'It is the right of every creature across the universe to survive, multiply and perpetuate its species. How else does the predator exist? We are all predators, Doctor. We kill, we devour, to live. Survival is all, you agree?'

'Oh yes, I do, I do. And on your argument I have a perfect right to dispose of you.'

'Of course. The law is survival of the fittest.'

The Nucleus and the Fourth Doctor, The Invisible Enemy (1977)

In 1838, the English naturalist Charles Darwin happened to read An Essay on the Principle of Population (1798), a book by the economist Robert Malthus, who warned that, without something to stop it, the number of people alive doubled every twenty-five years, and that meant there kept being catastrophes when there wasn't enough food for everyone. Darwin realised that if the same principle applied to animals and there were often times when there wasn't enough food, there would be a constant compet.i.tion going on between the animals. Having even a small advantage could make a difference between life and death.

Now, imagine a clone race such as the Sontarans facing that kind of shortage. The Sontarans are all identical, so they all stand an equal chance of getting their three-fingered hands on the small amount of food. Being Sontarans, there might be a big fight between them with plenty of explosions, but it would basically be a fair contest.

That's not what happens with animals and plants that aren't clones. Imagine a group of human beings in the same situation, fighting over food. The humans are a varied group: some old and some young; some male and some female; some tall and some short; some healthy and some not. We can see how some of these humans would have an advantage over the others: we'd expect the young and healthy ones to grab the food.

But another important factor is where the contest takes place. If the food is somewhere up high, tall people will have an advantage, while if it's somewhere down low, they won't. The phrase 'survival of the fittest' is often used to mean that the strong survive, but really it's about who happens to be best suited to the circ.u.mstances in that particular time and place.

Darwin realised what it would mean if this sort of contest happened reasonably often. While the losers of the contest would have a higher chance of dying before they had children, the winners would be more likely to live on. They would have children, who would inherit a mix of the characteristics from their parents. If the food is high up, being tall provides a survival advantage so the taller people are more likely to survive and pa.s.s on that characteristic of tallness to their children.

But these shortages are periodic. In the next contest, if the food was still high up then the tallest of those children would have the advantage and they would be the ones to survive and have children of their own. With each generation, if height provides a key advantage in surviving and reproducing then there would be a selection bias for being tall.

In fact, the same forces are driving all living organisms: affecting not only the people doing the eating but the living things they're eating, too. That has a dramatic impact on the way subsequent generations develop. We'll use an imaginary thought experiment to explain the idea.

Imagine a whole lot of apple trees growing in the wild. The apples contain seeds to grow the next generation of apple trees. But people also like to eat the apples, and if all a tree's apples are eaten it won't produce new trees. (Actually, that's not what happens in reality which we'll come back to in a moment.) A tree that produces apples higher up in its branches would have an advantage over apple trees around it as people would pick more of the apples they could reach that is, the ones from trees where the apples weren't so high. The fruits of the trees with apples higher up in their branches would be more likely to survive and they'd pa.s.s on to the next generation of apple trees the characteristic of producing apples high up in their branches.

If there weren't enough apples for all the people, we'd see two things happen over time. First, trees with apples low in their branches would be less likely to produce offspring, so we would expect there to be more trees that grew apples higher up in their branches. Secondly, people who couldn't reach the branches of the apple trees would be less likely to produce offspring. Over time, the average height of people taking part would be taller, and only the tallest of them would have the advantage, and be the ones more likely to survive and have children. Likewise, of the surviving apple trees with apples high up in their branches, only the ones with the apples most high up would produce more apple trees. And so on: with each generation, it would be the tallest people and the trees with apples in their highest branches that survived. Over many generations, there would be a gradual tendency towards taller humans and taller trees. It would be like an 'arms race', both sides unwittingly pushing the other to get taller.

Of course, this is just a simplified example, and there are many other factors to consider which is why people and apple trees haven't got infinitely taller over the years. In real life, an apple tree doesn't die if we eat its apples. In fact, apple trees might even have an advantage if their apples get eaten. While we can digest the soft fruit of the apple, the small, hard seeds pa.s.s through our bodies and emerge in our faeces. It takes about four hours for food to pa.s.s through our bodies, so it would be likely that by the time the seeds emerged, the person would have moved some distance from the apple tree. That means an apple tree whose apples are eaten can disperse seeds over a wide area and the faeces provides those seeds with fertiliser, too, giving them another head start in life. Over many generations, there would be a general tendency for apple trees to produce tastier apples, because they're the ones more likely to get eaten and so have that advantage.

