The Human Body (1998) s01e06 Episode Script
As Time Goes By
If we take a line of people, one from each year of life from birth to a hundred, what we see is the remarkable development of human ageing.
As we journey through the first stages of our lives, our bodies develop to meet the challenges of each new age.
Year by year, we're continually developing, growing stronger, becoming more intellectually alert and more sexually mature.
All these changes bring us to the point where we can reproduce, and so pass on our genes to the next generation.
But what's particularly remarkable is that we go on.
0n beyond the child-bearing years and the years it takes to raise our children.
0n into the later years.
It's remarkable because in this respect we're unlike any other animals.
In the wild, animals don't grow old, but we humans have evolved to live long lives - longer than any other mammal, in fact.
Why is something of a mystery, but for humans at least, there might be something more to growing old than a slow decline.
(HEARTBEATS) Believe it or not, that's my heart beating there.
I'm wired up to a cardiac monitor.
At the beginning of the century, there was a theory about ageing which was very popular.
It was called the ''Rate of Living'' theory.
It argued that each animal had a finite number of heartbeats, about one to two billion.
So, for example, a hare, whose heart beats very fast, might live from three to five years, a victim of live fast die young.
A tortoise, whose heart beats very slowly, might live for over a century.
I'm not quite sure where that leaves me.
Anyway, the theory was wrong.
The truth is we don't really understand very much about the ageing process.
But what we do know is very interesting.
We'll see why a great French painter gradually altered the way he painted his garden, and how an eighty-year-old cowboy can still ride a horse.
How sex hormones affect the way we age.
What we have in common with Voyager.
Why very familiar sounds are heard differently as time goes by.
And why I don't look the way I did when I first heard this music.
(LIVELY MUSIC) But science can only give us part of the picture.
So we'll be following the story of two people who are actually living through the experience of growing older.
They are an elderly couple living in a farming community in mid-west America.
Bud Mather is still herding cattle at the age of almost eighty.
He and his wife Viola have grown old together on their farm in Kansas.
We've been here ever since we married, 45 years ago.
And we've never been gone more than 20 days, I guess.
This place when we got married was just a little four-room side house.
It didn't have any water in it, it didn't have any bathroom.
We just had to carry all the water in and out, and er, that's why you married me, so I'd cook for you because you couldn't cook.
And you didn't want to haul the water.
You married me so I'd take care of you, didn't ya? Yeah.
I'll be 78 in November, and Vi, she was 63 in August.
That's about 14 years difference in us.
When we was going together, they always kid me about it: (C0UNTRY AND WESTERN MUSIC) Ageing is a process they've shared since they first met each other at a dance at the old schoolhouse almost half a century ago.
Boy, this place is a mess now, ain't it? I can't believe look at the terrible shape it's in.
All the windows are gone.
Wasn't like this when we met 50 - Yeah.
- Seems like 50, doesn't it? - 46 years ago.
- 46 years ago, yeah.
- We met here at a dance, remember that? - Oh, yeah.
On the night of the dance that we met, this place was full of people.
Music, laughter, and people having a good time.
And he came over and asked me to dance.
I thought he had the bluest eyes I had ever seen.
And I fell in love with those blue eyes.
She sure looked wonderful to me.
Her skin was real fair and smooth.
And I guess I fell in love with her then.
As we grow on through the years, we get a little more here and a little more there, and a little less here and a little less there.
But I don't think our love has got any less.
If anything, it's got deeper, more understanding of one another.
Naturally, over the years, Bud and Viola have experienced some of the changes that tend to happen to us as we grow older.
The first time I realised I needed glasses, we were driving in the car.
Bud was driving and it was late in the evening.
Well, I got the glasses, I put them on, and I have never taken them off again.
Eyesight is so remarkably and wonderfully complex that it's arguably our most important sense.
Using a special camera, it's possible to look directly into the eye.
The pupil has been enlarged so that we're looking straight through the lens of the eye, at the pink retina at the back - pink because of the blood vessels immediately behind it.
This is what produces the familiar red eye in flash photos.
It's the retina that carries the receptors that register the light from the scene in front of us.
For such a complex system, it's remarkable how much our brains have to compensate for what we actually see.
The lens in our eye produces an image on the retina which is actually upside down.
0ur brains correct this by telling us that what we're seeing is the right way up.
The lens itself is pretty basic.
It tends to produce an image which is blurred, particularly around the edges.
But when it comes to focusing, it's not just the lens that matters, it's the retina.
0n the retina, the white spot with blood vessels emerging is the optic nerve.
This carries the visual signals to the brain.
But the part that does the most work is the small dark circular area in the centre of the picture.
It's called the macula and it's only about one to two millimetres across.
Right in the middle of that area, best seen in a green light, is a small pit with a yellow spot in the middle.
It's called the fovea - this is what gives us a focused image.
Every time we look at something and see it in sharp focus, it's not the rest of the retina that's doing it, it's just that small spot, only a fifth of a millimetre across, that gives us that focused image.
But because it's so small, what it sees in focus is only a small part of the scene in front of us.
So the way we view something like this is to move the eyes around, seeing one small focused area after another.
The brain receives these images and persuades us that everything in front of us is in focus.
But it's an illusion.
As we get older, our brain has to do more and more work.
When we're children, the lens in our eye has a very pale blue colour.
By middle age, in many people the coloration is getting stronger.
It's going yellow, and by old age it can even be brown.
But at all stages, the brain corrects and takes out that coloration, so that we're totally unaware of it.
And there's something else going on.
At the front of the eye is the coloured iris, and immediately behind it is the lens.
In the eye of a small child, the lens is almost completely clear.
But it's at the start of an extremely gradual process that affects us all.
An eye at the age of eleven.
By twenty it's become a little bit cloudy.
At 45, more so.
At 80, definitely.
As we get older, we don't notice this cloudiness developing, because we simply can't remember how clearly we could once see things.
But if it gets too cloudy, there's a limit to what the brain can adjust for.
It doesn't happen to most people, but the lens in an extremely cloudy condition is called a cataract.
It's so cloudy, other people can see it.
The effects of cataracts were particularly important for what happened in this garden some years ago.
Earlier this century, the man who lived here at Giverny in Northern France experienced many of these changes to his eyesight.
In his case it was particularly important, because he was one of the greatest French painters of all time, Claude Monet.
0ver the years, he painted many scenes in these gardens.
Not only are they masterpieces, but they provide a remarkable record of what was happening as age affects eyesight.
What is noticeable is that his paintings mysteriously began to change.
They became redder and redder.
The reason - he was developing cataracts in both his eyes.
These not only affected the sharpness of what he could see, but the way he saw the colours.
