players who are broken down and rebuilt by microgravity or the repeated impact of ball and racket and we see how the world writes itself upon us. Like Ulysses we never stop moving or changing: our bodies are designed to learn from experience and adapt to threats we can have no knowledge of until we meet them. Our ability to learn is what allows us to move into the untravelled future: I may never ‘drink delight of battle with my peers, far on the ringing plains of windy Troy’, but we are all constantly learning to overcome the particular challenges life throws at us. And don’t worry, you can teach an old dog new tricks: we will see Chris valiantly attempting to prove this as he goes up against an 8-year-old in a juggling competition.
Parts of growth are about learning: we grow stronger, or we grow tougher in certain ways as we learn from the environment. But the growth chapter centres around the most striking aspect of growth: a typical human will increase its size by over 20 times from birth to adulthood and this vast increase must occur without interrupting learning or jeopardising survival. Growth is extraordinarily energy expensive and most of this energy in the early and most rapid phases of growth comes entirely from breast milk. Breast milk is the only stuff on the planet that has evolved specifically to feed humans, it has determined our ability to grow through our entire evolutionary history and yet we have only just managed to understand the role of its main ingredient.
We will meet one of the tallest families in the world and through them examine what drives us upwards, what advantages and disadvantages it might confer, and see the extraordinary mechanics of growth, the architectural equivalent of building a miniature building and then expanding 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.
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
