Secrets of the Human Body. Andrew Cohen

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 worth saying that there is a lot of bad science based on an evolutionary perspective. It usually involves scientists discovering something about the way people behave towards their mates, friends or enemies and then inferring causality from hypothesised ancestral sabre-tooth tiger encounters. This is often not very useful because we don’t understand much about how we used to interact with sabre-tooth tigers (probably very little). Bruce German and his team at UC Davis do not involve sabre-tooth tigers in their evolutionary perspective. They study the genes, enzymes and constituents of milk. Not all genes in the human body are treated equally by evolution. There are many ancient genes that remain stable, barely changing over long periods of evolutionary time. But the genes involved in breast milk production have been under intense evolutionary scrutiny since it first evolved. Because while milk is great for the offspring, it’s pretty bad for the mother. This is because it is massively expensive for her to produce. And this was the reasoning that the team at UC Davis started with: human breast milk has evolved to be perfect nutrition for a human infant but it needs to be extremely efficient because it is so costly for the mother.

      How costly? 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. This will gradually increase with demand to almost a litre a day if exclusive breast feeding continues, assuming the mother is herself well-nourished and hydrated. As drinks go it is rich in energy containing around 65 calories per 100 ml. Unsurprisingly this is about the same as whole milk from a cow. By comparison a sugary cola drink will have about 40 calories per 100 ml. The cost to the mother is immense. She will produce milk by breaking down her own body. Even if milk was made 100 per cent efficiently, it would still be a huge number of calories stripped from the mother, but to calculate the true cost you have to first work out the efficiency of milk production. And it’s not a trivial calculation. Experiments have been done all around the world using isotope labelled water, special metabolic chambers and biochemical calculations about milk composition. Estimates vary, but 1 calorie of milk takes about 1.2 calories of energy from the mother. All this adds up such that, on average, exclusively breast feeding a 6-month-old child will demand a large burger’s worth of energy from the mother each day.