The mechanism for this extraordinary process is transdifferentiation the ability of stem cells to specialise into particular forms and then turn back again into stem cells. Turritopsis dohrnii seem able to grow old and regenerate many times, which is why they are more commonly known as immortal jellyfish.

If human beings could somehow tap into that mechanism of transdifferentiation, could we regenerate our whole bodies, too? In Doctor Who, that's what happens in The Lazarus Experiment (2007), in which 76-year-old Professor Richard Lazarus is 'reborn' as a man of about 40.

'Hypersonic sound waves to destabilise the cell structure, then a metagenic program to manipulate the coding in the protein strands. Basically, he hacked into his own genes and instructed them to rejuvenate.'

The Tenth Doctor, The Lazarus Experiment (2007)

Of course, as the story makes clear, such technological innovation comes with risks. If we can make our cells change, how do we make sure they change only by the right amount or into the right things? As we saw in Chapter 14, our bodies comprise a whole series of complicated systems and processes and if we get those out of sync, the result can be deadly.

In Doctor Who, the result can be even worse than that. When Lazarus manipulates his own cells, something in his DNA is activated that causes continual change, transforming him into a huge monster. In another story, Mawdryn Undead (1983), the Fifth Doctor meets aliens who have stolen a Time Lord machine designed to help when their regenerations go wrong. With it, the aliens hope to be able to regenerate themselves but instead the result is to lock them into a continual cycle of degeneration and renewal. In eternal agony but unable to die, the aliens ask the Doctor to help them end their self-inflicted torment.

In real life, research is ongoing into the ways that stem cells might help us regenerate our bodies safely. But though we cannot yet regenerate our entire bodies, we partially regenerate all the time. Scars, bruises and sunburn are all signs of your skin regenerating, working to heal different kinds of damage. It's thought we replace all of our skin every two weeks or so.

Other parts of our bodies can regenerate, too. The endometrium in the uterus of human women entirely regenerates as part of a monthly cycle. (It also regenerates cyclically in the females of great apes, though at different rates to humans.) We seem able to regrow the tips of our fingers and toes if they are damaged, and human livers have been known to regrow themselves from just a quarter of the original tissue.

Individual cells in our bodies are continually dying and being replaced and it's sometimes claimed that we regenerate all our cells over a period of seven to ten years. If that were true then are we the same person now that we were eleven years ago?

'You take a broom, you replace the handle, and then later you replace the brush. And you do that over and over again. Is it still the same broom? Answer? No, of course it isn't.'

The Twelfth Doctor, Deep Breath (2014)

Versions of this question have been debated for a long time. The Greek historian Plutarch included it in his book, Lives of the n.o.ble Greeks and Romans, in the first century AD. In Plutarch's version, the hero Theseus owns a ship, and when its timbers decay they are replaced. After several hundred years of these repairs none of the original timbers remain. Plutarch tells us that the philosophers of his time were divided about whether that meant it was the same ship.

Whatever the case for Theseus's ship, it seems that we humans don't in fact regenerate all our cells. Cardiomyocyte cells in the heart are replaced very slowly, and the rate decreases as we age. Even those of us who live to be very old are unlikely to have replaced more than half our cardiomyocyte cells by the time we die. Scientists have also attempted to study whether neurons in the human brain regenerate or stay the same throughout our lives.

Surprisingly, the way they do this is to use nuclear weapons. The atomic bombs dropped on j.a.pan at the end of the Second World War in 1945 and subsequent testing of ever more powerful nuclear weapons increased levels of carbon-14 a radioactive isotope in the atmosphere. The levels have decreased ever since, but scientists know by exactly how much.

Carbon-14 in the atmosphere finds its way into our food and, once eaten, into our cells as they are created. The amount of carbon-14 in a human cell is therefore determined by the amount in the atmosphere when the cell was formed. That means we can measure the amount of carbon-14 in a cell, compare it to the known rates in the atmosphere over the years since 1945, and deduce the year in which the cell was created.

