Secrets of the Human Body. Andrew Cohen

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СКАЧАТЬ years. Our bodies cannot be in the business of just replicating viral DNA for free. Otherwise we would become walking virus factories. That’s the trouble with viruses: you give them an inch and they take a mile: these bits of mobile DNA can ruin our useful DNA. So we have developed genes called zinc-finger genes that act to bind to the retrotransposon DNA and stop it replicating. So far, so good. But remember we’re in an arms race. They don’t quit! These retrotransposons are forever adapting and breaking free of the zinc fingers. Over many generations they find a way to start replicating again. And so our zinc fingers expand to suppress them again. Just the way that when the cheetahs get faster to catch the antelope the next generation of antelope get faster too. We are in an arms race inside our own bodies against parts of ourselves. The lovely part of this – which seems on the face of it to be extremely annoying, like a rumbling civil war or secession movement – is that the technologies we develop to suppress the retrotransposons turn out to be useful ways of regulating other parts of our genome. Much in the way that military technologies can often have beneficial civilian uses: advances in aviation and radar and so on. Our internal battles make us stronger.

      ‘WE ALL CARRY IN OUR BODIES REPTILIAN GENES AND FISH GENES. FOR MOST OF THE ENZYMES WE MAKE, WE HAVE THE SAME SET OF GENES AS FISH.’

      SUSUMU OHNO

      This is one of many examples of the vast complexity of our genome, which contains so much of our ancient past, and it creates a kind of fog of war. Complexity is an excellent way of keeping secrets.

      There is a detailed explanation of DNA and the nature of our genome on pp. 130–3. But it is far from a simple blueprint. When it was decoded (one of the few vast public projects that arrived on time and under budget) it seemed we were on the brink of some new moment in science. We had finally been given the keys to the kingdom. But instead we were presented with further secrets.

      The variation in genome sizes is bizarre: the lungfish, a particularly unglamorous and uncomplex vertebrate, has a genome 40 times larger than ours. It is reasonable to ask why ours is so small? (We don’t know.) But it is also reasonable to ask why it is so much bigger than it apparently needs to be? We have vast quantities of genetic material that doesn’t code for proteins. It doesn’t seem to do anything. It is reasonable to suggest that these sequences may have a function … but that it is a secret. But in 1972 it was written off as ‘junk DNA’ – a term coined by Susumu Ohno, one of twentieth-century America’s greatest scientists.

      Susumu Ohno was a Japanese-American geneticist, and was said to have ‘thought at least half of the thoughts’ that form the basis of modern genetic research. He was showered with prizes and honorary degrees for his work on genetics and on his death in 2000 the Emperor of Japan sent his family condolences, a rare honour. His brilliance is not in question but he seemed to have an impish sense of humour. It may be that he used the term ‘junk DNA’ facetiously: he certainly seemed to believe that there were secrets locked up in our genes that might only be revealed in unusual ways.

      In 1986 he gave an interview to the Chicago Tribune about his attempts to convert the human genetic code into musical form so that the vast repetitive sequences of code could be experienced in new ways. Whether or not this shed any truly useful light on the secrets of the genome, it does force you to encounter the rhythmic, repetitive nature of the code of life, and his arrangements have a surreal beauty that is hard to deny. In the same interview, he said the following: ‘It is surprising that our ancient genes are not expressed more often. I think it’s possible that babies sometimes are born with tails. But the surgeons just snip them off, and we never hear about them … we all carry in our bodies reptilian genes and fish genes. For most of the enzymes we make, we have the same set of genes as fish.’

      There is no medical conspiracy to conceal the tails we are born with – or if there is, I’m out of the loop – but it’s a great line from a revered scientist and captures the bizarre accumulated nature of what is sometimes described as a conventional blueprint: the instructions to build a human. Our genome is more like the instructions to build a human, and also the instructions to build a ton of other ancient equipment and viruses, and also the instructions on how to prevent anyone using the wrong sets of instructions.

      So it seems like it’s not junk – the bundles of DNA allowing for complex evolution keep the secrets of our past. Even once we have the entire code in a computer database, as we do now, we barely understand its functions. This is also why this book is not an anatomy book: you can have a lot of facts, in this case all the facts, and still they can conceal secrets.

      This book is arranged around three themes: learning, survival and growth. There is a chapter devoted to each. Every aspect of the human body is shaped by the need to achieve these things. Human bodies, like all organisms, have one central job: to ensure the survival of their genes. Learning, growth and survival are all vital to achieve this goal. Counterintuitively, these three tasks may be even more important than reproduction. Reproduction is a useful but not essential task for gene survival: you don’t need to reproduce to ensure your genes survive. It helps because it puts more copies of them out there but if you look after your relatives, who share your genes, and help them survive and reproduce you are still giving your genes an advantage over your non-relatives. If you have an identical twin then your sibling will share all your genes, and their children will be – according to any genetic test – your children (my son Julian calls Chris ‘Uncle Dad’ and occasionally just ‘Dad’). But you can’t be any use to your family members unless you have managed to grow, learn and survive. These, the secrets of the human body, are the secret ways in which we all achieve these things in spite of the thousands of things that try to stop us every second of every day.

      Growth, learning and survival are interconnected: they are each part of a complex network of processes and forces that make us who we are. And they cannot happen independently. As we learn, our bodies grow and our survival is predicated on our ability to learn from threats and to learn how best to exploit our ecological niche.

      Learning is not simply the acquisition of facts and memories. The ‘Learn’ chapter is about the incorporation – the literal embodiment – of the physical and social world into ourselves. Ulysses, in Tennyson’s poem, reflects on his life and his worn-out body and his future:

      I am a part of all that I have met;

      Yet all experience is an arch wherethrough

      Gleams that untravelled world whose margin fades

      Forever and forever when I move.

      There is an ambiguity here: our bodies imprint themselves everywhere we go and our lives are incorporated into us. We shape the world as it literally shapes us. Our brains lay down our experiences not through some ineffable neural magic but with visible layers of fatty myelin – white matter – coating our nerves. In the ‘Learn’ chapter we will see how scientists are now able to watch our memories – or the proteins that write them in our brains – travel the length of neurons to be stored for retrieval. And we will see how we can track the timing and locations of our short- and long-term memories: the different processes by which we transiently remember a name at a party and store the feeling of our first day at school for the rest of our lives. We will meet unique people, each of them a Ulysses of their own world, exploring and pushing the boundaries of human experience: Danny MacAskill, the uniquely skilled cyclist; Deb Roy, the scientist with (surely) the largest home video collection in the world; Akash Vukoti, the youngest ever participant in the US spelling bee championships with a vocabulary that would shame most adults; Henry Molaison, the man who lost the ability to make new memories. These people in these stories will show us how our brains and therefore our selves are shaped by what we learn. Ulysses describes experience as an arch, a robust yet delicate structure built of the world through which we travel. Our bodies are literally built of our experiences. They learn and adapt to the untravelled world that faces all of us: our bones are shaped by the forces we experience: in the ‘Learn’ СКАЧАТЬ