We can see examples of how particular advantages have, over millions of years, shaped all the species on our planet in all manner of different ways. Giraffes have especially long necks to reach the food in the branches of especially tall trees. The cheetah is the fastest animal on land, able to run at up to 120 kilometres (75 miles) per hour in short bursts and that speed has developed because it eats antelope and hares, which can run quickly, too.

Darwin didn't discover evolution which is the name we give to descent with modification over successive generations. It was already well known that people could selectively breed plants and animals for particular, favoured traits. For example, a farmer with lots of cows might allow only those that produced the most milk to have children. Over many generations, this would produce cows with much higher yields of milk. It's thought that the domestication of many animals and plants by early humans was done in this manner though it's not known if it was done intentionally.

Instead, Darwin's great revelation was to understand that selection happened naturally, too. Contests for survival meant that over many generations and millions of years, life develops to better match its environment. Apple trees, cheetahs and people don't choose which variations give them an advantage, it's just that those with an advantage whatever that advantage might be in a given situation are more likely to survive and have children. Darwin called this mechanism of evolution 'natural selection'.

He didn't realise it, but this mechanism was later able to explain why cloned potatoes are so vulnerable to blight. We saw that a single potato could produce 1,225 cloned potatoes in just two generations but that those potatoes would all be 'genetically identical' (a term we'll explain in a moment). Under favourable conditions, this gives our clone potatoes a huge advantage as they don't need to reproduce with another potato to produce offspring. But if there's a parasite, organism, predator or unfavourable change in the environment that is effective at killing potatoes, the whole population is equally susceptible. A blight organism which has evolved to be good at attacking the original plant will be just as good at attacking all the cloned plants, too. The entire crop would have no defence and so would rapidly be consumed by the blight. In fact, this can happen with crops that aren't cloned but have little variation.

But now imagine 1,225 potatoes that are all varied in some way. Some potatoes might have particular traits or genes a term we'll discuss shortly that are better at resisting blight, and that would make them more likely to survive and pa.s.s on that resistance to the next generation of potatoes. Variation makes them harder to kill off.

That's all very well for potatoes, but we now know that this survival-through-variation is what happens with fast-evolving human diseases, too. It's why you can catch influenza the flu virus every year, even if you were vaccinated the year before. Influenza evolves very rapidly and a flu jab that stops one kind of influenza won't necessarily stop the new kind that comes round the next year. Doctors are concerned that many other serious diseases such as tuberculosis are also evolving in such a way that the medicines we currently use to treat them will one day no longer work against them.

'I thought this far in the future they'd have cured everything.'

'The human race moves on but so do the viruses. It's an ongoing war.'

Rose Tyler and the Tenth Doctor, New Earth (2006)

Evolution doesn't just show us how plants, animals and diseases are in compet.i.tion with one another. It also reveals how we are related.

In Listen (2014), Clara and the Doctor meet Colonel Orson Pink, a man from about a hundred years in Clara's future. Clara is struck by how much Orson looks like a man she knows in her own time Danny Pink. Then she discovers that Orson owns a plastic toy soldier, which he says is a 'family heirloom'. The toy is broken the soldier isn't carrying a gun which makes it distinctive. Clara recognises it as the same toy she once gave to Danny.

So, Danny and Orson share three things: the same looks, the same surname and the same toy soldier. The implication is that all three things have been inherited, meaning Orson is related to Danny.

Inheritance explains why people in families look similar. All living organisms (and many viruses) have a sort of instruction manual embedded into each of their cells, a coiling molecule called deoxyribonucleic acid, or DNA. These instructions aren't written in English. Instead, DNA is made up of a long sequence of four particular molecules guanine, adenine, thymine and cytosine. We might think of the molecules as letters in an alphabet G, A, T and C which can spell out words.

Just as we can use the four letters E, I, L and V to spell out English words with different meanings Evil, Live, Veil, Vile the particular sequence of G, A, T and C dictates the different ways cells will develop and function. Although there are only four letters in this language of DNA, those letters can be used any number of times. Most cells in human beings contain DNA with sequences of 6 billion letters about 10,000 times as many letters as appear in this book! That's a lot of information, just right for making something as complicated as you or me.

