The Body: A Guide for Occupants
LONG AGO, WHEN I was a junior high school student in Iowa, I remember being taught by a biology teacher that all the chemicals that make up a human body could be bought in a hardware store for $5.00 or something like that. I don’t recall the actual sum. It might have been $2.97 or $13.50, but it was certainly very little even in 1960s money, and I remember being astounded at the thought that you could make a slouched and pimply thing such as me for practically nothing.
It was such a spectacularly humbling revelation that it has stayed with me all these years. The question is, was it true? Are we really worth so little?
Many authorities (for which possibly read “science majors who don’t have a date on a Friday”) have tried at various times, mostly for purposes of amusement, to compute how much it would cost in materials to build a human. Perhaps the most respectable and comprehensive attempt of recent years was done by Britain’s Royal Society of Chemistry when, as part of the 2013 Cambridge Science Festival, it calculated how much it would cost to assemble all the elements necessary to build the actor Benedict Cumberbatch. (Cumberbatch was the guest director of the festival that year and was, conveniently, a typically sized human.)
Altogether, according to RSC calculations, fifty-nine elements are needed to construct a human being. Six of these—carbon, oxygen, hydrogen, nitrogen, calcium, and phosphorus—account for 99.1 percent of what makes us, but much of the rest is a bit unexpected. Who would have thought that we would be incomplete without some molybdenum inside us, or vanadium, manganese, tin, and copper? Our requirements for some of these, it must be said, are surpassingly modest and are measured in parts per million or even parts per billion. We need, for instance, just 20 atoms of cobalt and 30 of chromium for every 999,999,999½ atoms of everything else.
The biggest component in any human, filling 61 percent of available space, is oxygen. It may seem a touch counterintuitive that we are almost two-thirds composed of an odorless gas. The reason we are not light and bouncy like a balloon is that the oxygen is mostly bound up with hydrogen (which accounts for another 10 percent of you) to make water—and water, as you will know if you have ever tried to move a wading pool or just walked around in really wet clothes, is surprisingly heavy. It is a little ironic that two of the lightest things in nature, oxygen and hydrogen, when combined form one of the heaviest, but that’s nature for you. Oxygen and hydrogen are also two of the cheaper elements within you. All your oxygen will set you back just $14 and your hydrogen a little over $26 (assuming you are about the size of Benedict Cumberbatch). Your nitrogen (2.6 percent of you) is a better value still at just forty cents for a body’s worth. But after that it gets pretty expensive.
You need about thirty pounds of carbon, and that will cost you $69,550, according to the Royal Society of Chemistry. (They were using only the most purified forms of everything. The RSC would not make a human with cheap stuff.) Calcium, phosphorus, and potassium, though needed in much smaller amounts, would between them set you back a further $73,800. Most of the rest is even more expensive per unit of volume, but fortunately only needed in microscopic amounts. Thorium costs over $3,000 per gram but constitutes just 0.0000001 percent of you, so you can buy a body’s worth for thirty-three cents. All the tin you require can be yours for six cents, while zirconium and niobium will cost you just three cents apiece. The 0.000000007 percent of you that is samarium isn’t apparently worth charging for at all. It’s logged in the RSC accounts as costing $0.00.*1
Of the fifty-nine elements found within us, twenty-four are traditionally known as essential elements, because we really cannot do without them. The rest are something of a mixed bag. Some are clearly beneficial, some may be beneficial but we are not sure in what ways yet, others are neither harmful nor beneficial but are just along for the ride as it were, and a few are just bad news altogether. Cadmium, for instance, is the twenty-third most common element in the body, constituting 0.1 percent of your bulk, but it is seriously toxic. We have it in us not because our body craves it but because it gets into plants from the soil and then into us when we eat the plants. If you are from North America, you probably ingest about eighty micrograms of cadmium a day, and no part of it does you any good at all.
