as soon as that virus enters into the cell, the TRIM21 is ready 354 00:33:30,640 --> 00:33:33,480 to recognise the antibodies and destroy the virus. 355 00:33:44,040 --> 00:33:48,320 By working together, the defence proteins and recycling shredders 356 00:33:48,320 --> 00:33:51,760 can destroy an army of viruses in just a few hours. 357 00:34:11,680 --> 00:34:17,160 But it only takes a single virus to take control of an entire cell... 358 00:34:19,200 --> 00:34:21,600 ..spreading infection throughout the body. 359 00:34:24,480 --> 00:34:26,920 With no antibodies attached, 360 00:34:26,920 --> 00:34:29,960 this virus has evaded the cell's shredders. 361 00:34:39,920 --> 00:34:43,760 Nothing now stands between it and its target. 362 00:34:57,080 --> 00:35:01,640 The virus is now just one thousandth of a millimetre from the nucleus. 363 00:35:06,120 --> 00:35:10,960 But if it is to achieve its ultimate goal, it first has to get inside. 364 00:35:14,320 --> 00:35:17,120 Compared to the cell, the virus is tiny. 365 00:35:17,120 --> 00:35:20,520 But really they're just different versions of the same machine, 366 00:35:20,520 --> 00:35:22,760 and its only job is to copy itself. 367 00:35:22,760 --> 00:35:26,400 But the virus needs to take advantage of our cell mechanism 368 00:35:26,400 --> 00:35:27,760 for its own selfish ends. 369 00:35:31,280 --> 00:35:35,360 At the heart of every cell lies the nucleus, 370 00:35:35,360 --> 00:35:37,280 and it is a world all of its own. 371 00:35:43,320 --> 00:35:46,880 Its surface is made of the same molecules as the cell membrane. 372 00:35:48,920 --> 00:35:52,920 But entry into this world is governed by completely different gateways. 373 00:35:55,920 --> 00:35:57,520 Across the surface, 374 00:35:57,520 --> 00:36:01,680 protein arms search for molecules to draw inside nuclear pores. 375 00:36:06,360 --> 00:36:09,920 Through these gateways, billions of chemical messages 376 00:36:09,920 --> 00:36:13,120 and instructions pass between the DNA and the cell. 377 00:36:15,760 --> 00:36:18,960 But only if they are recognised by the protein arms. 378 00:36:25,720 --> 00:36:30,000 But once again, the viral shell carries a counterfeit pass. 379 00:36:34,280 --> 00:36:38,520 The arms lock on, but the virus is too large to be ferried inside. 380 00:36:44,600 --> 00:36:47,440 Thinking that they have hit an obstruction... 381 00:36:49,360 --> 00:36:52,760 ..the motor proteins shunt the virus into reverse. 382 00:37:17,080 --> 00:37:19,040 Pulled in two directions... 383 00:37:23,200 --> 00:37:25,600 ..the virus is ripped apart. 384 00:37:33,600 --> 00:37:39,880 But what looks like a catastrophe for the virus is, in fact, its masterstroke. 385 00:37:58,600 --> 00:38:05,000 Now the single strand of DNA it held inside is carried through the pore, 386 00:38:05,000 --> 00:38:08,080 and into the cell's control centre. 387 00:38:27,640 --> 00:38:31,960 Inside the human cell nucleus there are about 23,000 genes. 388 00:38:31,960 --> 00:38:35,920 They code for thousands and thousands of biochemical pathways. 389 00:38:35,920 --> 00:38:38,400 The virus has just got 40, 390 00:38:38,400 --> 00:38:41,000 but with those 40 it can do remarkable things. 391 00:38:42,800 --> 00:38:45,680 It's so tiny, just a piece of DNA, 392 00:38:45,680 --> 00:38:49,840 a couple of proteins to make its shell, and yet it can take over 393 00:38:49,840 --> 00:38:53,200 and wreak havoc in a huge human cell. 394 00:38:53,200 --> 00:38:54,640 It's brilliant. 395 00:39:06,440 --> 00:39:09,840 The adenovirus has proven itself a master of deception... 