0:00:02 > 0:00:05Do you ever stop to contemplate the wonder of your body?
0:00:05 > 0:00:06No?
0:00:06 > 0:00:08No, neither do I.
0:00:08 > 0:00:12But, actually, it's the most complicated piece of engineering
0:00:12 > 0:00:16known to us, and it throws up some very puzzling questions.
0:00:16 > 0:00:18Why do I catch colds?
0:00:19 > 0:00:21Where does wind come from?
0:00:21 > 0:00:25And what will we look like in 1,000 years?
0:00:25 > 0:00:27For the answers, stick with me,
0:00:27 > 0:00:32as we've uncovered the Things You Need To Know about the human body.
0:00:32 > 0:00:38Right, let's get this show on the road, starting at the beginning...
0:00:38 > 0:00:40Daddy, where did I come from?
0:00:40 > 0:00:44So, your dad told you that he and your mum loved each other,
0:00:44 > 0:00:46and decided to have a baby.
0:00:47 > 0:00:50Actually, it was much more complicated than that.
0:00:52 > 0:00:56If your father was an average guy, about 280 million of his sperm
0:00:56 > 0:00:59entered the race to fertilise your mother's egg.
0:01:03 > 0:01:06They were blasted in at an impressive 28mph,
0:01:06 > 0:01:09but almost instantly, 210 million had dropped out.
0:01:12 > 0:01:13What happened?
0:01:15 > 0:01:18Well, up to 70% of human sperm is dodgy.
0:01:18 > 0:01:21Some have three tails.
0:01:21 > 0:01:24There are two-headed sperm chasing their tails,
0:01:24 > 0:01:26and others with no tails at all.
0:01:26 > 0:01:30Humans compare pretty badly with the animal kingdom.
0:01:30 > 0:01:37Let's compare me with a rat and a pig.
0:01:37 > 0:01:41100% of the rat's sperm will be strong swimmers
0:01:41 > 0:01:44compared with only about 30% of mine.
0:01:44 > 0:01:47And the pig will produce two pints of sperm,
0:01:47 > 0:01:50where as I will produce about a teaspoonful.
0:01:53 > 0:01:57Anyway, back to the sperm's voyage through your mother.
0:01:57 > 0:02:03Those that made it into her cervix found themselves in a sticky maze.
0:02:03 > 0:02:05This mucus protects a uterus from germs,
0:02:05 > 0:02:09but it's also a hostile environment for sperm.
0:02:09 > 0:02:11Most will never get through.
0:02:13 > 0:02:18The 100,000 that did then had to race across the uterus -
0:02:18 > 0:02:20an exhausting marathon.
0:02:22 > 0:02:26By the fallopian tubes, we're down to just 200,
0:02:26 > 0:02:28and half of them swim up the wrong tube.
0:02:29 > 0:02:33Meanwhile, the egg is only fertile for 12 to 24 hours.
0:02:33 > 0:02:38Chemicals in the female body encourage the sperm to keep swimming.
0:02:40 > 0:02:43As they near the egg, they turn up the wick and accelerate.
0:02:48 > 0:02:51But there can only be one winner.
0:02:51 > 0:02:53As one super-sperm enters the egg,
0:02:53 > 0:02:57a chemical message seals its surface shut.
0:02:59 > 0:03:02So, if you ever feel down, remember this -
0:03:02 > 0:03:06you are the product of an Olympian sperm.
0:03:06 > 0:03:09Your father's sperm and your mother's egg
0:03:09 > 0:03:13contained all the information needed to make you.
0:03:13 > 0:03:17We are all unique products of our parents' genetic codes.
0:03:17 > 0:03:19In which case...
0:03:19 > 0:03:21How did I get my granny's chin?
0:03:22 > 0:03:27Since way back when, families have argued about who the kids look like.
0:03:27 > 0:03:30So, it was a nasty shock for your parents
0:03:30 > 0:03:34when you were born with your granny's smooth, pointy chin,
0:03:34 > 0:03:36instead of their elegant clefts.
