Secrets of the Human Body. Andrew Cohen

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СКАЧАТЬ 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 ability for self-transformation in both its structure and function, adapting constantly to the demands made on it. In engineering terms it would be a plane strengthened by turbulence, a car that got faster the more you drove it and used less fuel per mile, a computer that got quicker and more accurate with age. On top of that these machines would be able to fuel and reproduce themselves. As we will see in this chapter, attempts at bioengineering reveal just how superb and precise a designer nature is compared to even our most impressive innovations.

      For most animals on the planet the journey to maturity is a quick and efficient process. Many mammals, including dogs and cats, reach adulthood within six months of birth, blue whales (the largest animals on the planet) are able to reproduce as quickly as five years after birth and our nearest cousin the chimpanzee completes its journey to maturation years ahead of us, being fully grown, sexually mature and reproducing on average by the age of 13. It seems an elongated childhood is a uniquely human process. No other animal on the planet takes longer to reach maturity and no other animal goes through such a convoluted stop–start process of growth and development than a human child. In many parts of the developed world the average age of a first-time mother is well into her thirties. Many of the reasons behind this elongated adolescence are of course cultural, heavily influenced by the way we structure our societies and the growing balances between the lives of men and women. But underneath the social influences there is an intriguing biological story with mysteries that we are still trying to understand. Why, for instance, do we have two discrete growth spurts, one just after birth and one on average more than 10 years later, just before puberty? What is the reason for this elongated lull in our growth? And what triggers the sudden onset of puberty? All of these are questions that until very recently we have struggled to answer, but in the last few years many of the secrets of the journey from baby to reproduction are beginning to be revealed.

      SELF-REPLICATING MACHINES

      A quick interesting side note while we are talking about this is that the concept of self-replicating machines has a long history in both fact and fiction with perhaps the most famous being devised by the Hungarian mathematician, scientist and general genius John von Neumann. Von Neumann machines that could explore and colonise whole galaxies have been the subject of much conjecture for decades and the fact we have never found one has been used by some as evidence that advanced civilisations are absent from this and perhaps every other nearby galaxy.

      MIRACLE MILK

      There is only one substance on earth that is specifically produced as a nourishing food: milk.

      A few fruits have evolved to be palatable, persuading animals to eat them and distribute seeds in faeces, but these are just sweet-treats. And yes, we can extract nutrition from animal flesh and a handful of plants and fungi, but our relationship with these foodstuffs is more complex, more competitive. They didn’t evolve specifically to be food. Milk, and only milk, did.

      Breast milk is what makes mammals, mammals. There are other characteristics that most mammals share but the platypus, and a few of its Australasian friends, mess things up, with their egg-laying and lack of placentas. But even the platypus makes milk.

      Milk is extraordinary stuff. It is, most obviously, a complete source of nutrition. It contains fat, protein, carbohydrate, water, vitamins, minerals, amino acids and fatty acids, all in an available form, tailored perfectly to each stage of development. You can build a human child for several years entirely on breast milk. But it’s not just food. It also contains an immune system in the form of antibodies, and a cocktail of hormones and other factors that regulate and stimulate infant development. And far from being a single substance, it is constantly changing, as the child grows, between left and right breast and throughout the day. Even within a single feed the milk shows complex fluctuations in what it contains in terms of lipids, carbohydrates and total calories.

      Milk composition. Note how much more protein there is in cow’s milk and how much less HMO mass.

      Breast milk is made when a set of genes are turned on in the cells of the mother’s breast during pregnancy. These genes encode proteins, including the enzymes that turn the mother’s body into the end product. The genes for milk production have been selected by evolution over around 150–200 million years since the first shrew-like creatures gave their young primitive breast milk from barely modified sweat glands. Yes, for all its erotic and maternal associations, the breast is a modified sweat gland. It’s impossible to know what that first milk, produced somewhere around the late Jurassic era, would have consisted of, but considering that modern shrews can barely get enough calories to sustain themselves for more than a few hours, that early milk may have been more about the transfer of antibodies, to fight infection, than calories.

      THE COST OF A PINT OF MILK

      For Bruce German, a chemist at the University of California, Davis, milk was the obvious starting point to understand nutrition. Nutritional science made a few huge leaps early in the twentieth century, before stalling around the 1970s. It had been established that to stay alive, we need three macronutrients (carbohydrate, protein and fat) and all but invisible amounts of a few vitamins, minerals, amino acids and essential fatty acids. Biochemists figured out the chemical reactions that are required to turn what we put into our mouths into flesh and energy. They discovered the enzymes that enable these reactions to happen. They described how the molecular constituents of our cells are recycled and replaced in response to the world around us. But the ideal human diet continues to elude detailed description. There are some broad brushstrokes that we’re confident about: eat lots of plants. Meat seems to be OK in small amounts. Fish is good. Refined sugar may be bad. But dig a little deeper and confusion reigns: saturated fats were bad, then good, then bad. Oily fish, vitamin supplements, low-carb vs. high-carb? These questions still generate inconsistent answers.

      A MOTHER WILL, ON AVERAGE, MAKE ABOUT 750 ML (ALMOST A PINT AND A HALF!) OF BREAST MILK PER DAY FOR THE FIRST FIVE MONTHS AFTER BIRTH.

      ‘Milk offered the opportunity to take an evolutionary perspective. What food should we eat?’ says Bruce. It’s СКАЧАТЬ