396 00:39:11,800 --> 00:39:16,360 ..continually exploiting the cell's processes to further its own deadly aims. 397 00:39:17,760 --> 00:39:20,760 But its greatest trick is yet to come. 398 00:39:27,240 --> 00:39:30,880 The cell's DNA machines have no way of telling the difference 399 00:39:30,880 --> 00:39:34,240 between its own DNA and the DNA of the virus. 400 00:39:37,400 --> 00:39:40,960 Blindly, they set about converting its deadly code 401 00:39:40,960 --> 00:39:44,400 into thousands of instructions for the cell to act upon... 402 00:39:53,320 --> 00:39:56,440 ..blueprints for the cell's own destruction. 403 00:40:27,000 --> 00:40:30,760 But the machines that turn the blueprints into proteins 404 00:40:30,760 --> 00:40:32,720 lie outside the nucleus. 405 00:40:37,000 --> 00:40:40,480 Out in the main body of the cell, the instructions are met 406 00:40:40,480 --> 00:40:44,200 by a squadron of mobile protein factories, called ribosomes. 407 00:40:47,480 --> 00:40:50,280 The ribosomes precisely follow the instruction 408 00:40:50,280 --> 00:40:52,440 and start to construct viral proteins. 409 00:40:53,680 --> 00:40:56,720 Each is carefully folded into a specific shape, 410 00:40:56,720 --> 00:40:58,760 with a unique job to do. 411 00:40:58,760 --> 00:41:02,800 These large cellular machines, ribosomes, are absolutely fundamental 412 00:41:02,800 --> 00:41:05,880 to life, and very similar forms of them are found in every type 413 00:41:05,880 --> 00:41:07,680 of living cell on the planet. 414 00:41:07,680 --> 00:41:10,520 They read the genetic information and they decode it, 415 00:41:10,520 --> 00:41:13,040 bringing in the building blocks that make up proteins 416 00:41:13,040 --> 00:41:15,920 and sticking them together to make these functional molecules 417 00:41:15,920 --> 00:41:18,280 that are going to work inside the living cell. 418 00:41:28,360 --> 00:41:30,560 Only these functional molecules 419 00:41:30,560 --> 00:41:33,880 are the kit of parts needed to build an enemy army. 420 00:41:55,560 --> 00:41:57,960 But the army will not be built out here. 421 00:42:08,440 --> 00:42:13,280 The raw material for the new army is drawn back inside the nucleus... 422 00:42:16,760 --> 00:42:18,760 ..ready for construction. 423 00:42:33,520 --> 00:42:35,840 With its mission reaching its climax, 424 00:42:35,840 --> 00:42:39,160 the virus turns its attention to the cell's DNA, 425 00:42:39,160 --> 00:42:41,200 halting any process it doesn't need. 426 00:42:45,480 --> 00:42:48,240 The virus has taken complete control. 427 00:42:55,640 --> 00:42:59,000 And yet the cell still has a small window of opportunity. 428 00:43:02,200 --> 00:43:04,680 Before all normal activity stops, 429 00:43:04,680 --> 00:43:08,520 it has just enough time to send a message to the outside world. 430 00:43:27,200 --> 00:43:30,920 This parcel contains fragments of the viral army. 431 00:43:36,960 --> 00:43:40,000 The parcel merges with the cell membrane, 432 00:43:40,000 --> 00:43:43,080 and the enemy fragments are pushed to the surface, 433 00:43:43,080 --> 00:43:46,160 flags warning of the invasion that has taken place. 434 00:43:50,960 --> 00:43:54,040 If patrolling white blood cells spot the distress signal... 435 00:43:56,040 --> 00:44:01,120 ..they will destroy the cell, along with the entire alien army inside. 436 00:44:01,120 --> 00:44:07,640 If not, the infection will spread from cell to cell, to cell. 437 00:44:16,800 --> 00:44:19,440 After just one day of occupation, 438 00:44:19,440 --> 00:44:22,240 the virus has complete control over the cell. 439 00:44:23,440 --> 00:44:27,560 With routine maintenance halted, the cell has started to decay. 