In 1923, Monet had an operation to remove the cloudy lens from his right eye.
It instantly changed the way he saw things.
Shortly afterwards, he painted this scene with the eye that still had the cataract in it, and then again, the same scene with the other eye where the cataract had been removed.
What he saw with the cataract eye was this.
And what he saw with the other eye where the cataract had been removed was this.
It has much more blue in it.
A close-up of the tree makes it obvious.
The left eye with the cataract still in, and the right eye with it removed.
Monet was so horrified by the colours he had used when his vision was affected by the cataracts, he wanted to alter many of his paintings, and even destroyed some of them.
Coping with the changes that accompany ageing needn't be depressing.
The effects of growing older certainly don't bother these people too much.
They're Bud and Viola's friends, and they all go back to that same dance at the school hall, where Bud and Viola met nearly fifty years ago.
It's been really nice to get together.
It's about like it was back at the old schoolhouse.
Same bunch.
Well, we had a lot of fun then.
A lot of things we can't talk about.
There are people who hate to turn 40, 50 and 60, because they think that they're getting old.
Look at Bud Mather.
Now, he isn't old.
I mean the years might say that he's old, but he's got a lot in him yet.
You know what I remember about Bud? He had the prettiest, waviest hair.
- And that's different now, isn't it? - Yes, it is.
And his skin was smooth, no wrinkles.
And he didn't have any little pot here.
And he didn't have any false teeth.
Bud's skin might not be as soft as it once was, but he's still not doing that badly considering his job.
As a cattle rancher, his occupational hazard is the sun.
It can be extremely damaging to skin, because it gradually destroys what supports it.
Even those who tan easily will have skins prematurely wrinkled.
Regardless of any damage caused by the sun, normal ageing produces changes in the skin.
This ultrasound probe sends out high-frequency sound that bounces back from the layers just under the surface.
A young person will, on average, have skin about half a millimetre thick.
An older person tends to have thinner skin.
A young person's skin is on the left of the screen, and an old person on the right.
Typically, it's about twenty five percent thinner.
The reason is the older skin has lost substances known as collagen and elastin.
These are proteins which provide the underlying framework of a young, healthy skin.
Sunlight accelerates the loss of these proteins, and it's thought smoking has a similar effect.
The skin becomes less elastic, and the continual flexing of facial muscles gradually produces wrinkles.
It's been estimated that two hundred thousand frowns are enough to etch in one brow line.
0ne way to test the age of the skin is to pinch the back of the hand and watch what happens.
Young skin springs back quickly.
By the time we've reached our thirties and forties, it's getting a little slower.
And old skin definitely takes its time.
0ne thing's certain, though - they may not be very popular with some people, but nobody's ever died simply because they've got a few wrinkles.
- Morning.
- Morning.
- Seems kind of windy and cold today.
- Yes, it is.
- Well, Bud, how are you? - Oh, I'm doing all right.
- How are you, Gerald? - Good, Bud.
Hope we don't get any more rain.
Like many older men, Bud doesn't overwork his barber.
While his friend Gerald still has a full head of hair, Bud has been both bald and grey for some time now.
(''WHEN I'M SIXTY F0UR'' PLAYS) Baldness is caused by the male sex hormone testosterone, but it's nothing to do with virility.
It might be that a bald man has hair follicles which are just very sensitive to the hormone.
In other words, even small amounts of testosterone can make the hair fall out.
So, unfortunately, the only way to guarantee you don't go bald is to choose your parents with great care or get castrated before reaching puberty.
Women don't usually lose their hair until after the menopause.
They make small amounts of the male hormone, testosterone, and with the reduction of female hormones at the menopause, the effect of their testosterone becomes more marked, and this could be the cause of hair loss.
For the same reason, some women grow a light beard at that time.
A hair grows at the rate of a centimetre a month, so for those of us who have a full head of hair, what that means is that during this programme, we will grow a metre and a half on our scalp.
Around one thousand kilometres over a lifetime.
Whether your hair curls or not depends to some extent on where you're from.
Asian hair is circular in cross section, so it tends to hang straight.
Black people have hair which is a flat oval, so it naturally tends to form tight curls.
Caucasians or white Europeans have hair halfway between the two, so they tend to have slightly wavy hair.
White Europeans tend to go grey earlier in life than black or Asian people.
Again, it's down to genetics.
Despite all the stories about people going completely grey overnight, it's impossible.
However, it can happen over a few days, as long as you're partly grey to start with.
It's thought a sudden shock can sometimes make the coloured hair fall out, leaving the grey behind.
- Think that'll be good enough for you, Gerald? - Yeah, looks good.
Well, I hope so, I'm running out of hair.
That's the time to quit, when you run out of hair.
When you get to Bud, you're gonna have to find his to cut it.
Just about as long as it takes on mine.
But while testosterone makes men lose hair from their head, it produces more vigorous growth in other places.
Though it's not always welcome, it tends to be in their ears and noses.
Why testosterone has such differing effects on hair growth is a total mystery.
And body hair is not the only thing.
0ur noses appear to go on growing after we've reached maturity.
But it's not clear if it's real growth or a kind of stretching and sagging.
And the same thing happens to our ears.
All in all, these gradual changes mean that, eventually, we just have to accept the loss of our youthful looks.
I'm making this cake for my forty-fifth wedding anniversary.
I'm going to put some pink roses on it, and little green leaves, and I'm going to put this wedding ornament on it that was on my original cake, when Bud and I got married 45 years ago.
Now, the ribbon's a little tattered and torn, but Bud and I are a little tattered and torn, too, so we'll use that on there, I think.
0n the whole, both Bud and Viola enjoy robust good health.
But as they've got older, they've occasionally had to deal with something serious.
I have always had wonderful health, all of my life, until I was 58 years old, and at that time I came down with spinal meningitis.
In fact, I was doing a big wedding cake for some people, four hundred guests, and I became very ill that day.
So ill in fact that Viola became unconscious.
She was rushed to hospital and, apart from anything else, it looked as if she and Bud would miss a holiday together.
Then, whilst she was in the coma, one of the children came up with a bright idea.
and she opened her eyes and that brought her to.
My eyes flew open and I came out of the coma.
I couldn't believe it.
I couldn't let Bud take another woman on a beautiful trip.
I wanted to go on that trip myself.
0ne trip Bud and Viola are about to make will involve taking that cake to the family reunion for their forty-fifth wedding anniversary.
They're off to meet children they haven't seen for years and grandchildren they've never met.
But it's going to be the trip of a lifetime, because they're also going to visit Alaska.
We've packed enough stuff we can homestead up there.
For years they've longed to see the place, and plan to do it now before it's too late.