0:03:39 > 0:03:42Your DNA is to blame.
0:03:42 > 0:03:47In 1953, James Watson and Francis Crick discovered that DNA,
0:03:47 > 0:03:50or Deoxyribonucleic acid,
0:03:50 > 0:03:53is contained in a structure called the double helix.
0:03:53 > 0:03:56Lying at the centre of virtually all our cells.
0:03:58 > 0:04:01Within your DNA are around 23,000 genes,
0:04:01 > 0:04:03which you inherited in two packs -
0:04:03 > 0:04:07one from your mother, and one from your father.
0:04:08 > 0:04:12Your genes come in pairs, one from each parent.
0:04:13 > 0:04:18Some pairs are what's called dominant recessive.
0:04:18 > 0:04:21A dominant gene - like the one that causes a cleft chin -
0:04:21 > 0:04:23always trumps its recessive pair,
0:04:23 > 0:04:26making sure its characteristic is turned on.
0:04:29 > 0:04:33But hang on, if your parents both have dominant cleft chins,
0:04:33 > 0:04:36then how can yours be smooth?
0:04:36 > 0:04:40Well, actually, we only inherit half of each of our parents' genes.
0:04:40 > 0:04:43That's one gene from each of their pairs.
0:04:46 > 0:04:50So we get half of our mum's genes and half of our dad's,
0:04:50 > 0:04:56so it must follow that we get a quarter of each of our grandparents'.
0:04:56 > 0:04:59Now here comes the important bit -
0:04:59 > 0:05:02genes passed down from our grandparents
0:05:02 > 0:05:08may be turned off in our parents, but turned on again in us.
0:05:08 > 0:05:12To see how this works, let's take a look at your family tree.
0:05:14 > 0:05:17Your dad got a dominant cleft gene from your grandfather,
0:05:17 > 0:05:21with a paired recessive gene from your grandmother.
0:05:22 > 0:05:24The same is true for your mother.
0:05:26 > 0:05:29So, in both of them, the dominant cleft gene was switched on.
0:05:32 > 0:05:35The eggs and sperm your parents produce could only carry
0:05:35 > 0:05:37one gene from each pair.
0:05:40 > 0:05:43It just so happened that the egg and sperm that made you
0:05:43 > 0:05:46both got the recessive chin gene,
0:05:46 > 0:05:50so you could only get your granny's pointy, smooth chin.
0:05:52 > 0:05:55The genetic mix your family gave you is unique,
0:05:55 > 0:05:59but the genes themselves aren't uniquely yours.
0:05:59 > 0:06:04That's because DNA is the blueprint for all life,
0:06:04 > 0:06:08from bacteria to human beings.
0:06:08 > 0:06:11All your organs are made up of cells,
0:06:11 > 0:06:16instructed by the genes within them to work together to keep you alive.
0:06:16 > 0:06:19We share this basic method of staying alive
0:06:19 > 0:06:23with every animal, and plant, on earth.
0:06:24 > 0:06:28Which means you share 95% of your genes with rats,
0:06:28 > 0:06:3360% with chickens,
0:06:33 > 0:06:35and 50% with bananas.
0:06:36 > 0:06:41Not all your features are decided by a single gene.
0:06:41 > 0:06:45Genes are divided across structures called chromosomes,
0:06:45 > 0:06:48and your eye colour, for example, was created by genes
0:06:48 > 0:06:52from different chromosomes working together in harmony.
0:06:52 > 0:06:57Now, this is very complex science, but here is a simple question...
0:06:57 > 0:07:01How are you seeing this?
0:07:01 > 0:07:02In Ancient Greece,
0:07:02 > 0:07:07some people believed that eyes worked by projecting beams of light.
0:07:07 > 0:07:09This idea was knocked on the head
0:07:09 > 0:07:12by the 11th century scientist, Alhazen.
0:07:12 > 0:07:16He figured out that we see by the light entering our eyes.
0:07:18 > 0:07:23Objects reflect light rays which are focused on to the retina,
0:07:23 > 0:07:27a layer at the back of the eyeball that's packed with 125 million
0:07:27 > 0:07:31light receptor cells called cones and rods.