440 00:44:29,160 --> 00:44:35,040 And all activity is now focused on building the brand new viral army 441 00:44:35,040 --> 00:44:36,840 inside the nucleus. 442 00:44:45,840 --> 00:44:47,880 The new army self-assembles. 443 00:44:52,120 --> 00:44:55,560 How do viruses know how to invade our cells, how to break 444 00:44:55,560 --> 00:44:57,600 and enter the nucleus itself? 445 00:44:57,600 --> 00:45:03,960 We know that viruses and cells co-evolved together over long periods of time, but it's more than that. 446 00:45:03,960 --> 00:45:06,760 We're actually surprisingly closely related. 447 00:45:06,760 --> 00:45:09,400 It turns out that the viruses that attack us 448 00:45:09,400 --> 00:45:12,800 are actually made from bits and pieces of our own cells. 449 00:45:12,800 --> 00:45:14,280 As our cells were evolving, 450 00:45:14,280 --> 00:45:17,440 as our nucleus itself was first coming to be, 451 00:45:17,440 --> 00:45:20,720 so these viruses were cobbled together from bits and pieces, 452 00:45:20,720 --> 00:45:22,800 and they can attack our nucleus 453 00:45:22,800 --> 00:45:25,200 because they're made of the same stuff. 454 00:45:26,640 --> 00:45:29,880 Already built into its surface are the binding sites 455 00:45:29,880 --> 00:45:31,720 for the cell's motorised legs. 456 00:45:37,000 --> 00:45:39,040 Fibres snap into place, 457 00:45:39,040 --> 00:45:42,520 arming each virus with the keys to enter other cells. 458 00:45:47,080 --> 00:45:50,640 But these shells are harmless without its instructions. 459 00:46:04,520 --> 00:46:08,280 The final component is loaded - identical copies 460 00:46:08,280 --> 00:46:10,720 of the virus's deadly DNA. 461 00:46:16,200 --> 00:46:18,480 Carried by powerful motors, 462 00:46:18,480 --> 00:46:22,280 long strands of DNA are fed into every single virus. 463 00:46:42,240 --> 00:46:46,680 All this is the result of one single virus 464 00:46:46,680 --> 00:46:49,120 getting through our cell's defences. 465 00:46:55,640 --> 00:46:59,360 It's been two days since the virus entered the body, 466 00:46:59,360 --> 00:47:03,320 and the nucleus, once the centre of cellular organisation, 467 00:47:03,320 --> 00:47:07,000 now harbours an army of 10,000 deadly viruses. 468 00:47:20,880 --> 00:47:23,200 But before it can begin its conquest, 469 00:47:23,200 --> 00:47:25,040 it has to overcome two barriers. 470 00:47:26,840 --> 00:47:30,160 The army is trapped inside the tough nuclear membrane, 471 00:47:30,160 --> 00:47:32,360 held at the centre of the cell itself. 472 00:47:34,840 --> 00:47:37,440 And then there is the skin of the cell itself. 473 00:47:40,520 --> 00:47:44,000 The protein factories outside the nucleus are instructed 474 00:47:44,000 --> 00:47:45,720 to build viral saboteurs. 475 00:47:55,280 --> 00:47:58,080 The first are released into the decaying cell 476 00:47:58,080 --> 00:48:00,000 and target its cytoskeleton. 477 00:48:05,480 --> 00:48:07,560 The effects are cataclysmic. 478 00:48:12,000 --> 00:48:14,240 Without support... 479 00:48:14,240 --> 00:48:16,280 the cell starts to collapse. 480 00:48:24,200 --> 00:48:28,360 Now the virus turns its attention to the nuclear membrane. 481 00:48:34,520 --> 00:48:36,560 A second protein is released. 482 00:48:40,240 --> 00:48:45,080 Called the Adenovirus Death Protein, it burrows into the membrane... 