But before that, they've decided to stop over to see the big city.
They've heard and read about it all their lives, but have never seen it.
They're going to visit New York for the first time.
- Look at that skyline.
- Look at that big tower on top there.
Look at that, two of them, see.
Oh, my gosh.
- Isn't that marvellous? - Well, I don't know.
I wouldn't care how marvellous it is, I'd rather go watch the pasture.
Oh, but we never saw anything like this.
- No.
- Never.
- I don't suppose we will again here.
- Such tall buildings.
- Isn't this bridge something? - All the cars.
- Glad you're not driving today? - Yes, I am.
I'm glad you aren't driving today, too.
- I'd be a nervous wreck by the time we got - Well, I'd be more nervous.
And the Waldorf Astoria.
We've heard of that all our lives, - that famous building.
- That's a big hotel, ain't it? Although being in the big city is both fascinating and exciting, for an elderly couple, it can also be a little bewildering, partly because, in contrast to the relative quiet of the plains of Kansas, here it's noisy.
Very noisy.
And as we get older, our hearing starts to fade a little.
Like all our senses, hearing is both wonderful and extraordinary in the way it works.
The fleshy outer ear collects the sounds that surround us, and channels them down the ear canal to vibrate the ear drum.
0n the other side of the drum, the vibrations are transmitted via the middle ear by minute bones.
These are the only bones that stop growing soon after birth.
An adult has ear bones the same size as a new-born baby.
0ne of them is the smallest bone in the body.
Known as the stirrup, it's about the size of a grain of rice.
The stirrup rests on the oval window of the inner ear and passes vibrations through it.
0n the other side is something rather intriguing.
Called the cochlea, it's a small bony spiral tube.
It's the key to how we hear the vast range of sounds we do.
If we zoom right the way in with an electron microscope, we should see something remarkable: the secret of how sound is transmitted to the brain.
There are rows of minute hairs only a few thousandths of a millimetre high.
As noise vibrates them, they send electrical signals to the brain which we experience as sound.
It's fascinating to watch and we're about to see it happen.
Take a closer look at these V shapes.
They are in fact clusters of three lines of hairs which are part of a built-in amplifying system.
Now, if we take a look below, we should see the rest of the amplifier.
There we are.
The hairs are sticking out of a cell underneath shaped like a sausage.
It's actually been possible to isolate one of these hair cells, so let's see what it does when we play it some music.
You might recognise the tune.
(''R0CK AR0UND THE CL0CK'' PLAYS) The joint's jumping, literally.
The excited response of these hair cells amplifies the faint vibrations that arrive from outside.
And they do it so well that we can actually hear the sound of a pin drop.
Unfortunately, from the moment we are born, one by one hair cells start to die.
And those that register high frequenices die off first.
The damage here is where hair cells have failed.
Because of the loss of these hair cells, by the age of ten we've heard a greater range of sound than we'll ever hear in the rest of our lives.
As we get older, some people continue to hear relatively well, but in others, so many of the amplifying hair cells have gone that they can only hear loud noises.
Most cannot hear high frequencies any more.
So the sound of the New York subway which younger people might hear as something like this (L0UD METALLIC WH00SHING) .
.
would instead be heard like this.
(DULLER, MUFFLED WH00SHING) I'd like to get off this.
Oh, no, I could ride this all day.
0bviously, if older people's hearing is fading a little and their eyes are not as good as they were, their brains just have to cope as best they can.
And some think this is why older people can sometimes appear easily confused and bewildered.
It's not so much that their intelligence is waning, although some brain cells will have been lost.
The brain has to work harder to make sense of the limited information it gets from the outside world.
This leaves fewer brain cells for memory and decision-making, and might account for the confusion, especially in strange and noisy places.
I never thought I'd see New York.
So many people, cars, and so much noise and everything, that you just can't quite sort it all out.
Well, I can't.
My neck's almost sore from looking up all the time, they're so tall in the big city.
But I really enjoyed seeing it.
Couldn't have dreamed of coming to some place like this when I was a child.
It was a faraway, distant place.
It's wonderful to be able to do this in our lifetime.
Later on the trip, Bud and Viola meet up with some of their children and grandchildren.
By the age of 70, we'll have lost about a third of our muscle strength.
But we needn't have.
Regular exercise will help us retain it.
So, as a physically active cattle rancher, it's perhaps not surprising that Bud does as well as his grandchildren.
But there's something else that can happen, and we can see it with the help of a special camera.
A thermal camera can detect differences in the heat emerging from various parts of the body.
And it's used by doctors to detect something that's different, something that's wrong.
Margaret was out walking one day, when she felt a sudden sharp pain in her leg.
To the naked eye, there's not much difference between her two knees, but, to the thermal camera, there definitely is.
0ne knee has a cool blue and green colour.
But the other knee has a different colour.
It has a hotspot in red.
Let's take a more detailed look at that knee joint.
Where the thigh bone and the shin bone meet, they're protected by a membrane of cartilage.
Take away the bones for a moment, and we're left with that cup-shaped membrane.
If we take a look inside, yes, with wear and tear, a small hole has developed in the membrane, and the two bones have come in contact.
As they touch each other and grind together, the surfaces are damaged, and without treatment the joint can become extremely painful.
It's called osteo-arthritis, and affects many elderly people.
Although physically very fit, Bud has osteo-arthritis in both knees.
- No more chocolate.
- No more chocolate for you.
Although they're joking, Viola's distorted image in the mirror is very similar to what happens to women when they put on weight.
Fat tends to settle around the hips and bottom, and there's a difference between the sexes.
When a rather plump man is scanned in a special X-ray machine, it's possible to see the outline of the layers of fat.
Men tend to put it on around the waist, so they become apple-shaped.
Some believe this happens because of a drop in the sex hormone testosterone.
It's thought it can reduce as a result of age and stress.
Fat around the waist tends to get into the bloodstream easily, increasing the risk of diabetes and heart disease in men.
The inside walls of young arteries are smooth and clean.
But after years of a diet high in saturated fat, old arteries can start to look clogged.
This can trigger blood clots and block the vital circulation to the heart.
Female sex hormones tend to make fat accumulate around the bottom, as it did at puberty when a woman first developed her adult shape.
This is a safer place, in that the fat here is inclined to stay put and not circulate in the blood.
Women become pear-shaped in comparison to men.
Bud and Viola journey on to the family reunion and Alaska.
All in all, ageing means lots of changes as we go through our lives, but perhaps the more interesting question is not so much how we change, but rather why do we change at all? Ageing is one of the greatest paradoxes of human biology.
It turns out that our bodies are constantly renewing themselves.
0ur skin is continuously being replaced.