A surprising amount of what goes on at this elemental level is still being worked out. Pluck almost any cell from your body, and it will have a million or more selenium atoms in it, yet until recently nobody had any idea what they were there for. We now know that selenium makes two vital enzymes, deficiency in which has been linked to hypertension, arthritis, anemia, some cancers, and even, possibly, reduced sperm counts. So, clearly it is a good idea to get some selenium inside you (it is found particularly in nuts, whole wheat bread, and fish), but at the same time if you take in too much you can irremediably poison your liver. As with so much in life, getting the balances right is a delicate business.
Altogether, according to the RSC, the full cost of building a new human being, using the obliging Benedict Cumberbatch as a template, would be a very precise $151,578.46. Labor and sales tax would, of course, boost costs further. You would probably be lucky to get a take-home Benedict Cumberbatch for much under $300,000—not a massive fortune, all things considered, but clearly not the meager few dollars that my junior high school teacher suggested. That said, in 2012 Nova, the long-running science program on PBS, did an exactly equivalent analysis for an episode called “Hunting the Elements” and came up with a figure of $168 for the value of the fundamental components within the human body, illustrating a point that will become inescapable as this book goes on, namely that where the human body is concerned, the details are often surprisingly uncertain.
But of course it hardly really matters. No matter what you pay, or how carefully you assemble the materials, you are not going to create a human being. You could call together all the brainiest people who are alive now or have ever lived and endow them with the complete sum of human knowledge, and they could not between them make a single living cell, never mind a replicant Benedict Cumberbatch.
That is unquestionably the most astounding thing about us—that we are just a collection of inert components, the same stuff you would find in a pile of dirt. I’ve said it before in another book, but I believe it’s worth repeating: the only thing special about the elements that make you is that they make you. That is the miracle of life.
We pass our existence within this warm wobble of flesh and yet take it almost entirely for granted. How many among us know even roughly where the spleen is or what it does? Or the difference between tendons and ligaments? Or what our lymph nodes are up to? How many times a day do you suppose you blink? Five hundred? A thousand? You’ve no idea, of course. Well, you blink fourteen thousand times a day—so much that your eyes are shut for twenty-three minutes of every waking day. Yet you never have to think about it, because every second of every day your body undertakes a literally unquantifiable number of tasks—a quadrillion, a nonillion, a quindecillion, a vigintillion (these are actual measures), at all events some number vastly beyond imagining—without requiring an instant of your attention.
In the second or so since you started this sentence, your body has made a million red blood cells. They are already speeding around you, coursing through your veins, keeping you alive. Each of those red blood cells will rattle around you about 150,000 times, repeatedly delivering oxygen to your cells, and then, battered and useless, will present itself to other cells to be quietly killed off for the greater good of you.
Altogether it takes 7 billion billion billion (that’s 7,000,000,000,000,000,000,000,000,000, or 7 octillion) atoms to make you. No one can say why those 7 billion billion billion have such an urgent desire to be you. They are mindless particles, after all, without a single thought or notion between them. Yet somehow for the length of your existence, they will build and maintain all the countless systems and structures necessary to keep you humming, to make you you, to give you form and shape and let you enjoy the rare and supremely agreeable condition known as life.
That’s a much bigger job than you realize. Unpacked, you are positively enormous. Your lungs, smoothed out, would cover a tennis court, and the airways within them would stretch nearly from coast to coast. The length of all your blood vessels would take you two and a half times around Earth. The most remarkable part of all is your DNA (or deoxyribonucleic acid). You have a meter of it packed into every cell, and so many cells that if you formed all the DNA in your body into a single strand, it would stretch ten billion miles, to beyond Pluto. Think of it: there is enough of you to leave the solar system. You are in the most literal sense cosmic.
But your atoms are just building blocks and are not themselves alive. Where life begins precisely is not so easy to say. The basic unit of life is the cell—everyone is agreed on that. The cell is full of busy things—ribosomes and proteins, DNA, RNA, mitochondria, and much other cellular arcana—but none of those are themselves alive. The cell itself is just a compartment—a kind of little room: a cell—to contain them, and of itself is as nonliving as any other room. Yet somehow when all of these things are brought together, you have life. That is the part that eludes science. I kind of hope it always will.