0:07:33 > 0:07:38It's the red, blue and green cone cells that give you colour vision.
0:07:38 > 0:07:41In total, we see ten million different shades.
0:07:43 > 0:07:46What's more, your eyes are perfectly positioned to give you
0:07:46 > 0:07:49two separate fields of vision.
0:07:49 > 0:07:53Each eye shows your brain the same object from a different angle,
0:07:53 > 0:07:58allowing you to see in 3D and judge distance and depth.
0:07:58 > 0:08:02Human eyes move at over 30 mph,
0:08:02 > 0:08:06giving you more information than the rest of your senses combined.
0:08:07 > 0:08:09Impressive, till the sun drops
0:08:10 > 0:08:13and human vision starts to look rather shoddy -
0:08:13 > 0:08:16something you'll have noticed if you stub your toe
0:08:16 > 0:08:19when you get up to go to the lavvy in the middle of the night,
0:08:19 > 0:08:22while your cat wanders around as if he owns the place.
0:08:22 > 0:08:24Your eyes are missing something.
0:08:24 > 0:08:29Line up a human, a cat, a sheep, a seal and a deer,
0:08:29 > 0:08:32and it's only human eyes that don't glint at night.
0:08:34 > 0:08:39That's because the others have some awesome night-vision technology -
0:08:40 > 0:08:43a layer of shiny cells called the tapetum lucidum.
0:08:46 > 0:08:50When just a tiny bit of light trickles into the eye,
0:08:50 > 0:08:52this layer reflects it back,
0:08:52 > 0:08:57giving the receptor cells a second chance to pick up the light.
0:08:57 > 0:09:00So, it's a myth that your cat sees in total darkness,
0:09:00 > 0:09:03but it can see in one-sixth of the light that you can.
0:09:09 > 0:09:11I love cats.
0:09:11 > 0:09:13It's a pity they don't really like me.
0:09:13 > 0:09:16Actually, they don't really like anybody,
0:09:16 > 0:09:18they just want some food.
0:09:18 > 0:09:24Food is what keeps your cells working and keeps you on the road.
0:09:24 > 0:09:28Cells need fuel - that's why we eat, and this can cause pollution.
0:09:28 > 0:09:30So...
0:09:30 > 0:09:32where does wind come from?
0:09:33 > 0:09:38You eat 2,000 to 3,000 calories a day.
0:09:38 > 0:09:41You also let loose up to two and half pints of wind.
0:09:43 > 0:09:47We're told the Emperor Claudius thought releasing wind so important,
0:09:47 > 0:09:50he encouraged it at banquets.
0:09:51 > 0:09:55He was right - it's an essential by-product of digestion.
0:09:58 > 0:10:00Within ten seconds of swallowing,
0:10:00 > 0:10:04your stomach is whisking your food into a liquid called chyme,
0:10:04 > 0:10:07which is pushed into your small intestine,
0:10:07 > 0:10:09where most of the goodness is absorbed.
0:10:13 > 0:10:16The small intestine is actually huge -
0:10:16 > 0:10:19a folded, 21-foot tube which, pressed flat,
0:10:19 > 0:10:23would be bigger than a tennis court.
0:10:24 > 0:10:28So, while you're busy sipping coffee after your dinner,
0:10:28 > 0:10:32your stomach is already churning your shepherd's pie up into a soup,
0:10:32 > 0:10:35which your small intestine can break down
0:10:35 > 0:10:38into nutrients your cells can use.
0:10:38 > 0:10:39Unfortunately...
0:10:39 > 0:10:41HE BREAKS WIND
0:10:41 > 0:10:42there are some leftovers.
0:10:42 > 0:10:46Two and a half pints of watery gruel.
0:10:46 > 0:10:48This is poured into your large intestine,
0:10:48 > 0:10:54home to billions of bacteria known as gut flora.
0:10:54 > 0:10:58These account for over two pounds of your body weight.
0:10:58 > 0:11:01To your bacteria, this gruel is a feast.