Most of the dust we find at home is dead skin that's fallen off us as new cells are substituted.
Just rubbing your arm produces a minor dust storm of dead skin cells.
Likewise, the lining of the gut is continuously replaced, so that there is in effect a total change every three days.
All this is done by making copies of cells.
The new cells then take the place of the old cells.
It's going on all the time.
The blood is replaced three times a year, parts of the skeleton every four years.
So extraordinary is this process of copying and renewal that very little of our bodies is actually more than ten years old.
But if our bodies are perpetually being renewed in this way, why do we gradually start to look old? Why don't we all look the way we did when we were young and in our prime? You can understand why we don't look the way we did as a child, after all we were still growing.
But having reached maturity, why don't we continue to look that way for the rest of our lives? Well, one possible explanation as to why I no longer look as I did when I was a medical student may be due to faults in the renewal process itself.
Renewal involves making copies: copies of skin cells, blood cells, bone cells, all sorts of cells.
And copying can have its problems.
Suppose we recorded this programme onto this cassette and then copied from this one onto this, and did so backwards and forwards maybe half a dozen times.
What would be the effect of all that copying? Well, we've done it, so let me show you.
You'd get an image which looks something like this.
The copying process is not perfect.
Mistakes are made.
And although I'm still recognisable, the picture's starting to look the worse for wear.
In terms of the human body, we call these mistakes ageing.
Moreover, the older the person, i.
e.
the copying machine, the more frequent are the mistakes.
And of course, if we carry on copying for too long, we'll eventually reach the point where we disappear altogether.
That's one theory why we age.
But another has it that we grow old because of something that is actually vital for life - oxygen.
We tend to think of oxygen as healthy and essential for life.
But this is the other side of it.
This blazing inferno, like all fires, requires oxygen for its power.
And despite this protective suit, I'm well aware of the power.
It's getting hotter every minute.
Just like all fires, we too require oxygen.
0ur bodies need it every second of our lives.
Yet it turns out that over years, oxygen can be as damaging and dangerous as these flames.
Steel can resist fire, but not the effects of oxygen, because that leads to rust.
Rust is caused by a highly reactive type of oxygen called free radicals.
0ver time, free radicals have the power to corrode and destroy even the largest steel structures.
Some of the oxygen we breathe in turns into the dangerous free radical form.
This type of oxygen has a habit of ripping bits out of molecules, damaging them in the process.
As these loose cannon roam around our bodies, they can play havoc as they start to damage our cells and tissues.
Many now believe years of accumulated damage by free radicals are an important cause of ageing.
And what's more, it's not just breathing oxygen that puts us at risk.
We can get free radicals into our bodies in all sorts of ways.
Too much sun increases them, and we can pick them up from tobacco smoke and air pollution, and eating things like barbecued meat.
But some foods are rich in anti-oxidents, and these can destroy free radicals.
These foods include fresh fruit and vegetables, and, I'm glad to say, a glass of red wine.
Free radical damage is one theory.
There are many more.
With the onslaught of free radicals and the like, the question inevitably arises, how do we manage to live for so long? The answer might be out in space.
0ne possible reason is because we're built rather like the thing that all this is listening to.
Right now, this satelitte dish is peering at the far edge of our own solar system.
It's listening to the faint signals of the most distant object ever made by humans, about six and a half billion miles away.
It's the Voyager space probe, and it started its journey twenty years ago on a voyage of discovery.
Two of them were launched, and their original mission was to relay information about the planets Jupiter and Saturn.
By 1981, they'd achieved that original goal.
But they were built so well that they went on to do much more.
Amazingly, they're still sending back information as they head out into interstellar space.
But the point is, to be doubly sure they achieved their original goal, they were over-engineered, built so robustly that they're capable of surviving into very old age.
Just like us, in fact, though our original goal was to make sure we have children.
But can over-engineering really explain why we live so very long? After all, we have by far the longest lifespan of any other mammal, and we live way beyond an age when we can first have children.
Well, it could, but first we need to think about what we've been over-engineered to do.
Perhaps it's not just to have children and become parents, but to become grandparents.
Arriving at the family reunion for their forty-fifth anniversary, Bud and Viola encounter something that would be very puzzling in other animals - grandchildren.
Arguably, in evolutionary terms, all we have to do is live long enough to have children, and rear them successfully so that they carry on our genes.
That's what other animals do.
Yet Bud and Viola's children have had children of their own, and Bud and Viola are still around.
So if we're capable of living long enough to become grandparents, how has this affected our bodies and the way we age? Well, one thing, and it seems rather strange, happens to women as they get older.
It's the menopause, and it's virtually unique in the animal world.
For women, it marks the end of their child-bearing years, and it's a bit of a mystery.
Men can often produce fertile sperm throughout their lifetime.
At the age of a hundred, a man is, in theory at least, capable of siring a child.
But that's definitely not the case for a woman.
At birth, she has about two million eggs in her two ovaries.
From puberty, they start to be released.
At the rate of one per month, there should be enough to last a lifetime and longer, but there aren't.
Most of the eggs die, until, around the age of fifty, the body stops releasing eggs altogether.
In effect, the store has been used up.
Some scientists believe that the menopause has evolved as we've come to live so long.
The menopause helps us make the most of our long lives by becoming effective grandparents.
The reason is, to continue to give birth to children in an ageing body is dangerous for both mother and child.
Better for her to stop having her own children, and instead to concentrate on taking care of her grandchildren.
After all, they are carrying her genes.
So the menopause helps us make the most of old age.
I said humans are almost unique in this, but not quite, because interestingly there's one other animal where the female has a menopause, and that's the pilot whale.
Many elderly female whales spend many years caring for the offspring of their offspring - being grannies, in fact.
It's controversial, but some feel this supports the idea that there's a similar purpose for the human menopause.
At the end of their trip, Bud and Viola have reached Alaska.
The natural cycle of life is birth and then eventually death.
I guess I sit here around this beautiful surrounding here in Alaska I can see before my eyes the creation going on.
The small trees that are coming on, the tall trees that are a symbol of middle life, and then I can see the dead trees, - all within a few feet of me.
- That's the process we go through.
There's no other way of getting around it.
They've never found a youth pill that'll keep you going.
(VI0LA) I don't worry about dying, I think it's just the next step in life, and life goes on after that, so it's out of my hands, and when that happens, whichever one of us is left, the other one will find the strength to cope.
(BUD) When my time comes, there's nothing I can do about it but go.
And I want to stick around as long as I can.
We tend to think that the human capacity for art, science and technology is what marks us out.
But though we don't often see it this way, perhaps our ability to live to a ripe old age is the human body's greatest achievement.