This technique was used by Jonas Frisen, a professor of stem cell research at the Karolinska Inst.i.tute in Sweden, to test cells in various parts of the human brain. In 2013, Frisen and his team published evidence that perhaps 700 neurons are created in the hippocampus of the adult brain every day. It seems that not all neurons are replaced, and that neurons created in later life do not survive as long as those we are born with. In short, we are still not quite sure how and to what extent our brains regenerate but scientists are working to find out.

Even so, there's a big difference between the kind of regeneration that repairs our bodies and what happens to the Doctor in Doctor Who when he becomes an entirely new person.

'If I'm killed before regeneration then I'm dead. Even then, even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away and I'm dead.'

The Tenth Doctor, The End of Time (20092010).

At the end of The Time of the Doctor (2013), the Eleventh Doctor looks old and wrinkled, his memory is failing and he has to use a walking stick. He has used the last of his allotted twelve regenerations, and he is going to die. However, the Time Lords then give him a whole new regenerative cycle and something odd happens.

When Clara finds him in the TARDIS, the Eleventh Doctor looks young again. The regenerative process has healed him but it can't stop there. He just has time to say goodbye before he becomes someone else.

That also happens in The End of Time: before the Tenth Doctor becomes the Eleventh Doctor, regenerative energy heals his cuts and bruises and he has time to journey in the TARDIS and look in on his old friends. In Journey's End, the Doctor used regenerative energy to heal himself from the effects of being shot by a Dalek and was then able to stop the process before he became someone else as we've already discussed, by siphoning off the rest of the regeneration energy into his spare hand.

The suggestion is that regeneration involves a great deal of energy which is difficult to control. That energy can also be used to destroy s.p.a.ceships in The Time of the Doctor, and wrecks the control room of the TARDIS in The End of Time. It seems that the Doctor often has trouble controlling the outcome of his own regenerations, and despite claiming several times that he's always wanted ginger hair, he has yet to manage it. In fact he always seems rather surprised by his new appearance and when different incarnations of the Doctor have met each other for example in The Three Doctors (19721973) or The Day of the Doctor (2013) they rarely approve of each other's personality quirks or fashion sense.

Perhaps because they're triggered by violent or traumatic situations, the Doctor's regenerations often leave him in a weakened state and can lead to erratic and confused behaviour. In The Christmas Invasion, the Tenth Doctor is unconscious for hours while the alien Sycorax threaten London, while in The Twin Dilemma (1984), the newly regenerated Sixth Doctor even tries to strangle his companion Peri.

It seems that the Time Lords are aware that regeneration might not always go smoothly. In Castrovalva (1982), the Fifth Doctor seeks refuge in a part of the TARDIS called the Zero Room which seems designed to provide a calming environment in which Time Lords can recover from the stress of regeneration. As we discussed in Chapter 14, the Time Lords also have an arrangement with the Sisterhood of Karn, who provide them with a restorative elixir to help when regenerations go wrong as we see in The Brain of Morbius (1976).

Others seem able to control the wild energy of regeneration. In The Christmas Invasion, alien 'pilot fish' are attracted to the newly regenerated Doctor because he is bursting with energy he says they could use him as a power source. In Let's Kill Hitler (2011), River Song is able to heal the Doctor by using up her all remaining regenerations in one go. The Doctor returns the favour in The Angels Take Manhattan (2012), using a small amount of regenerative energy to heal River's broken arm.

For other Time Lords, regeneration seems not to be a random process at all. In The War Games (1969), they offer the Doctor a choice of faces for his next incarnation. In Destiny of the Daleks (1979), the Time Lady Romana seems to regenerate effortlessly, appearing in a selection of different bodies before deciding on one that she likes (which looks just like a woman she met in the previous story). Perhaps the Doctor would find regeneration easier if he didn't do it in the middle of a crisis.

Even so, the Doctor, seems to exert a certain amount of control over his new form as he sometimes adopts the faces of people he's met in the past. Actors Colin Baker and Peter Capaldi both played other characters in Doctor Who before being cast as the Doctor, which means that the Sixth Doctor looks like a Time Lord called Maxil from Arc of Infinity (1983) and the Twelfth Doctor looks like a human, Caecilius, from The Fires of Pompeii (2008).