0:11:01 > 0:11:05As it pours in, they set to work digesting the plant fibres
0:11:05 > 0:11:11your small intestine can't and extracting vitamins and fatty acids.
0:11:11 > 0:11:15This vital work produces gas, including carbon dioxide,
0:11:15 > 0:11:20hydrogen, methane and the odorous hydrogen sulphide...
0:11:21 > 0:11:25which builds up in your large intestine, along with solid waste,
0:11:25 > 0:11:30up to 50% of which is dead bacteria and cells from your own body.
0:11:34 > 0:11:36Your intestinal muscles then squeeze
0:11:36 > 0:11:40to create a zone of high pressure that moves the gas and waste
0:11:40 > 0:11:42down towards your rectum...
0:11:44 > 0:11:48forcing you to let out wind around ten times a day.
0:11:50 > 0:11:51FART SOUNDS
0:11:51 > 0:11:56Embarrassing when not expected, but at least you can do it on the move,
0:11:56 > 0:11:59whereas getting rid of solid waste takes time.
0:11:59 > 0:12:02According to a recent survey, by the end of our lives
0:12:02 > 0:12:05we'll have spent 90 days sitting on the lavvy...
0:12:05 > 0:12:07STRAINING NOISES
0:12:08 > 0:12:14..giving us plenty of opportunities for a nice quiet read.
0:12:14 > 0:12:18Now, if I'm Mr Average, I'm made up of two stone of protein,
0:12:18 > 0:12:22two stone of fat, and around eleven pints of blood.
0:12:22 > 0:12:24Sounds disgusting.
0:12:24 > 0:12:26But to other life forms,
0:12:26 > 0:12:29our bodies are actually an irresistible all-day buffet,
0:12:29 > 0:12:32which begs a rather unusual question...
0:12:32 > 0:12:36Is there life on me?
0:12:36 > 0:12:42As the proud owner of a human body, you see yourself as an individual.
0:12:42 > 0:12:44But, if an alien examined you,
0:12:44 > 0:12:49he'd see a walking zoo of bacteria and parasites.
0:12:49 > 0:12:53Your bacteria outnumber your cells ten to one.
0:12:53 > 0:12:57Numerically, you're 90% microbe.
0:12:57 > 0:13:01You house around 500 species in your gut,
0:13:01 > 0:13:05128 in your lungs,
0:13:05 > 0:13:08and up to 200 in your mouth.
0:13:08 > 0:13:11Gross!
0:13:11 > 0:13:15But as we've seen in the intestine, some bacteria are vital.
0:13:15 > 0:13:18More than you can say for...
0:13:18 > 0:13:19parasites.
0:13:19 > 0:13:25At some point, face mites will almost certainly feed off your skin.
0:13:25 > 0:13:29Right now, they may diving down into your follicles,
0:13:29 > 0:13:33using their needle-like mouths to hoover up some of the eight pounds
0:13:33 > 0:13:36of skin cells you lose every year.
0:13:36 > 0:13:38SOUND OF A VACUUM CLEANER
0:13:38 > 0:13:40Pull the plug, it's eating me alive!
0:13:40 > 0:13:46Scary, but better than feeding bloodsucking bedbugs.
0:13:46 > 0:13:50Vampire fiction is, of course, all the rage,
0:13:50 > 0:13:54but the real twilight bloodsuckers are rather less romantic.
0:13:54 > 0:13:58A few years ago, we believed we had the bedbug beaten.
0:13:58 > 0:14:03But now, all across the world, the bedbug is back.
0:14:03 > 0:14:09Bedbugs use their six legs to scuttle out as you sleep.
0:14:09 > 0:14:12One feeding tube administers anaesthetic
0:14:12 > 0:14:17to keep you in the land of nod, while the other sucks your blood.
0:14:19 > 0:14:20Nasty.
0:14:20 > 0:14:24But nowhere near as bad as hosting a tapeworm.
0:14:24 > 0:14:29The tapeworm gets in as larvae in undercooked meat.