- Happy anniversary! - Happy anniversary! (T0GETHER) Onetwothree
As we journey through the first stages of our lives, our bodies develop to meet the challenges of each new age.
Year by year, we're continually developing, growing stronger, becoming more intellectually alert and more sexually mature.
All these changes bring us to the point where we can reproduce, and so pass on our genes to the next generation.
But what's particularly remarkable is that we go on.
0n beyond the child-bearing years and the years it takes to raise our children.
0n into the later years.
It's remarkable because in this respect we're unlike any other animals.
In the wild, animals don't grow old, but we humans have evolved to live long lives - longer than any other mammal, in fact.
Why is something of a mystery, but for humans at least, there might be something more to growing old than a slow decline.
(HEARTBEATS) Believe it or not, that's my heart beating there.
I'm wired up to a cardiac monitor.
At the beginning of the century, there was a theory about ageing which was very popular.
It was called the ''Rate of Living'' theory.
It argued that each animal had a finite number of heartbeats, about one to two billion.
So, for example, a hare, whose heart beats very fast, might live from three to five years, a victim of live fast die young.
A tortoise, whose heart beats very slowly, might live for over a century.
I'm not quite sure where that leaves me.
Anyway, the theory was wrong.
The truth is we don't really understand very much about the ageing process.
But what we do know is very interesting.
We'll see why a great French painter gradually altered the way he painted his garden, and how an eighty-year-old cowboy can still ride a horse.
How sex hormones affect the way we age.
What we have in common with Voyager.
Why very familiar sounds are heard differently as time goes by.
And why I don't look the way I did when I first heard this music.
(LIVELY MUSIC) But science can only give us part of the picture.
So we'll be following the story of two people who are actually living through the experience of growing older.
They are an elderly couple living in a farming community in mid-west America.
Bud Mather is still herding cattle at the age of almost eighty.
He and his wife Viola have grown old together on their farm in Kansas.
We've been here ever since we married, 45 years ago.
And we've never been gone more than 20 days, I guess.
This place when we got married was just a little four-room side house.
It didn't have any water in it, it didn't have any bathroom.
We just had to carry all the water in and out, and er, that's why you married me, so I'd cook for you because you couldn't cook.
And you didn't want to haul the water.
You married me so I'd take care of you, didn't ya? Yeah.
I'll be 78 in November, and Vi, she was 63 in August.
That's about 14 years difference in us.
When we was going together, they always kid me about it: (C0UNTRY AND WESTERN MUSIC) Ageing is a process they've shared since they first met each other at a dance at the old schoolhouse almost half a century ago.
Boy, this place is a mess now, ain't it? I can't believe look at the terrible shape it's in.
All the windows are gone.
Wasn't like this when we met 50 - Yeah.
- Seems like 50, doesn't it? - 46 years ago.
- 46 years ago, yeah.
- We met here at a dance, remember that? - Oh, yeah.
On the night of the dance that we met, this place was full of people.
Music, laughter, and people having a good time.
And he came over and asked me to dance.
I thought he had the bluest eyes I had ever seen.
And I fell in love with those blue eyes.
She sure looked wonderful to me.
Her skin was real fair and smooth.
And I guess I fell in love with her then.
As we grow on through the years, we get a little more here and a little more there, and a little less here and a little less there.
But I don't think our love has got any less.
If anything, it's got deeper, more understanding of one another.
Naturally, over the years, Bud and Viola have experienced some of the changes that tend to happen to us as we grow older.
The first time I realised I needed glasses, we were driving in the car.
Bud was driving and it was late in the evening.
Well, I got the glasses, I put them on, and I have never taken them off again.
Eyesight is so remarkably and wonderfully complex that it's arguably our most important sense.
Using a special camera, it's possible to look directly into the eye.
The pupil has been enlarged so that we're looking straight through the lens of the eye, at the pink retina at the back - pink because of the blood vessels immediately behind it.
This is what produces the familiar red eye in flash photos.
It's the retina that carries the receptors that register the light from the scene in front of us.
For such a complex system, it's remarkable how much our brains have to compensate for what we actually see.
The lens in our eye produces an image on the retina which is actually upside down.
0ur brains correct this by telling us that what we're seeing is the right way up.
The lens itself is pretty basic.
It tends to produce an image which is blurred, particularly around the edges.
But when it comes to focusing, it's not just the lens that matters, it's the retina.
0n the retina, the white spot with blood vessels emerging is the optic nerve.
This carries the visual signals to the brain.
But the part that does the most work is the small dark circular area in the centre of the picture.
It's called the macula and it's only about one to two millimetres across.
Right in the middle of that area, best seen in a green light, is a small pit with a yellow spot in the middle.
It's called the fovea - this is what gives us a focused image.
Every time we look at something and see it in sharp focus, it's not the rest of the retina that's doing it, it's just that small spot, only a fifth of a millimetre across, that gives us that focused image.
But because it's so small, what it sees in focus is only a small part of the scene in front of us.
So the way we view something like this is to move the eyes around, seeing one small focused area after another.
The brain receives these images and persuades us that everything in front of us is in focus.
But it's an illusion.
As we get older, our brain has to do more and more work.
When we're children, the lens in our eye has a very pale blue colour.
By middle age, in many people the coloration is getting stronger.
It's going yellow, and by old age it can even be brown.
But at all stages, the brain corrects and takes out that coloration, so that we're totally unaware of it.
And there's something else going on.
At the front of the eye is the coloured iris, and immediately behind it is the lens.
In the eye of a small child, the lens is almost completely clear.
But it's at the start of an extremely gradual process that affects us all.
An eye at the age of eleven.
By twenty it's become a little bit cloudy.
At 45, more so.
At 80, definitely.
As we get older, we don't notice this cloudiness developing, because we simply can't remember how clearly we could once see things.
But if it gets too cloudy, there's a limit to what the brain can adjust for.
It doesn't happen to most people, but the lens in an extremely cloudy condition is called a cataract.
It's so cloudy, other people can see it.
The effects of cataracts were particularly important for what happened in this garden some years ago.
Earlier this century, the man who lived here at Giverny in Northern France experienced many of these changes to his eyesight.
In his case it was particularly important, because he was one of the greatest French painters of all time, Claude Monet.
0ver the years, he painted many scenes in these gardens.
Not only are they masterpieces, but they provide a remarkable record of what was happening as age affects eyesight.
What is noticeable is that his paintings mysteriously began to change.
They became redder and redder.
The reason - he was developing cataracts in both his eyes.
These not only affected the sharpness of what he could see, but the way he saw the colours.
In 1923, Monet had an operation to remove the cloudy lens from his right eye.
