'The Time Lords are an immensely civilised race. We can control our own environment, we can live for ever, barring accidents.'

The Second Doctor, The War Games (1969)

There are several ways that medical scientists can think about ageing. One is in terms of the general wear and tear that a body undergoes during a lifetime, which is determined to some extent by lifestyle and chance. For a time, your body can withstand and even repair much of the daily damage that it incurs but eventually the effects acc.u.mulate until something tips it over the edge the straw that breaks the camel's back But if we could entirely limit the damage done to a body, how long might a person live for? The longest human lifespan on record (with good evidence to confirm the claim) is that of a French woman, Jeanne Calment, who lived for 122 years and 164 days between 1875 and 1997. In her long life, she had watched the Eiffel Tower being built, and once even sold coloured pencils to the artist Vincent van Gogh 100 years later, she remembered him as 'dirty, badly dressed and disagreeable'.

A scientist who tried to investigate the secret of Calment's longevity could find nothing exceptional about the way she lived, but it was discovered that several members of her family had also been long-lived: her brother died at 97, her father at 94 and her mother at 86, while ancestors in the seventeenth and eighteenth centuries had lived into their 70s well above the average of the time. While Calment's environment she lived all her long life in Arles in southern France might have helped, it seemed her longevity was inherited.

Another way to think about ageing is that there are biological limitations a bit like the biological clocks we discussed in Chapters 7 and 8. Senescence, from the Latin word to mean 'growing old', is the scientific name for ageing either of cells or a whole organism. In cellular senescence, there seems to be a limit on how many times cells can divide and reproduce. Research suggests that this is the result of telomeres, which 'cap' the end of our DNA, a little like the plastic tips at the end of a shoelace. When we're young, a telomere contains thousands of repet.i.tions of a six-letter DNA sequence. When a cell divides, some of the telomere is lost and after several divisions, too much has been lost for the cell to divide again. It can still play a useful part in the body senescent cells seem to play a role in battling a number of diseases but recent experiments suggest that removing senescent cells from the body can increase resistance to age-related diseases, suggesting that cellular senescence is directly linked to ageing. However, the senescence of a whole organism is much more difficult to understand because it is so complicated, involving many separate processes all happening at once.

People often equate the process of ageing with age-related diseases, such as heart disease, cancer, arthritis or dementia. There are ways to combat these diseases. As we saw in Chapter 8, it's been shown that an active mind can reduce the risks of dementia. But what we understand about telomeres suggests that lifespan is hardwired into our DNA.

It has also been suggested that all living creatures have a limited average number of heartbeats or breaths so that lifespan is defined by the speed of our biological clocks, called our 'metabolic rate'. If this is true, the slower our clocks run, the more time it takes us to reach our limit of heartbeats, so the longer we live. There is evidence to support this theory in general: we can compare the heart rates of different animals that have different lifespans. However, different species seem to have different limits. In fact, lifespan is probably a mixture of many different factors.

In Doctor Who, it seems that the Time Lords have worked out how to control all these different factors. Of course we know that Time Lords are able to regenerate, restoring their ageing or damaged bodies to a new, healthy state perhaps by resetting their biological clocks (we'll discuss the process in Chapter 15). But even between regenerations Time Lords seem to age much more slowly than humans a single incarnation of a Time Lord can live for hundreds of years. In The Time of the Doctor (2013), the Eleventh Doctor lives in a town called Christmas for a thousand years. He ages, but at a much slower rate than a human.

That said, this rate of ageing in Time Lords seems related to regeneration. In The Sound of Drums (2007), the Master can use what he calls the Doctor's 'biological code' to age him by a hundred years a process that would kill most humans. In the next episode, Last of the Time Lords (2007), the Master ages the Doctor even more, turning him into a small, wizened creature. But the Master claims he's done this by suspending the Doctor's 'capacity to regenerate', which somehow lets us see all 900 years of the Doctor's life at once.

Whatever the mechanisms by which humans and Time Lords age, the different rates of ageing causes problems for the Doctor when he travels with human companions.

'I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone who you-'

'What, Doctor?'

'You can spend the rest of your life with me, but I can't spend the rest of mine with you. I have to live on. Alone. That's the curse of the Time Lords.'

The Tenth Doctor and Rose Tyler, School Reunion (2006)

If we didn't outlive our loved ones if they all had long lives, too there would still be problems. In fact, some scientists think death might play an important role in evolution.

As we saw in Chapter 11, food and resources in any environment are often limited. The theory is that once a living organism has raised its offspring to adulthood, it will then compete with them for those limited resources. It would seem to be an advantage to the species if not the individual for the parent to die once the offspring is fully grown, as that means the offspring doesn't have to compete for the limited food, and so is more likely to produce offspring of its own. Again, other factors may also play a part: in some species such as humans, different generations tend to cooperate rather than compete. But in a crisis, human adults will often put children's needs ahead of their own. We're used to the idea of putting 'women and children first' and perhaps that's not just part of our culture, but also hardwired into our cells. Dying so others can live isn't just about morality it's a good tactic for the long-term survival of the species or the genes we share with our family or kin groups.