0:14:29 > 0:14:32Once inside, it clings to your intestines,
0:14:32 > 0:14:37spending up to 25 years happily bathing in your undigested food.
0:14:39 > 0:14:44By absorbing your nutrients through its skin, it can grow up to 30 feet.
0:14:48 > 0:14:51With any luck you'll manage to avoid tapeworm,
0:14:51 > 0:14:55unless you're one of those blokes who believes he can do a barbecue.
0:14:55 > 0:14:59Viruses, on the other hand, are most definitely going to get you,
0:14:59 > 0:15:03and when they do, they will give you anything from a cold to Ebola.
0:15:03 > 0:15:08And yet, technically, viruses have no life of their own,
0:15:08 > 0:15:10so why do they cause so much trouble?
0:15:10 > 0:15:12More to the point...
0:15:12 > 0:15:15Why do I catch colds?
0:15:18 > 0:15:20See that guy with the blocked nose?
0:15:20 > 0:15:26When he sneezes, 40,000 droplets will fly twelve feet in the air,
0:15:26 > 0:15:28infecting up to 150 people.
0:15:28 > 0:15:29Ugh!
0:15:29 > 0:15:31You should have ducked.
0:15:32 > 0:15:34A cold virus just can't live without you.
0:15:34 > 0:15:37WOMAN SCREAMS
0:15:37 > 0:15:38With no cells of its own,
0:15:38 > 0:15:41it needs to take over your cells and replicate.
0:15:41 > 0:15:43Here's something to think about -
0:15:43 > 0:15:48the virus can only travel around inside a blob of mucus,
0:15:48 > 0:15:51and that means that when you catch a sniffle,
0:15:51 > 0:15:54someone else's snot has been up your nose.
0:15:54 > 0:15:55Anyway...
0:15:55 > 0:16:00your immune system is constantly on the prowl for attackers like these.
0:16:00 > 0:16:04If it wasn't, you could end up with fatal pneumonia.
0:16:06 > 0:16:10So, when it spots a viral invasion, it grabs a sample
0:16:10 > 0:16:14and takes it to the nearest lymph node, home of your killer T cells.
0:16:16 > 0:16:20Here, a T cell first identifies the invader,
0:16:20 > 0:16:25and then deploys an army of tailor-made immune cells to your nose.
0:16:28 > 0:16:30These provide specialist backup
0:16:30 > 0:16:34for the standard immune cells already fighting your cold.
0:16:35 > 0:16:38Your nose has become a battleground.
0:16:40 > 0:16:43Meanwhile, to stop infection spreading to your lungs,
0:16:43 > 0:16:46you're manufacturing a daily pint of mucus.
0:16:48 > 0:16:53This snot gives you a headache, while the virus irritates your nose,
0:16:53 > 0:16:57so you have to blow it around 45 times a day.
0:17:01 > 0:17:05Beating a cold takes you about seven days,
0:17:05 > 0:17:08and you'll catch about four a year.
0:17:09 > 0:17:12This is the golden age for the cold virus.
0:17:14 > 0:17:18It hops on planes with its human hosts, visits new cities,
0:17:18 > 0:17:22and finds hundreds of new homes with every sneeze.
0:17:25 > 0:17:28As it replicates, it mutates,
0:17:28 > 0:17:32so by next year it may be back in a different form.
0:17:32 > 0:17:34SNEEZING
0:17:34 > 0:17:37Children catch more colds than adults
0:17:37 > 0:17:41because their immune systems are less experienced.
0:17:41 > 0:17:44We also take ages to grow up -
0:17:44 > 0:17:48the human being is the only species with a long adolescence -
0:17:48 > 0:17:53and this life phase developed around 500,000 years ago,
0:17:53 > 0:17:58just before the human brain developed into its current large size.
0:17:58 > 0:18:01And all of this makes it much easier to answer our next question...
0:18:01 > 0:18:05Why are teenagers so moody?
