The Body: A Guide for Occupants

by Bill Bryson

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

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.

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

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

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

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

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.

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

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.

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

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

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

called the stratum corneum, is made up entirely of dead cells. It is an arresting thought that all that makes you lovely is deceased. Where body meets air, we are all cadavers. These outer skin cells are replaced every month. We shed skin copiously, almost carelessly: some twenty-five thousand flakes a minute, over a million pieces every hour. Run a finger along a dusty shelf, and you are in large part clearing a path through fragments of your former self. Silently and

you burn the contents of a vacuum cleaner bag, the predominant odor is that unmistakable scorched smell that we associate with burning hair. That’s because skin and hair are made largely of the same stuff: keratin.

Sometimes the pores become blocked with little plugs of dead skin and dried sebum in what is known as a blackhead. If the follicle additionally becomes infected and inflamed, the result is the adolescent dread known as a pimple. Pimples plague young people simply because their sebaceous glands—like all their glands—are highly active. When the condition becomes chronic, the result is acne, a word of very uncertain derivation.

Curiously, we don’t have any receptors for wetness. We have only thermal sensors to guide us, which is why when you sit down on a wet spot, you can’t generally tell whether it really is wet or just cold. Women

of skin about a millimeter thick from the arm of a cadaver. It was so thin as to be translucent. “That,” he said, “is where all your skin color is. That’s all that race is—a sliver of epidermis.” I mentioned this to Nina Jablonski when we met in her office in State College, Pennsylvania, soon afterward. She gave a nod of vigorous assent. “It is extraordinary how such a small facet of our composition is given so much importance,” she said. “People act as if skin color is a determinant

Biologically, there is actually no such thing as race—nothing in terms of skin color, facial features, hair type, bone structure, or anything else that is a defining quality among peoples. And yet look how many people have been enslaved or hated or lynched or deprived of fundamental rights through history because of the color of their skin.” A tall, elegant

“Melanin is a superb natural sunscreen,” says Jablonski. “It is produced in cells called melanocytes. All of us, whatever our race, have the same number of melanocytes. The difference is in the amount of melanin produced.” Melanin often responds to sunlight in a literally patchy way, resulting in freckles, which are technically known as ephelides. Skin color is a classic example of what is known as convergent evolution—that is, similar outcomes that have evolved in two or more locations. The people of, say, Sri Lanka and Polynesia have light brown skin not because of any direct genetic link but because they independently evolved brown skin to deal with the conditions of where they lived. It used to be thought that depigmentation probably took perhaps ten thousand to twenty thousand years, but now thanks to genomics we know it can happen much more quickly—in probably just two or three thousand years. We also know that it has happened repeatedly. Light-colored skin—“de-pigmented skin,” as Jablonski calls it—has evolved at least three times on Earth. The lovely range of hues humans boast is an ever-changing process. “We are,” as Jablonski puts it, “in the middle of a new experiment in human evolution.” It has been suggested that light skin may be a consequence of human migration and the rise of agriculture. The argument is that hunter-gatherers got a lot of their vitamin D from fish and game and that these inputs fell sharply when people started growing crops, especially as t...

Vitamin D is vital to health. It helps to build strong bones and teeth, boosts the immune system, fights cancers, and nourishes the heart. It is thoroughly good stuff. We can get it in two ways—from the foods we eat or through sunlight. The problem is that too much UV exposure damages DNA in our cells and can cause skin cancer. Getting the right amount is a tricky balance. Humans have addressed the challenge by evolving a range of skin tones to suit sunshine intensity at different latitudes. When a human body adapts to altered circumstances, the process is known as phenotypic plasticity. We alter our skin color all the time—when we tan or burn beneath a bright sun or blush from embarrassment. The red of sunburn is because the tiny blood vessels in the affected areas become engorged with blood, making the skin hot to the touch. The formal