'Have you seen this face before? ... It's funny, because I'm sure that I have. You know, I never know where the faces come from. They just pop up. Zap. Faces like this one ... Why did I choose this face? It's like I'm trying to tell myself something.'

The Twelfth Doctor, Deep Breath.

Why would the Doctor's unconscious make him look like Caecilius? We've not yet been given a definite answer to that question, but one possibility is what Caecilius represents.

In The Fires of Pompeii, the Tenth Doctor and his companion Donna visit the Roman town in AD 79, and meet Caecilius and his family. However, when the nearby volcano erupts, the Doctor knows that the town and its people will be smothered by hot ash and pumice. He ensures that established history is not changed, then he and Donna leave in the TARDIS.

Donna, horrified at the fate of all those people they're leaving behind, begs him to go back. The Doctor insists that he can't just as he can't change history to save his own people, his own family, from the Time War. Donna continues to plead with him.

'Just someone. Please. Not the whole town. Just save someone.'

Donna n.o.ble, The Fires of Pompeii.

He relents. They save Caecilius and his family while the rest of Pompeii is lost. So is the Twelfth Doctor's appearance his unconscious way of ensuring that he remembers that act of mercy and Donna's insistence that he save those he can?

Whatever the case, the suggestion is that when the Doctor regenerates, it involves something more than his cells starting over again in a random manner. Somehow, his experience is part of the process, too, shaping the new man he'll become. The things he has learnt and suffered, the people he's loved and lost, are able to physically change him.

The Twelfth Doctor in the latest episodes is thousands of years older than the First Doctor in An Unearthly Child (1963), and yet he seems younger, more energised, more daft, more exciting, more alive. That's not solely down to his ability to regenerate. What keeps the Doctor young is his att.i.tude: his eagerness to engage and question, to be astounded by what he learns, to change.

We might even argue that the secret to the Doctor's longevity is that he's a scientist.

'I held back death...'

Doctor Who presents many remarkable ways to hold back the ageing process. Here are some examples.

Regenerate.

The Time Lords of Gallifrey can regenerate twelve times though in special circ.u.mstances they can also be granted a whole new regenerative cycle. The Master is offered a new regenerative cycle in The Five Doctors (1983); the Doctor is given one in The Time of the Doctor (2013).

Manipulate your cells.

Perhaps related to regeneration (because the research seems to have been guided by the Master), Professor Richard Lazarus was able to manipulate his cells to lose some thirty years in The Lazarus Experiment (2007) but there were nasty side effects.

Surgery In The End of the World (2005) and New Earth (2006), the Lady Ca.s.sandra seems to have been sustained by continual, ever more drastic surgical interventions.

Stop time In The Pirate Planet (1978), the evil Queen Xanxia is suspended in the last seconds of life using time dams but the process requires so much energy that her people ransack entire planets to supply it.

Chemical supplements In The Caves of Androzani (1984), the bats of the planet Androzani Minor leave deposits of a substance called Spectrox which, when properly refined, can 'hold back the ravages of time' and keep people young.

Steal other people's youth In The Talons of Weng-Chiang (1977), war criminal Magnus Greel uses a futuristic process called 'organic distillation' to drain the 'life essence' from young women and transfer it to himself.

Steal other people's bodies The Master, having used up his allotted twelve regenerations, takes over other people's bodies in The Keeper of Traken (1981) and the television movie Doctor Who (1996).

Upgrade to a computer When River Song and her friends die in Silence in the Library / Forest of the Dead (2008), the Doctor uploads her into the computer 'data core', where she can 'live' in a virtual world for ever after.

Upgrade to a Cyberman.

In Dark Water / Death in Heaven (2014), the minds of the dead are uploaded to a computer and then downloaded into Cybermen. Led by a Cyber-converted Danny Pink, these Cybermen die to save the Earth but one, apparently a Cyber-converted Brigadier Alistair Lethbridge-Stewart survives.

Become immortal.