483 00:48:46,920 --> 00:48:48,440 ..and weakens it. 484 00:49:01,840 --> 00:49:05,640 The nucleus can no longer contain the bulging army. 485 00:49:31,000 --> 00:49:34,680 Beyond the nucleus, the cell is a wasteland... 486 00:49:42,200 --> 00:49:46,440 ..unrecognisable from the harmonious, buzzing community 487 00:49:46,440 --> 00:49:48,000 of just 48 hours ago. 488 00:49:56,000 --> 00:50:00,240 The cell is now completely helpless to stop the virus army 489 00:50:00,240 --> 00:50:02,680 from flooding into surrounding tissue... 490 00:50:07,160 --> 00:50:10,640 ..attacking neighbouring cells 491 00:50:10,640 --> 00:50:13,760 and spreading infection throughout the body. 492 00:50:20,880 --> 00:50:24,200 The battle for this cell is over. 493 00:50:38,840 --> 00:50:41,400 But the war has only just begun. 494 00:50:56,520 --> 00:50:59,360 While the virus has been busy inside the cell, 495 00:50:59,360 --> 00:51:02,760 our antibodies have adapted and now come back in force, 496 00:51:02,760 --> 00:51:07,200 carrying new receptors, tailor-made to lock onto the escaping army. 497 00:51:10,160 --> 00:51:14,520 Yet even in these numbers, they cannot stop every virus. 498 00:51:17,400 --> 00:51:19,280 But they are not alone. 499 00:51:20,720 --> 00:51:24,920 The cell's dying message to the outside world was not in vain. 500 00:51:24,920 --> 00:51:28,520 Giant white blood cells flock to the stricken cell 501 00:51:28,520 --> 00:51:31,240 to devour the escaping hordes. 502 00:51:31,240 --> 00:51:35,400 They too are learning how to tackle this particular invader. 503 00:51:35,400 --> 00:51:38,400 Once the virus has been detected by the immune system, 504 00:51:38,400 --> 00:51:41,480 there's a heightened level of security inside your body, 505 00:51:41,480 --> 00:51:44,680 and one of the results of this is that the cells that make antibodies, 506 00:51:44,680 --> 00:51:46,720 and make the right antibody for that virus, 507 00:51:46,720 --> 00:51:48,800 will make lots of copies of themselves, 508 00:51:48,800 --> 00:51:52,800 and then they will start pumping out up to 5,000 antibodies per second 509 00:51:52,800 --> 00:51:55,600 to flood your bloodstream, the spaces between your cells, 510 00:51:55,600 --> 00:51:57,680 so as the viruses emerge from dying cells, 511 00:51:57,680 --> 00:52:01,600 they can get tagged by antibodies, then destroyed by white blood cells. 512 00:52:02,760 --> 00:52:04,560 Taking no chances, 513 00:52:04,560 --> 00:52:08,400 white blood cells engulf nearby cells that may have been infected. 514 00:52:13,280 --> 00:52:18,960 Meanwhile, surrounding healthy cells make the ultimate sacrifice, 515 00:52:18,960 --> 00:52:22,640 destroying themselves to stop the spread of the virus. 516 00:52:26,080 --> 00:52:29,440 It is only at this stage that we become aware of the battle 517 00:52:29,440 --> 00:52:31,400 taking place inside us. 518 00:52:31,400 --> 00:52:35,640 Increasing blood flow brings more white blood cells to the battleground, 519 00:52:35,640 --> 00:52:38,800 causing our nasal tissue to become inflamed. 520 00:52:38,800 --> 00:52:43,040 What we feel is a blocked nose is, in fact, the clearest sign 521 00:52:43,040 --> 00:52:44,760 of a viral onslaught. 522 00:52:46,200 --> 00:52:48,120 Once you've had an infection, 523 00:52:48,120 --> 00:52:51,640 one cell, that makes the antibody for that infection, will be kept 524 00:52:51,640 --> 00:52:53,840 inside your bone marrow for the rest of your life. 525 00:52:53,840 --> 00:52:56,680 So that if you ever get another infection with the same virus, 526 00:52:56,680 --> 00:52:58,880 the immune system already knows how to respond, 527 00:52:58,880 --> 00:53:03,920 it knows what antibody to make and it can respond very quickly and stop you getting sick. 528 00:53:07,040 --> 00:53:10,800 Working together, the body's immune system finally prevents 529 00:53:10,800 --> 00:53:12,840 the viral infection from spreading. 