0:18:06 > 0:18:08At around eight or nine, an alarm goes off
0:18:08 > 0:18:10in the hypothalamus region of the brain,
0:18:10 > 0:18:14waking up powerful hormones and triggering seismic changes,
0:18:14 > 0:18:18which eventually erupt as breasts,
0:18:18 > 0:18:21body hair, and bodily functions.
0:18:21 > 0:18:23GROANING
0:18:23 > 0:18:26Along with volcanic surges of temper,
0:18:26 > 0:18:30which, until recently, were also put down to hormones.
0:18:30 > 0:18:34Now experts believe the brain is to blame.
0:18:34 > 0:18:38We used to think the human brain was mature by 18 months.
0:18:40 > 0:18:41Not any more.
0:18:41 > 0:18:44When scientists used magnetic resonance imaging
0:18:44 > 0:18:46to scan the brains of young adolescents,
0:18:46 > 0:18:49they discovered multiplying connections
0:18:49 > 0:18:53between the cells of the prefrontal cortex.
0:18:56 > 0:18:58That's the bit of the brain
0:18:58 > 0:19:01that makes decisions and controls emotion.
0:19:01 > 0:19:04So, what happens when we ask a teenage
0:19:04 > 0:19:07and an adult brain a straightforward question?
0:19:07 > 0:19:12The young brain's freshly grown connections really slow it down.
0:19:12 > 0:19:16The teenage brain - here's one we found earlier -
0:19:16 > 0:19:20finds it tough to control impulses and emotions,
0:19:20 > 0:19:24and this explains why teenagers think going out drinking
0:19:24 > 0:19:28with their mates is a viable alternative to, say, exam revision,
0:19:28 > 0:19:31and that it explains why they can't understand
0:19:31 > 0:19:34that this will upset their parents.
0:19:34 > 0:19:38Because research show that young adolescent brains struggle
0:19:38 > 0:19:41to read facial expressions.
0:19:41 > 0:19:45Thankfully, by adulthood, the troublesome extra connections are pruned.
0:19:47 > 0:19:50Mind you, hormones aren't entirely off the hook
0:19:50 > 0:19:52when it comes to teenage moods.
0:19:52 > 0:19:58Hormones produce female curves and male muscles,
0:19:58 > 0:20:02but also an oily substance called sebum,
0:20:02 > 0:20:05which means 80% of teenage skin suffers from...
0:20:07 > 0:20:09whiteheads, blackheads,
0:20:09 > 0:20:11yellow, pus-sy pimples,
0:20:11 > 0:20:15and, if they're really unlucky, bulging nodules.
0:20:15 > 0:20:18At the exact moment they want to look attractive,
0:20:18 > 0:20:20teenagers start resembling pizza.
0:20:20 > 0:20:22And if they do manage to get a date,
0:20:22 > 0:20:25their brains are so emotionally inept
0:20:25 > 0:20:27they're almost bound to mess it up.
0:20:27 > 0:20:32Teenagers can be forgiven for not being able to handle alcohol.
0:20:32 > 0:20:35Adults, you'd think, would know better,
0:20:35 > 0:20:39but I'm afraid we don't, because every day in Britain,
0:20:39 > 0:20:45500,000 of us go to work with a hangover.
0:20:45 > 0:20:47Although, mind you, it's not just us.
0:20:47 > 0:20:49Human beings have been boozing like mad
0:20:49 > 0:20:52for thousands and thousands of years,
0:20:52 > 0:20:56which gives my next question truly international significance.
0:20:57 > 0:20:58Mmm.
0:21:00 > 0:21:04Why am I hung-over?
0:21:04 > 0:21:05Headache?
0:21:05 > 0:21:06Dizziness?
0:21:06 > 0:21:07Nausea?
0:21:07 > 0:21:10The evidence suggests you drink alcohol.
0:21:10 > 0:21:16On average, we each sink more than a gallon of pure alcohol a year.
0:21:16 > 0:21:20Worldwide, this adds up to a trillion pints of beer,
0:21:20 > 0:21:24or 250 billion bottles of wine,
0:21:24 > 0:21:26or 100 billion bottles of vodka.