We inherit the DNA in our cells from our parents half from each. We can see how brothers and sisters have been 'built' from similar sets of instructions because they often look very similar.

In clones and identical twins, those instructions are exactly the same. Stretches of DNA that provide information for particular functions such as making eyes or ears are called 'genes', so scientists call twins (and clones) with exactly the same DNA 'genetically identical'. However, twins can often behave very differently from one another. Our genes don't fully determine the way our lives turn out other factors are important.

The Tenth Doctor's companion Martha Jones looks very like her cousin, Adeola (seen in Army of Ghosts (2006)) because both characters were played by actress Freema Agyeman. In fact, cousins can look similar to each other because they inherit half their DNA from the same source that is, cousins don't share the same parents but they share one set of grandparents. Second cousins (that is, the cousins of your cousins) are less likely to look like you because you inherit much less of your DNA from the same source as they do from a shared set of great-grandparents. The less DNA we share with someone, the further back our most recent common ancestor must have lived. It has been estimated that the most recent common ancestor of all human beings alive today may have lived as recently as between 2,000 and 4,000 years ago not the first ever human, but the most recent individual who would appear on the family tree of everyone. But we don't just share DNA with human beings. As we said before, DNA is found in the cells of all living organisms and many viruses.

Darwin didn't know about DNA; the structure of it wasn't discovered until 1953, seventy years after his death. Yet he realised that his theory of evolution by natural selection explained the great diversity of life on the planet. When he published his theory in 1859, he called it On the Origin of Species species being the basic units of biological cla.s.sification, given in a two-part Latin name such as 'h.o.m.o sapiens' for you and me.

Darwin argued and DNA supports the theory that all species on Earth are part of one huge family. It has been estimated that humans could share as much as ninety-eight per cent of our DNA with chimpanzees, and it's thought we shared a common ancestor with them some 6 million years ago. By using fossil evidence, we can estimate the rate at which genes have evolved over time creating a 'molecular clock' with which we can deduce when different species diverged from one another. It's estimated that we shared a common ancestor with cheetahs and other types of cat some 85 million years ago, and with plants such as the apple tree and potato at some point in the Mesoproterozoic Era, some 1 billion to 1.6 billion years ago.

Darwin also argued that since we're so closely related to other apes we might learn much about ourselves from ape behaviour. In fact, studying apes has shed light on how evolution might develop behaviour as well as physical characteristics. We can see how working together as a group, agreeing systems of leadership and even friendship and kindness might have given our ancestors an advantage in the contest to survive. It's even been argued that heroism and self-sacrifice are evolutionary traits by dying to save others, an individual can ensure the survival of their family or kinship group if not themselves. The more we've studied apes and their closeness to human beings, their ability to think rationally and with self-awareness, even to use sign language, the more there have been calls to grant legal protections to them. The Ninth Doctor clearly doesn't think we've evolved all that far from our cousins.

'Listen, if I did forget some kid called Mickey-'

'He's not a kid!'

'-it's because I'm busy trying to save the life of every stupid ape blundering about on top of this planet, all right?

The Ninth Doctor and Rose Tyler, Rose (2005)

Darwin's discovery had an extraordinary impact. In scientific terms, it led to whole new areas of scientific research, and further discoveries such as the structure of DNA and why cloned potatoes are vulnerable to blight. But it affected more than science. The Doctor Who story Ghost Light (1989) captures something of this. It is set in 1883, the year after Darwin's death, but his theory is cited in arguments about religion, cla.s.s, politics and race.

The theory of evolution changed the world. After Darwin, economists argued that, just as compet.i.tion in the natural world causes each new generation to be better adapted to its environment, compet.i.tion was good for business, and those businesses that survived would evolve to better fit their markets. Today, some economists argue that if organisations such as schools and hospitals have to compete to achieve the best results, they can improve the services that they offer to the public.

People also worried how the human race might evolve in future, and how they might help to ensure that we evolved in the right direction whatever that might be. Some even suggested that only those people with traits considered to be desirable should be allowed to have children an idea called eugenics. Many apparently respectable people including H.G. Wells and Winston Churchill spoke in support of eugenics in the early twentieth century, and eugenics programmes of different kinds operated in many countries. However, the terrible reality of what these ideas could mean in practice was shown during the Second World War, when the n.a.z.is systematically murdered millions of Jews and other people they considered to have undesirable traits.