It instantly changed the way he saw things.
Shortly afterwards, he painted this scene with the eye that still had the cataract in it, and then again, the same scene with the other eye where the cataract had been removed.
What he saw with the cataract eye was this.
And what he saw with the other eye where the cataract had been removed was this.
It has much more blue in it.
A close-up of the tree makes it obvious.
The left eye with the cataract still in, and the right eye with it removed.
Monet was so horrified by the colours he had used when his vision was affected by the cataracts, he wanted to alter many of his paintings, and even destroyed some of them.
Coping with the changes that accompany ageing needn't be depressing.
The effects of growing older certainly don't bother these people too much.
They're Bud and Viola's friends, and they all go back to that same dance at the school hall, where Bud and Viola met nearly fifty years ago.
It's been really nice to get together.
It's about like it was back at the old schoolhouse.
Same bunch.
Well, we had a lot of fun then.
A lot of things we can't talk about.
There are people who hate to turn 40, 50 and 60, because they think that they're getting old.
Look at Bud Mather.
Now, he isn't old.
I mean the years might say that he's old, but he's got a lot in him yet.
You know what I remember about Bud? He had the prettiest, waviest hair.
- And that's different now, isn't it? - Yes, it is.
And his skin was smooth, no wrinkles.
And he didn't have any little pot here.
And he didn't have any false teeth.
Bud's skin might not be as soft as it once was, but he's still not doing that badly considering his job.
As a cattle rancher, his occupational hazard is the sun.
It can be extremely damaging to skin, because it gradually destroys what supports it.
Even those who tan easily will have skins prematurely wrinkled.
Regardless of any damage caused by the sun, normal ageing produces changes in the skin.
This ultrasound probe sends out high-frequency sound that bounces back from the layers just under the surface.
A young person will, on average, have skin about half a millimetre thick.
An older person tends to have thinner skin.
A young person's skin is on the left of the screen, and an old person on the right.
Typically, it's about twenty five percent thinner.
The reason is the older skin has lost substances known as collagen and elastin.
These are proteins which provide the underlying framework of a young, healthy skin.
Sunlight accelerates the loss of these proteins, and it's thought smoking has a similar effect.
The skin becomes less elastic, and the continual flexing of facial muscles gradually produces wrinkles.
It's been estimated that two hundred thousand frowns are enough to etch in one brow line.
0ne way to test the age of the skin is to pinch the back of the hand and watch what happens.
Young skin springs back quickly.
By the time we've reached our thirties and forties, it's getting a little slower.
And old skin definitely takes its time.
0ne thing's certain, though - they may not be very popular with some people, but nobody's ever died simply because they've got a few wrinkles.
- Morning.
- Morning.
- Seems kind of windy and cold today.
- Yes, it is.
- Well, Bud, how are you? - Oh, I'm doing all right.
- How are you, Gerald? - Good, Bud.
Hope we don't get any more rain.
Like many older men, Bud doesn't overwork his barber.
While his friend Gerald still has a full head of hair, Bud has been both bald and grey for some time now.
(''WHEN I'M SIXTY F0UR'' PLAYS) Baldness is caused by the male sex hormone testosterone, but it's nothing to do with virility.
It might be that a bald man has hair follicles which are just very sensitive to the hormone.
In other words, even small amounts of testosterone can make the hair fall out.
So, unfortunately, the only way to guarantee you don't go bald is to choose your parents with great care or get castrated before reaching puberty.
Women don't usually lose their hair until after the menopause.
They make small amounts of the male hormone, testosterone, and with the reduction of female hormones at the menopause, the effect of their testosterone becomes more marked, and this could be the cause of hair loss.
For the same reason, some women grow a light beard at that time.
A hair grows at the rate of a centimetre a month, so for those of us who have a full head of hair, what that means is that during this programme, we will grow a metre and a half on our scalp.
Around one thousand kilometres over a lifetime.
Whether your hair curls or not depends to some extent on where you're from.
Asian hair is circular in cross section, so it tends to hang straight.
Black people have hair which is a flat oval, so it naturally tends to form tight curls.
Caucasians or white Europeans have hair halfway between the two, so they tend to have slightly wavy hair.
White Europeans tend to go grey earlier in life than black or Asian people.
Again, it's down to genetics.
Despite all the stories about people going completely grey overnight, it's impossible.
However, it can happen over a few days, as long as you're partly grey to start with.
It's thought a sudden shock can sometimes make the coloured hair fall out, leaving the grey behind.
- Think that'll be good enough for you, Gerald? - Yeah, looks good.
Well, I hope so, I'm running out of hair.
That's the time to quit, when you run out of hair.
When you get to Bud, you're gonna have to find his to cut it.
Just about as long as it takes on mine.
But while testosterone makes men lose hair from their head, it produces more vigorous growth in other places.
Though it's not always welcome, it tends to be in their ears and noses.
Why testosterone has such differing effects on hair growth is a total mystery.
And body hair is not the only thing.
0ur noses appear to go on growing after we've reached maturity.
But it's not clear if it's real growth or a kind of stretching and sagging.
And the same thing happens to our ears.
All in all, these gradual changes mean that, eventually, we just have to accept the loss of our youthful looks.
I'm making this cake for my forty-fifth wedding anniversary.
I'm going to put some pink roses on it, and little green leaves, and I'm going to put this wedding ornament on it that was on my original cake, when Bud and I got married 45 years ago.
Now, the ribbon's a little tattered and torn, but Bud and I are a little tattered and torn, too, so we'll use that on there, I think.
0n the whole, both Bud and Viola enjoy robust good health.
But as they've got older, they've occasionally had to deal with something serious.
I have always had wonderful health, all of my life, until I was 58 years old, and at that time I came down with spinal meningitis.
In fact, I was doing a big wedding cake for some people, four hundred guests, and I became very ill that day.
So ill in fact that Viola became unconscious.
She was rushed to hospital and, apart from anything else, it looked as if she and Bud would miss a holiday together.
Then, whilst she was in the coma, one of the children came up with a bright idea.
and she opened her eyes and that brought her to.
My eyes flew open and I came out of the coma.
I couldn't believe it.
I couldn't let Bud take another woman on a beautiful trip.
I wanted to go on that trip myself.
0ne trip Bud and Viola are about to make will involve taking that cake to the family reunion for their forty-fifth wedding anniversary.
They're off to meet children they haven't seen for years and grandchildren they've never met.
But it's going to be the trip of a lifetime, because they're also going to visit Alaska.
We've packed enough stuff we can homestead up there.
For years they've longed to see the place, and plan to do it now before it's too late.
But before that, they've decided to stop over to see the big city.
They've heard and read about it all their lives, but have never seen it.