0:21:28 > 0:21:33To find out how it makes you feel so bad, we need to flash back.
0:21:33 > 0:21:3690 seconds after your first sip, the alcohol hit your brain,
0:21:36 > 0:21:40and interfered with your neurotransmitters,
0:21:40 > 0:21:44making you talkative and self-confident.
0:21:44 > 0:21:48By drink two, your inhibitions were really dropping.
0:21:48 > 0:21:49Back inside your brain,
0:21:49 > 0:21:53a chemical called vasopressin would normally be sending a signal
0:21:53 > 0:21:57to your kidneys, to tell them how much water take from your blood.
0:21:59 > 0:22:01Alcohol switches this chemical off.
0:22:02 > 0:22:07So, your kidneys started channelling most of the water to your bladder.
0:22:07 > 0:22:13For every drink, you expelled four times as much in urine.
0:22:13 > 0:22:16By now, you were also tired and emotional.
0:22:16 > 0:22:20Thankfully, someone took you home, where you crashed out.
0:22:20 > 0:22:22And slept really badly.
0:22:22 > 0:22:26That's because alcohol suppresses the production of glutamine,
0:22:26 > 0:22:29one of the body's natural stimulants.
0:22:29 > 0:22:33When you stopped drinking, production revved up again,
0:22:33 > 0:22:36so you spent the night tossing and turning.
0:22:36 > 0:22:41Overnight, your dehydrated liver had to process the alcohol toxins,
0:22:41 > 0:22:44so it stole water from your brain, which shrank
0:22:44 > 0:22:48and began to pull on the membranes attaching it to your skull.
0:22:48 > 0:22:52Which is why you just woke up with a pounding head, dry mouth,
0:22:52 > 0:22:56and nausea from the after-effects of the toxins.
0:22:56 > 0:22:59A hangover can last for up to 24 hours,
0:22:59 > 0:23:04giving you plenty of time to think about what you did last night.
0:23:04 > 0:23:09If you can face it, a cooked breakfast might help.
0:23:09 > 0:23:11But chances are you can't,
0:23:11 > 0:23:14so you'll just have to spend the day feeling like death.
0:23:14 > 0:23:16But then one day you'll be ill
0:23:16 > 0:23:19and your body won't be able to make you better,
0:23:19 > 0:23:23and that brings us to the greatest question in life...
0:23:23 > 0:23:26Why do I have to die?
0:23:26 > 0:23:30For most of history, human life has been dangerous and short.
0:23:30 > 0:23:33Natural selection favoured the genes that made you strong,
0:23:33 > 0:23:36not the ones that helped you get old.
0:23:36 > 0:23:40Our bodies still peak between 20 and 35.
0:23:40 > 0:23:43After that, it's a slippery slope to the grave.
0:23:45 > 0:23:49To find out why, scientists are investigating our cells.
0:23:49 > 0:23:53When you're young and healthy, your cells divide and replicate
0:23:53 > 0:23:5650 billion times every day.
0:23:56 > 0:24:00But each individual cell can only split 50 times.
0:24:00 > 0:24:03When it hits this magic number, it's retired.
0:24:06 > 0:24:09As you age, cell death starts to outpace cell birth.
0:24:11 > 0:24:16And ageing cells are more vulnerable to attackers.
0:24:16 > 0:24:19Amongst the worst are free radicals -
0:24:19 > 0:24:22unstable oxygen molecules which stabilize themselves
0:24:22 > 0:24:25by stealing electrons from your cells...
0:24:27 > 0:24:32inflicting damage that can cause diseases like cancer.
0:24:32 > 0:24:34Free radicals aren't the only problem -
0:24:35 > 0:24:38old cells just get tired.
0:24:38 > 0:24:42The elderly cell enlarges, and becomes less efficient
0:24:42 > 0:24:45at turning oxygen and nutrients into energy.
0:24:45 > 0:24:48A bit like a person, really.
0:24:48 > 0:24:51You can see the results of ageing in your skin.
0:24:51 > 0:24:55Inside your body, the same thing is going on.
