Secrets of the Human Body. Andrew Cohen

every part of it over the course of 20 years while constantly improving its function. This remarkable growth occurs in two spurts during childhood and adolescence, and we will see how the demands of growth must be balanced with the immediate demands of learning and survival. But even once we have reached adult size we do not stop growing. We will meet Lew Hollander who, at 87 years old, is still competing in Iron Man triathlons and making demands on his body that require new growth. We will see how he forces us to consider the role of wear and tear in stimulating growth and the way in which the body never stops growing.

      The fact that we never stop growing – that our cells have the ability to divide trillions of times – provides an enticing opportunity: the possibility of human growth without a complete human body. We will meet Professor Harald Ott who is growing human hearts in his laboratory. If he succeeds it will be an almost unprecedented milestone in the history of medicine, and we will see how much his work relies on one of the most important but uncelebrated parts of the body: the extracellular matrix. This is a lattice of proteins and sugars that tells dividing cells where to sit and what to do. It binds us together so that we are not simply slime. We will see how the architecture of the heart allows it to pump blood so efficiently and unfailingly as it grows and also see how the forces that the heart itself generates are essential in its growth and function.

      Learning and growth are fundamental to our survival and to the survival of our genes, allowing us to repair damage, reduce threats and learn from previous encounters to interact with our environment in a more sophisticated way, and the ‘Survival’ chapter presents the challenges of doing this. Why must we learn and why must we grow? Because we are so delicate. Because we are unable to withstand even the smallest changes to our internal environment. But we need to have innate mechanisms to protect us from the vast variability in the world because there is so little margin for error and we need ways of incorporating this variability into our behaviour and reactions to threats.

      We begin the chapter with Chris witnessing the moment of conception in a Harley Street IVF clinic. The miracle of this moment is almost overshadowed by the miracle of human homeostasis: our ability to keep our internal environment constant in almost every way. That cell will not change temperature, pressure, acidity, oxygen concentration or anything else until its owner dies. And at least one of its owner’s cells will endure in that same environment indefinitely as long as they have a direct line of descendants.

      Of course scientists are rarely happy to simply agree that the internal environment of the human body is pretty consistent. They want to see just what it is possible to endure in the way of external changes. And so Chris and I went to Professor Mike Tipton’s extreme physiology lab in Southampton, where we were taken on a journey from the high Arctic to the desert to see just how much temperature variation our bodies could handle.

      It is easy to imagine as a modern human that we live our lives in rational ways, our decisions based on education and experience. We have brains developed to cope with very different times, and parts of our brains are very old indeed. We will see how the more recently evolved parts of our brain govern the older, more instinctive, parts of our brain. We live in a delicate balance with emotions like fear and disgust that serve to protect us but simultaneously can potentially disable us. Disgust is particularly complicated. It is our least-considered emotion, but most of the time our disgust sensors are turned up full volume. Just occasionally we have to turn them off entirely, to reproduce or eat. Chris and I encounter the most disgusting meal we have ever eaten and realise that food and sex are linked in ways you might never imagine (… and might prefer to continue not imagining for the sake of both your love life and your dinner table). We tour through fear: experiments we are no longer allowed to do show how our bodies fine-tune our sense of what to be afraid of. And we learn about the one thing we are all born frightened of and what happens if you completely lack the capacity for fear.

      This book isn’t just a catalogue of the intriguing or miraculous. We want to show you the secrets to the way that the human body is interrogated, the way in which it gives up information agonisingly slowly and reluctantly. Much of writing this book, and making the accompanying television programme, felt like bring-your-child-to-work-day. Chris and I got to be naive and to ask simple questions of amazing people. Questions like ‘why does it do that?’ and ‘why is it made that way?’ are the sorts of thing children ask but when you pose them to the world’s best scientists you get extraordinary answers. We were able to arrange absurd scenarios like having the most disgusting dinner party with a scientist we had only just met and persuaded colleagues to torture us to make a point about homeostasis. We got to watch a baby get made. Why isn’t this just a textbook? Because this mad cascade of events and facts needs meaning. We wanted to give you a way of thinking about yourself, and to let you in on some of the secrets that your body has been keeping from you.

       ONE

       GROW

      BABY TO BABY-MAKER

      The simple process of growing is an extraordinary thing. During a lifetime our growth rate is truly staggering: from starting out as a single fertilised egg at the moment of conception, we multiply into a mass of trillions of cells made up of over 200 different cell types organised into around 80 organs (the exact number depends on how you define an organ … not as straightforward as you might think!), and that’s before we are even born.

      At birth the average human weighs about 3.5 kg and is approximately half a metre in length from head to heel. On our journey from baby to adult we then go through an amazing transformation: we quadruple in height. We add, on average, 70–80 kg to our body weight, although far more than this is increasingly common. At our fastest rate of growth we can elongate by up to 1.5 cm in a single day. It’s a process that we hardly notice but exploring how, why and when we grow reveals an extraordinary secret inside our bodies, which ultimately leads to a transformation that every one of us goes through. In this chapter we will explore the latest understanding of that process of growth and the magic ingredient that fuels it through the beginning of our lives. We will uncover the mystery of human childhood, a childhood longer than any other creature on earth, and explore the mysterious moment that triggers the body of a child to suddenly transform itself into an adult. We will also discover that growing doesn’t just end at adulthood, because throughout our lives our bodies are endlessly replenishing and regenerating, and even in old age, in some ways, we still continue to grow. We are now using this knowledge at the very cutting edge of medical science to redefine our perception of human growth by learning how to replicate it, control it and ultimately build new human organs and tissues grown entirely in the laboratory.

      I was born at 13.45 on 18 August 1978 with Chris taking an extra seven minutes to emerge into the welcoming arms of a midwife at the Queen Charlotte Hospital in London. At birth I weighed in at 6 lb 12 oz (3.06 kg) with Chris a slightly chubbier 6 lb 14 oz (3.12 kg). Thirty-nine years later and the vital statistics have not played out in my favour. Chris is not only an entire half an inch taller than me at 6 ft 1 in (185 cm), but he is also from the last available records approximately 5 kg lighter than me as well. Small differences to you perhaps but when you’re an identical twin it’s these differences that really matter! But the changes in our height and weight are just two of the miraculous transformations that we have gone through over the last 39 years. Each one of us sees our body transform throughout our childhood and beyond under the influence of a multitude of different factors, a complex web of genes and environment that combine together to turn a baby into a baby-maker. As we’ll see in the ‘Learn’ chapter, the transformation of our brains from newborn to highly skilled adult is a miraculous journey of its own, but our bodies undergo an equally extraordinary transformation. The size, shape, strength, appearance and function of our bodies are completely altered through those first 18 or so years of our lives. It’s such a gradual process that until we compare young and old photographs we often miss just how comprehensive and extreme a physical change this is.

      To put the extraordinary nature of this process into a slightly different context, just imagine attempting to build a machine that has such an inherent