They're going to visit New York for the first time.
- Look at that skyline.
- Look at that big tower on top there.
Look at that, two of them, see.
Oh, my gosh.
- Isn't that marvellous? - Well, I don't know.
I wouldn't care how marvellous it is, I'd rather go watch the pasture.
Oh, but we never saw anything like this.
- No.
- Never.
- I don't suppose we will again here.
- Such tall buildings.
- Isn't this bridge something? - All the cars.
- Glad you're not driving today? - Yes, I am.
I'm glad you aren't driving today, too.
- I'd be a nervous wreck by the time we got - Well, I'd be more nervous.
And the Waldorf Astoria.
We've heard of that all our lives, - that famous building.
- That's a big hotel, ain't it? Although being in the big city is both fascinating and exciting, for an elderly couple, it can also be a little bewildering, partly because, in contrast to the relative quiet of the plains of Kansas, here it's noisy.
Very noisy.
And as we get older, our hearing starts to fade a little.
Like all our senses, hearing is both wonderful and extraordinary in the way it works.
The fleshy outer ear collects the sounds that surround us, and channels them down the ear canal to vibrate the ear drum.
0n the other side of the drum, the vibrations are transmitted via the middle ear by minute bones.
These are the only bones that stop growing soon after birth.
An adult has ear bones the same size as a new-born baby.
0ne of them is the smallest bone in the body.
Known as the stirrup, it's about the size of a grain of rice.
The stirrup rests on the oval window of the inner ear and passes vibrations through it.
0n the other side is something rather intriguing.
Called the cochlea, it's a small bony spiral tube.
It's the key to how we hear the vast range of sounds we do.
If we zoom right the way in with an electron microscope, we should see something remarkable: the secret of how sound is transmitted to the brain.
There are rows of minute hairs only a few thousandths of a millimetre high.
As noise vibrates them, they send electrical signals to the brain which we experience as sound.
It's fascinating to watch and we're about to see it happen.
Take a closer look at these V shapes.
They are in fact clusters of three lines of hairs which are part of a built-in amplifying system.
Now, if we take a look below, we should see the rest of the amplifier.
There we are.
The hairs are sticking out of a cell underneath shaped like a sausage.
It's actually been possible to isolate one of these hair cells, so let's see what it does when we play it some music.
You might recognise the tune.
(''R0CK AR0UND THE CL0CK'' PLAYS) The joint's jumping, literally.
The excited response of these hair cells amplifies the faint vibrations that arrive from outside.
And they do it so well that we can actually hear the sound of a pin drop.
Unfortunately, from the moment we are born, one by one hair cells start to die.
And those that register high frequenices die off first.
The damage here is where hair cells have failed.
Because of the loss of these hair cells, by the age of ten we've heard a greater range of sound than we'll ever hear in the rest of our lives.
As we get older, some people continue to hear relatively well, but in others, so many of the amplifying hair cells have gone that they can only hear loud noises.
Most cannot hear high frequencies any more.
So the sound of the New York subway which younger people might hear as something like this (L0UD METALLIC WH00SHING) .
.
would instead be heard like this.
(DULLER, MUFFLED WH00SHING) I'd like to get off this.
Oh, no, I could ride this all day.
0bviously, if older people's hearing is fading a little and their eyes are not as good as they were, their brains just have to cope as best they can.
And some think this is why older people can sometimes appear easily confused and bewildered.
It's not so much that their intelligence is waning, although some brain cells will have been lost.
The brain has to work harder to make sense of the limited information it gets from the outside world.
This leaves fewer brain cells for memory and decision-making, and might account for the confusion, especially in strange and noisy places.
I never thought I'd see New York.
So many people, cars, and so much noise and everything, that you just can't quite sort it all out.
Well, I can't.
My neck's almost sore from looking up all the time, they're so tall in the big city.
But I really enjoyed seeing it.
Couldn't have dreamed of coming to some place like this when I was a child.
It was a faraway, distant place.
It's wonderful to be able to do this in our lifetime.
Later on the trip, Bud and Viola meet up with some of their children and grandchildren.
By the age of 70, we'll have lost about a third of our muscle strength.
But we needn't have.
Regular exercise will help us retain it.
So, as a physically active cattle rancher, it's perhaps not surprising that Bud does as well as his grandchildren.
But there's something else that can happen, and we can see it with the help of a special camera.
A thermal camera can detect differences in the heat emerging from various parts of the body.
And it's used by doctors to detect something that's different, something that's wrong.
Margaret was out walking one day, when she felt a sudden sharp pain in her leg.
To the naked eye, there's not much difference between her two knees, but, to the thermal camera, there definitely is.
0ne knee has a cool blue and green colour.
But the other knee has a different colour.
It has a hotspot in red.
Let's take a more detailed look at that knee joint.
Where the thigh bone and the shin bone meet, they're protected by a membrane of cartilage.
Take away the bones for a moment, and we're left with that cup-shaped membrane.
If we take a look inside, yes, with wear and tear, a small hole has developed in the membrane, and the two bones have come in contact.
As they touch each other and grind together, the surfaces are damaged, and without treatment the joint can become extremely painful.
It's called osteo-arthritis, and affects many elderly people.
Although physically very fit, Bud has osteo-arthritis in both knees.
- No more chocolate.
- No more chocolate for you.
Although they're joking, Viola's distorted image in the mirror is very similar to what happens to women when they put on weight.
Fat tends to settle around the hips and bottom, and there's a difference between the sexes.
When a rather plump man is scanned in a special X-ray machine, it's possible to see the outline of the layers of fat.
Men tend to put it on around the waist, so they become apple-shaped.
Some believe this happens because of a drop in the sex hormone testosterone.
It's thought it can reduce as a result of age and stress.
Fat around the waist tends to get into the bloodstream easily, increasing the risk of diabetes and heart disease in men.
The inside walls of young arteries are smooth and clean.
But after years of a diet high in saturated fat, old arteries can start to look clogged.
This can trigger blood clots and block the vital circulation to the heart.
Female sex hormones tend to make fat accumulate around the bottom, as it did at puberty when a woman first developed her adult shape.
This is a safer place, in that the fat here is inclined to stay put and not circulate in the blood.
Women become pear-shaped in comparison to men.
Bud and Viola journey on to the family reunion and Alaska.
All in all, ageing means lots of changes as we go through our lives, but perhaps the more interesting question is not so much how we change, but rather why do we change at all? Ageing is one of the greatest paradoxes of human biology.
It turns out that our bodies are constantly renewing themselves.
0ur skin is continuously being replaced.
Most of the dust we find at home is dead skin that's fallen off us as new cells are substituted.