530 00:53:13,840 --> 00:53:17,080 It's one more battle in an unending war. 531 00:53:19,720 --> 00:53:23,800 The struggle between viruses and ourselves is evolution, 532 00:53:23,800 --> 00:53:28,120 but it's co-evolution - both sides have to change. 533 00:53:28,120 --> 00:53:31,840 It's a bit like an arms race - one party gets better weapons, 534 00:53:31,840 --> 00:53:33,680 the other party has to match them. 535 00:53:36,600 --> 00:53:40,400 Even though the individual cells are fighting this epic battle 536 00:53:40,400 --> 00:53:44,160 against viruses, remember, you have trillions of cells. 537 00:53:44,160 --> 00:53:49,560 And so even if one cell loses its war, most of the time the organism wins and we get better. 538 00:54:04,120 --> 00:54:05,520 The war is over. 539 00:54:08,080 --> 00:54:09,640 For now. 540 00:54:17,840 --> 00:54:20,440 Although many cells have been lost, 541 00:54:20,440 --> 00:54:23,640 there are many more healthy cells waiting to replace them. 542 00:54:31,240 --> 00:54:36,000 And at the heart of each one lies an identical copy of our DNA. 543 00:54:41,160 --> 00:54:43,040 Inherited from our parents, 544 00:54:43,040 --> 00:54:45,840 and their parents over countless generations, 545 00:54:45,840 --> 00:54:50,600 our DNA connects us to a family tree that stretches back 546 00:54:50,600 --> 00:54:54,040 over three billion years, to the very first cell... 547 00:54:56,040 --> 00:55:00,480 ..a cell that existed long before humans, long before mammals, 548 00:55:00,480 --> 00:55:02,080 long before the dinosaurs. 549 00:55:04,600 --> 00:55:09,400 It's a lineage that connects us to every living creature and plant on Earth. 550 00:55:09,400 --> 00:55:13,520 We are all descended from a single prehistoric ancestor, 551 00:55:13,520 --> 00:55:18,560 a cell containing the single strand of DNA that started it all. 552 00:55:22,680 --> 00:55:25,480 But the virus is as old as we are. 553 00:55:25,480 --> 00:55:28,920 It has evolved alongside us, forcing us to adapt, 554 00:55:28,920 --> 00:55:32,240 to change or die in a deadly game of cat and mouse. 555 00:55:37,280 --> 00:55:41,480 This eternal arms race has driven our evolution 556 00:55:41,480 --> 00:55:43,080 and made us both stronger. 557 00:55:46,760 --> 00:55:51,040 We wouldn't be what we are today were it not for this battle 558 00:55:51,040 --> 00:55:52,880 with our ancient enemy. 559 00:55:56,000 --> 00:55:59,600 The story of the cell is a story of innovation and change, 560 00:55:59,600 --> 00:56:03,560 and because viruses continuously force cells to change, 561 00:56:03,560 --> 00:56:08,440 they actually aid their adaptation to different environments. 562 00:56:08,440 --> 00:56:11,360 And for that reason they've also helped shape us, 563 00:56:11,360 --> 00:56:12,880 they've made us who we are. 564 00:56:21,200 --> 00:56:23,400 Every minute of every day, 565 00:56:23,400 --> 00:56:27,360 this battle with the virus rages within seven billion of us. 566 00:56:32,720 --> 00:56:37,560 Though we are rarely aware of it, we fight each other, 567 00:56:37,560 --> 00:56:40,000 change each other, 568 00:56:40,000 --> 00:56:42,040 improve each other. 569 00:56:51,440 --> 00:56:54,440 Subtitles by Red Bee Media Ltd