In The Brain of Morbius (1976), the Fourth Doctor meets the Sisterhood of Karn, whose elixir can prolong life for centuries. As the Doctor explains, the fact that the sisterhood can all live very long lives means that nothing ever changes in the sisterhood, which leaves them unprepared for a contest with the evil Morbius. As the Doctor tells the sisterhood's leader, Maren:

'Death is the price we pay for progress.'

The Fourth Doctor, The Brain of Morbius

Maren later concedes the point and dies to save the Doctor's life. Change is how we survive. Of course, on several different occasions including when he visits Karn again in The Night of the Doctor the Doctor survives death by changing his whole persona...

'Looks like we're joining a party!'

Nyssa was first out of the TARDIS. They had landed in a side street of a sprawling alien city, and as she stepped into the main thoroughfare she was struck by the colour, bustle and noise of a metropolis in celebratory mood.

She was reminded of the Festival of the Harvest Moon back on Traken, but on a much larger scale. Everywhere there were flags, balloons, banners and ribbons. Many of the hundreds of people who thronged the street were dressed in gaudy, brightly coloured costumes with patterned face masks. Stalls and makeshift entertainments lined the middle of the street.

The Doctor, Adric and Tegan joined her and took in the scene.

'Is it some kind of carnival?' Tegan asked.

The Doctor said, 'A celebration of some kind, certainly.'

At the sound of approaching music, Nyssa looked round and saw a group of young men and women moving in their direction through the crowds. Four of them played strange musical instruments, spirals of metal whose surfaces responded melodically to the players' touch. The others were skipping and hopping to the music and handing out slips of printed, glossy paper.

A young man at the front of the small parade thrust one of the pieces of paper into Nyssa's hand. 'Happy Freedom Day, friend!' he cried.

'Freedom Day?' said Nyssa.

The young man looked curious. 'Don't you know?' Then he looked at her companions, nodded his understanding and spread his arms wide. 'Of course. You are visitors. Welcome to Lemaria!'

'Thank you,' said the Doctor.

Adric asked, 'What's Freedom Day?'

'Why, friend, it is the anniversary of Lemaria's liberation! Ten years since we were saved from the cruel alien race who invaded Lemaria and enslaved us!'

'What happened, exactly?' asked Tegan.

The young man saw that his friends had moved on some way into the crowds and he ran after them. He called back over his shoulder, 'Read the flyer! Come and see! You'll love it!'

Adric asked, 'What's a flyer?'

The Doctor pointed to the piece of paper in Nyssa's hand. 'That. It's an advertis.e.m.e.nt.'

Nyssa looked at the paper.

The

CAPITAL HISTORICAL SOCIETY.

presents

THE LIBERATION OF LEMARIA!.

A re-enactment

Performed every half hour ******************

Venue: The Public Amphitheatre, Capital Square Admission: FREE The Doctor said, 'Can I see?' and took the paper from Nyssa. He scanned it and said to Tegan and Adric, 'They're putting on a theatrical performance. A depiction of the events of ten years ago.'

Adric and Tegan stepped close to the Doctor to read the flyer. As they did, Nyssa looked around. And up.

Oh.

She took a few steps into the street to get a better look at the side of the building next to them.

The Doctor said, 'It must be an extremely concise history, if they're performing it every half hour.'

'Shall we have a look?' asked Adric.

The Doctor said, 'Amateur dramatics aren't really my thing.'

Nyssa called, 'Doctor?' and pointed up.

The Doctor, Tegan and Adric joined her and looked up, following her finger.

Tegan said, 'Well now.'

Adric said, 'Is that...?'

The Doctor said, 'Ah.'

On the wall was an enormous mural. From its depiction of hundreds of fallen, lizard-like bodies and exploding s.p.a.cecraft, and the accompanying portrayal of cheering Lemarians, this was clearly an ill.u.s.tration of the planet's freedom from the 'cruel alien race' that had enslaved them.

Which was fine, thought Nyssa. Except, at the very centre of the painting, taking pride of place, was the unmistakable likeness of a familiar blue box.

Adric answered his own question. 'It is. It's the TARDIS!'

Nyssa said, 'If it's the TARDIS, who's that?'

She pointed to the figure of a man standing in the open TARDIS doors, looking out with a wave and acknowledging the Lemarians' cheers. He was old. Really old. With thick shoulder-length white hair. He wore a dark frock coat over a dress shirt with a black ribbon tie, and checked trousers. She supposed there was only person it could be.

'He's you,' said Nyssa. 'In the future, when you're old.'

The Doctor sighed, and said, 'Well, yes. But also no.'

They found the venue for the performance in a communal square at the far end of the main street. Tiered stone seating rose in a semicircle in front of a simple wooden stage.