Just rubbing your arm produces a minor dust storm of dead skin cells.
Likewise, the lining of the gut is continuously replaced, so that there is in effect a total change every three days.
All this is done by making copies of cells.
The new cells then take the place of the old cells.
It's going on all the time.
The blood is replaced three times a year, parts of the skeleton every four years.
So extraordinary is this process of copying and renewal that very little of our bodies is actually more than ten years old.
But if our bodies are perpetually being renewed in this way, why do we gradually start to look old? Why don't we all look the way we did when we were young and in our prime? You can understand why we don't look the way we did as a child, after all we were still growing.
But having reached maturity, why don't we continue to look that way for the rest of our lives? Well, one possible explanation as to why I no longer look as I did when I was a medical student may be due to faults in the renewal process itself.
Renewal involves making copies: copies of skin cells, blood cells, bone cells, all sorts of cells.
And copying can have its problems.
Suppose we recorded this programme onto this cassette and then copied from this one onto this, and did so backwards and forwards maybe half a dozen times.
What would be the effect of all that copying? Well, we've done it, so let me show you.
You'd get an image which looks something like this.
The copying process is not perfect.
Mistakes are made.
And although I'm still recognisable, the picture's starting to look the worse for wear.
In terms of the human body, we call these mistakes ageing.
Moreover, the older the person, i.
e.
the copying machine, the more frequent are the mistakes.
And of course, if we carry on copying for too long, we'll eventually reach the point where we disappear altogether.
That's one theory why we age.
But another has it that we grow old because of something that is actually vital for life - oxygen.
We tend to think of oxygen as healthy and essential for life.
But this is the other side of it.
This blazing inferno, like all fires, requires oxygen for its power.
And despite this protective suit, I'm well aware of the power.
It's getting hotter every minute.
Just like all fires, we too require oxygen.
0ur bodies need it every second of our lives.
Yet it turns out that over years, oxygen can be as damaging and dangerous as these flames.
Steel can resist fire, but not the effects of oxygen, because that leads to rust.
Rust is caused by a highly reactive type of oxygen called free radicals.
0ver time, free radicals have the power to corrode and destroy even the largest steel structures.
Some of the oxygen we breathe in turns into the dangerous free radical form.
This type of oxygen has a habit of ripping bits out of molecules, damaging them in the process.
As these loose cannon roam around our bodies, they can play havoc as they start to damage our cells and tissues.
Many now believe years of accumulated damage by free radicals are an important cause of ageing.
And what's more, it's not just breathing oxygen that puts us at risk.
We can get free radicals into our bodies in all sorts of ways.
Too much sun increases them, and we can pick them up from tobacco smoke and air pollution, and eating things like barbecued meat.
But some foods are rich in anti-oxidents, and these can destroy free radicals.
These foods include fresh fruit and vegetables, and, I'm glad to say, a glass of red wine.
Free radical damage is one theory.
There are many more.
With the onslaught of free radicals and the like, the question inevitably arises, how do we manage to live for so long? The answer might be out in space.
0ne possible reason is because we're built rather like the thing that all this is listening to.
Right now, this satelitte dish is peering at the far edge of our own solar system.
It's listening to the faint signals of the most distant object ever made by humans, about six and a half billion miles away.
It's the Voyager space probe, and it started its journey twenty years ago on a voyage of discovery.
Two of them were launched, and their original mission was to relay information about the planets Jupiter and Saturn.
By 1981, they'd achieved that original goal.
But they were built so well that they went on to do much more.
Amazingly, they're still sending back information as they head out into interstellar space.
But the point is, to be doubly sure they achieved their original goal, they were over-engineered, built so robustly that they're capable of surviving into very old age.
Just like us, in fact, though our original goal was to make sure we have children.
But can over-engineering really explain why we live so very long? After all, we have by far the longest lifespan of any other mammal, and we live way beyond an age when we can first have children.
Well, it could, but first we need to think about what we've been over-engineered to do.
Perhaps it's not just to have children and become parents, but to become grandparents.
Arriving at the family reunion for their forty-fifth anniversary, Bud and Viola encounter something that would be very puzzling in other animals - grandchildren.
Arguably, in evolutionary terms, all we have to do is live long enough to have children, and rear them successfully so that they carry on our genes.
That's what other animals do.
Yet Bud and Viola's children have had children of their own, and Bud and Viola are still around.
So if we're capable of living long enough to become grandparents, how has this affected our bodies and the way we age? Well, one thing, and it seems rather strange, happens to women as they get older.
It's the menopause, and it's virtually unique in the animal world.
For women, it marks the end of their child-bearing years, and it's a bit of a mystery.
Men can often produce fertile sperm throughout their lifetime.
At the age of a hundred, a man is, in theory at least, capable of siring a child.
But that's definitely not the case for a woman.
At birth, she has about two million eggs in her two ovaries.
From puberty, they start to be released.
At the rate of one per month, there should be enough to last a lifetime and longer, but there aren't.
Most of the eggs die, until, around the age of fifty, the body stops releasing eggs altogether.
In effect, the store has been used up.
Some scientists believe that the menopause has evolved as we've come to live so long.
The menopause helps us make the most of our long lives by becoming effective grandparents.
The reason is, to continue to give birth to children in an ageing body is dangerous for both mother and child.
Better for her to stop having her own children, and instead to concentrate on taking care of her grandchildren.
After all, they are carrying her genes.
So the menopause helps us make the most of old age.
I said humans are almost unique in this, but not quite, because interestingly there's one other animal where the female has a menopause, and that's the pilot whale.
Many elderly female whales spend many years caring for the offspring of their offspring - being grannies, in fact.
It's controversial, but some feel this supports the idea that there's a similar purpose for the human menopause.
At the end of their trip, Bud and Viola have reached Alaska.
The natural cycle of life is birth and then eventually death.
I guess I sit here around this beautiful surrounding here in Alaska I can see before my eyes the creation going on.
The small trees that are coming on, the tall trees that are a symbol of middle life, and then I can see the dead trees, - all within a few feet of me.
- That's the process we go through.
There's no other way of getting around it.
They've never found a youth pill that'll keep you going.
(VI0LA) I don't worry about dying, I think it's just the next step in life, and life goes on after that, so it's out of my hands, and when that happens, whichever one of us is left, the other one will find the strength to cope.
(BUD) When my time comes, there's nothing I can do about it but go.
And I want to stick around as long as I can.
We tend to think that the human capacity for art, science and technology is what marks us out.
But though we don't often see it this way, perhaps our ability to live to a ripe old age is the human body's greatest achievement.
- Happy anniversary! - Happy anniversary! (T0GETHER) Onetwothree