What is perhaps most remarkable is that nothing is in charge. Each component of the cell responds to signals from other components, all of them bumping and jostling like so many bumper cars, yet somehow all this random motion results in smooth, coordinated action, not just across the cell but across the whole body as cells communicate with other cells in different parts of your personal cosmos.
The heart of the cell is the nucleus. It contains the cell’s DNA—three feet of it, as we have already noted, scrunched into a space that we may reasonably call infinitesimal. The reason so much DNA can fit into a cell nucleus is that it is exquisitely thin. You would need twenty billion strands of DNA laid side by side to make the width of the finest human hair. Every cell in your body (strictly speaking, every cell with a nucleus) holds two copies of your DNA. That’s why you have enough to stretch to Pluto and beyond.
DNA exists for just one purpose—to create more DNA. A DNA molecule, as you will almost certainly remember from countless television programs if not school biology, is made up of two strands, connected by rungs to form the celebrated twisted ladder known as a double helix. Your DNA is simply an instruction manual for making you. A length of DNA is divided into segments called chromosomes and shorter individual units called genes. The sum of all your genes is the genome.
DNA is extremely stable. It can last for tens of thousands of years. It is nowadays what enables scientists to work out the anthropology of the very distant past. Probably nothing you own right now—no letter or piece of jewelry or treasured heirloom—will still exist a thousand years from now, but your DNA will almost certainly still be around and recoverable, if only someone could be bothered to look for it.
DNA passes on information with extraordinary fidelity. It makes only about one error per every billion letters copied. Still, because your cells divide so much, that is about three errors, or mutations, per cell division. Most of those mutations the body can ignore, but just occasionally they have lasting significance. That is evolution.
All of the components of the genome have one single-minded purpose—to keep the line of your existence going. It’s a slightly humbling thought that the genes you carry are immensely ancient and possibly—so far anyway—eternal. You will die and fade away, but your genes will go on and on so long as you and your descendants continue to produce offspring. And it is surely astounding to reflect that not once in the three billion years since life began has your personal line of descent been broken. For you to be here now, every one of your ancestors had to successfully pass on its genetic material to a new generation before being snuffed out or otherwise sidetracked from the procreative process. That’s quite a chain of success.
What genes specifically do is provide instructions for building proteins. Most of the useful things in the body are proteins. Some speed up chemical changes and are known as enzymes. Others convey chemical messages and are known as hormones. Still others attack pathogens and are called antibodies. The largest of all our proteins is called titin, which helps to control muscle elasticity. Its chemical name is 189,819 letters long, which would make it the longest word in the English language except that dictionaries don’t recognize chemical names. Nobody knows how many types of proteins there are within us, but estimates range from a few hundred thousand to a million or more.
The paradox of genetics is that we are all very different and yet genetically practically identical. All humans share 99.9 percent of their DNA, and yet no two humans are alike. My DNA and your DNA will differ in three to four million places, which is a small proportion of the total but enough to make a lot of difference between us. You also have within you about a hundred personal mutations—stretches of genetic instructions that don’t quite match any of the genes given to you by either of your parents but are yours alone.
How all this works in detail is still largely a mystery to us. Only 2 percent of the human genome codes for proteins, which is to say only 2 percent does anything demonstrably and unequivocally practical. Quite what the rest is doing isn’t known. A lot of it, it seems, is just there, like freckles on skin. Some of it makes no sense. One particular short sequence, called an Alu element, is repeated more than a million times throughout our genome, including sometimes in the middle of important protein-coding genes. It is complete gibberish, as far as anyone can tell, yet it constitutes 10 percent of all our genetic material. No one has any idea why. The mysterious part was for a while called junk DNA but now is more graciously called dark DNA, meaning that we don’t know what it does or why it is there. Some is involved in regulating the genes, but much of the rest remains to be determined.
The body is often likened to a machine, but it is so much more than that. It works twenty-four hours a day for decades without (for the most part) needing regular servicing or the installation of spare parts, runs on water and a few organic compounds, is soft and rather lovely, is accommodatingly mobile and pliant, reproduces itself with enthusiasm, makes jokes, feels affection, appreciates a red sunset and a cooling breeze. How many machines do you know that can do any of that? There is no question about it. You are truly a wonder. But then so, it must be said, is an earthworm.