After the Second World War, international laws were brought in to prevent such abuses happening again but trying to influence human reproduction is something that still goes on today. Forced sterilisation a medical process that prevents people from having children on a widespread or systematic basis has been recognised as a Crime against Humanity with the jurisdiction of the International Criminal Court, but it still occurs. People have been sterilised because they have certain diseases, or represent 'undesirable' racial, s.e.xual or social groups. In some countries, criminals have been sterilised, as if criminality were somehow an inherited trait.

But surely this is missing the point of Darwin's discovery. Apple trees and cheetahs didn't consciously select the traits that gave them an advantage. We don't know what threats or pressures we'll face in future contests, so the traits we might think are desirable now won't necessarily help our species survive as we saw with the tasty cloned potatoes that were destroyed by blight.

Even so, natural selection can't help with big, fast changes such as pandemics of disease or catastrophes. In Chapter 5, we talked about the meteor collision that is thought to have wiped out the dinosaurs.

Except that some dinosaurs survived. Until the collision, it was an advantage to be a large, powerful dinosaur; afterwards, it's thought that dust and debris in Earth's atmosphere blocked the warmth of the Sun and created an 'impact winter'. In these changed conditions, large, powerful dinosaurs were at a disadvantage. Being small meant you could keep warm more easily and required less of the scarce food; being able to fly meant you cover more distance in looking for that food. So small, flying dinosaurs survived in these new conditions, and their descendants are still alive on Earth today we call them birds.

Perhaps something like that happens in the Doctor Who story New Earth (2006). In the year 5,000,000,023, Ca.s.sandra insists that she is the last human in existence. However, she has a very narrow sense of what 'human' means. Rose points out that there are plenty of others living on the planet New Earth, but Ca.s.sandra dismisses them out of hand.

'There's millions of humans out there. Millions of them.'

'Mutant stock.'

'They evolved, Ca.s.sandra. They just evolved, like they should. You stayed still. You got yourself all pickled and preserved, and what good did it do you?'

Rose Tyler and Ca.s.sandra, New Earth (2006)

At the end of the story, Ca.s.sandra dies, but humanity lives on because it has evolved. In fact, many of Doctor Who's most effective monsters are monstrous because they've tried to control evolution...

There was no doubt about it. Barry Jenkins was lost.

As water dripped from the roof of the arched tunnel, and effluent flowed along the narrow central channel beside the concrete walkway on which he was standing, he consulted his map again. It was as he had thought. That wall in front of him simply shouldn't exist. Maybe the lads in the maintenance depot were playing a joke on him because he was new? Maybe this was an initiation test to see if he could find his way through the maze of Central London's sewer system with a dud map?

Sighing, he trudged forward. He guessed he would just have to take the detour to the right and see where it led. Reaching the opening, he peered into it suspiciously. It was dark and narrow. He could only hope that ahead of him was a left turn, which at least would mean he'd again be going in roughly the right direction. Light from the lamp on his hard hat played along the wet wall, which, by rights, shouldn't be here. He tapped the wall, scowling.

Something tapped back.

He recoiled, then grinned. It must have been an echo. The lads at the depot would be hooting with laughter if they could see how jumpy he was. Setting his face determinedly, he marched into the narrow pa.s.sage. Sure enough, there was a blacker opening ahead on his left. Now he was getting somewhere.

Light slithered into the opening as he approached it. Aside from the drip of water all was silent. Barry peered into the blackness, craning forward so that the light could stretch as far as possible. And then he jumped as he heard scurrying sounds from the tunnel ahead.

Although he couldn't see anything, he knew what was causing the sounds. Rats. They didn't generally bother him they tended to flee from humans but on this occasion he felt uneasy. He wasn't sure why at first, and then he realised. It was because the sounds were not growing fainter, but louder.

The rats were moving towards him!

He waited, unsure what to do. Were the rats really heading in his direction or were the weird acoustics in the tunnel only making it seem that way? The light from his lamp probed the shadows ahead...

And then he saw them!

Red eyes. Glinting in the darkness. First one pair. Then another.