0:24:55 > 0:24:59At 40, your nervous system becomes less co-ordinated,
0:24:59 > 0:25:02giving you heartburn and constipation.
0:25:02 > 0:25:08By 60, your eyes let in two-thirds less light than they did at 20.
0:25:08 > 0:25:14And at 85, your heart can only beat a fifth as fast as a 20-year-old's.
0:25:14 > 0:25:15At which point, well,
0:25:15 > 0:25:20you and your magnificent body are just about to hit journey's end.
0:25:21 > 0:25:25Although science is helping more of us to live to be old,
0:25:25 > 0:25:28humans have a natural lifespan.
0:25:28 > 0:25:3085 is about it, really,
0:25:30 > 0:25:34and conventional medicine isn't likely to extend this very much.
0:25:34 > 0:25:39But of course, medicine is becoming more and more unconventional,
0:25:39 > 0:25:44so for our descendants, life and death may be very different.
0:25:44 > 0:25:48So what will we look like in 1,000 years' time?
0:25:48 > 0:25:53Two million years ago, Homo habilis made stone tools.
0:25:53 > 0:25:57These days, Homo sapiens design computer chips.
0:25:57 > 0:26:01Next, we'll be taking charge of our own evolution.
0:26:01 > 0:26:05In 2008, scientists announced that they had taken a rat's heart,
0:26:05 > 0:26:11stripped it to its scaffolding, and re-seeded it with cardiac cells.
0:26:11 > 0:26:15Within four days, the cells were contracting.
0:26:15 > 0:26:17Within eight, they had begun to pump.
0:26:17 > 0:26:21This was a big step on the road to replacement organs.
0:26:22 > 0:26:26Also in the pipeline are eyes, lungs and limbs.
0:26:27 > 0:26:30This technology could make us immortal
0:26:30 > 0:26:34IF we can also work out how to regenerate our brains.
0:26:35 > 0:26:38But that will take a while,
0:26:38 > 0:26:42because the brain is a mind-bogglingly complex thing.
0:26:42 > 0:26:46Inside here there are a hundred billion cells,
0:26:46 > 0:26:50all communicating through tiny waves of electricity.
0:26:50 > 0:26:54And somewhere within all those cells and all those connections
0:26:54 > 0:26:58is the mind, the essence of you and me.
0:27:00 > 0:27:05Scientists are determined to unlock the mysteries of the mind.
0:27:05 > 0:27:08When they do, we may achieve not just immortality,
0:27:08 > 0:27:11but a higher intelligence than we can imagine.
0:27:11 > 0:27:14Here's how -
0:27:14 > 0:27:17in 2004, a computer chip was implanted
0:27:17 > 0:27:20into the brain of a paralysed man.
0:27:20 > 0:27:23When he wanted to watch TV, the chip read his thoughts,
0:27:23 > 0:27:25and switched the channels.
0:27:26 > 0:27:30By 2050, it's predicted that computer-brain interaction
0:27:30 > 0:27:35will be so advanced, that we'll use PCs to store our surplus memories.
0:27:36 > 0:27:40Beyond that, we'll aim to devise technology to merge
0:27:40 > 0:27:44the creativity of human thought with the speed of computers.
0:27:44 > 0:27:48So, while your descendants might choose to look human,
0:27:48 > 0:27:51inside they could be more machine than man.
0:27:54 > 0:27:57So, back to my first question -
0:27:57 > 0:28:01do you ever contemplate the wonder of your body?
0:28:01 > 0:28:03Well, we just have,
0:28:03 > 0:28:05and now I'm incredibly proud to own one,
0:28:05 > 0:28:09a marvellous vehicle in which to journey through life.
0:28:09 > 0:28:12And one day that journey may last forever,
0:28:12 > 0:28:15but I'm afraid you and I were born too early for that.
0:28:15 > 0:28:18All we can do is sit back and enjoy the rest of the ride.
0:28:44 > 0:28:46Subtitles by Red Bee Media Ltd
0:28:46 > 0:28:48E-mail subtitling@bbc.co.uk