And how do we celebrate the glory of our existence? Well, for most of us by eating maximally and exercising minimally. Think of all the junk you throw down your throat and how much of your life is spent sprawled in a near-vegetative state in front of a glowing screen. Yet in some kind and miraculous way our bodies look after us, extract nutrients from the miscellaneous foodstuffs we push into our faces, and somehow hold us together, generally at a pretty high level, for decades. Suicide by lifestyle takes ages.
Even when you do nearly everything wrong, your body maintains and preserves you. Most of us are testament to that in one way or another. Five out of every six smokers won’t get lung cancer. Most of the people who are prime candidates for heart attacks don’t get heart attacks. Every day, it has been estimated, between one and five of your cells turn cancerous, and your immune system captures and kills them. Think of that. A couple of dozen times a week, well over a thousand times a year, you get the most dreaded disease of our age, and each time your body saves you. Of course, very occasionally a cancer develops into something more serious and possibly kills you, but overall cancers are rare: most cells in the body replicate billions and billions of times without going wrong. Cancer may be a common cause of death, but it is not a common event in life.
Our bodies are a universe of 37.2 trillion cells operating in more or less perfect concert more or less all the time.*2 An ache, a twinge of indigestion, the odd bruise or pimple, are about all that in the normal course of things announces our imperfectability. There are thousands of things that can kill us—slightly more than eight thousand, according to the International Statistical Classification of Diseases and Related Health Problems compiled by the World Health Organization—and we escape every one of them but one. For most of us, that’s not a bad deal.
We are not perfect by any means, goodness knows. We get impacted molars because we have evolved jaws too small to accommodate all the teeth we are endowed with. We have pelvises too small to pass children without excruciating pain. We are hopelessly susceptible to backache. We have organs that mostly cannot repair themselves. If a zebra fish damages its heart, it grows new tissue. If you damage your heart, well, too bad. Nearly all animals produce their own vitamin C, but we can’t. We undertake every part of the process except, inexplicably, the last step, the production of a single enzyme.
The miracle of human life is not that we are endowed with some frailties but that we aren’t swamped with them. Don’t forget that your genes come from ancestors who most of the time weren’t even human. Some of them were fish. Lots more were tiny and furry and lived in burrows. These are the beings from whom you have inherited your body plan. You are the product of three billion years of evolutionary tweaks. We would all be a lot better off if we could just start fresh and give ourselves bodies built for our particular Homo sapien needs—to walk upright without wrecking our knees and backs, to swallow without the heightened risk of choking, to dispense babies as if from a vending machine. But we weren’t built for that. We began our journey through history as unicellular blobs floating about in warm, shallow seas. Everything since then has been a long and interesting accident, but a pretty glorious one, too, as I hope the following pages make clear.
*1 The RSC calculations were done in British pounds and have been converted here into U.S. dollars at the rate that prevailed in the summer of 2013 of £1 = $1.57.
*2 That number is of course an educated guess. Human cells come in a variety of types, sizes, and densities and are literally uncountable. The figure of 37.2 trillion was arrived at in 2013 by a team of European scientists led by Eva Bianconi from the University of Bologna in Italy and was reported in the Annals of Human Biology.
Also by Bill Bryson
1 How to Build a Human
2 The Outside: Skin and Hair
3 Microbial You
4 The Brain
5 The Head
6 Down the Hatch: The Mouth and Throat
7 The Heart and Blood
8 The Chemistry Department
9 In the Dissecting Room: The Skeleton
10 On the Move: Bipedalism and Exercise
12 The Immune System
13 Deep Breath: The Lungs and Breathing
14 Food, Glorious Food
15 The Guts
17 Into the Nether Regions
18 In the Beginning: Conception and Birth
19 Nerves and Pain
20 When Things Go Wrong: Diseases
21 When Things Go Very Wrong: Cancer
22 Medicine Good and Bad
23 The End
Notes on Sources
About the Author
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