The flush we get when angry is a little counterintuitive. When the body is poised for a fight, it mostly diverts blood flow to where it is really needed—namely, the muscles—so why it would send blood to the face, where it confers no obvious physiological benefit, remains a mystery. One possibility suggested by Jablonski is that it helps in some way to mediate blood pressure. Or it could just serve as a signal to an opponent to back off because one is really angry. At all events, the slow evolution of different skin tones worked fine when people stayed in one place or migrated slowly, but nowadays increased mobility means that lots

regions like northern Europe and Canada, it isn’t possible in the winter months to extract enough vitamin D from weakened sunlight to maintain health no matter how pale one’s skin, so vitamin D must be consumed as food, and hardly anyone gets enough—and not surprisingly. To meet dietary requirements from food alone, you would have to eat fifteen eggs or six pounds of swiss cheese every day, or, more plausibly if not more palatably, swallow half a tablespoon of cod liver oil. In America, milk is helpfully supplemented with vitamin D, but that still provides only a third of daily adult requirements. In consequence, some 50 percent of people globally are estimated to be vitamin D deficient for at least part of the year. In northern climes, it may be as much as 90 percent. —

Lighter-colored eyes and hair evolved somewhere around the Baltic Sea about six thousand years ago. It’s not obvious why. Hair and eye color don’t affect vitamin D metabolism, or anything else physiological come to that, so there seems to be no practical benefit. The supposition is that these traits were selected for as tribal markers or because people found them more attractive. If you have blue or green eyes, it’s not because you have more of those colors in your irises than other people but because you simply have less of other colors. It is the paucity of other pigments that leaves the eyes looking blue or green.

Skin color has been changing over a much longer period—at least sixty thousand

Indigenous populations in South America, for instance, are lighter-skinned than would be expected at the latitudes they inhabit. That is because in evolutionary terms they are recent arrivals. “They were able to get to the tropics quite quickly and had lots of gear, including some clothing,” Jablonski told me. “So in effect they thwarted evolution.” Rather harder to explain have been the KhoeSan people of southern Africa. They have always lived under a desert sun and have never migrated any great distance, yet have 50 percent lighter skin than would be predicted by their environment. It now appears that a genetic mutation for lighter skin was introduced to them sometime in the last two thousand years by outsiders—but who these mysterious light-skinned outsiders

Hairless skin is called glabrous, and there isn’t much of it. Our only truly hairless parts are lips, nipples and genitalia, and the bottoms of our hands and feet. The rest of the body is covered with either conspicuous hair, called terminal hair, as on your head, or vellus hair, which is the downy stuff you find on a child’s cheek. We are actually as hairy as our cousins the apes. It’s

it is known from genetic studies that dark pigmentation dates from between 1.2 and 1.7 million years ago. Dark skin wasn’t necessary when we were still furry, so that would strongly suggest a time frame for hairlessness. Why we retained hair on some parts of our bodies is fairly straightforward with respect to the

Hair on the head acts as a good insulator in cold weather and a good reflector of heat in hot weather. According to Nina Jablonski, tightly curled hair is the most efficient kind “because it increases the thickness of the space between the surface of the hair and the scalp, allowing air to blow through.” A separate but no less important reason for the retention of head hair is that it has been a tool of seduction since time immemorial. Pubic and underarm

each grow about twenty-five feet of hair in a lifetime, but because all hair falls out at some point, no single

strand can ever get longer than about three feet. Hair grows by one third of a millimeter a day, but the rate of hair growth depends on your age and health and even

Similarly, no one has ever come close to explaining why our fingers wrinkle when we have long baths. The explanation most often given is that wrinkling helps them to drain water better and improves grip. But that doesn’t really make a great deal of sense. Surely the people who most urgently need a good grip

Much better to do as we do and seep watery fluids onto nearly bare skin, which cools the body as it evaporates, turning us into a kind of living air conditioner. As Jablonski has written, “The loss of most of our body hair and the gain of the ability

dissipate excess body heat through eccrine sweating helped to make possible the dramatic enlargement of our most temperature-sensitive organ, the brain.” That, she says, is how sweat helped to make you brainy. Even

Although salt is only a tiny part of your overall sweat, you can lose as much as three teaspoonfuls of it in a day in hot weather, which can be a dangerously high amount, so it is important to replenish salt as well as water. Sweating is activated by the release of adrenaline, which is why when you are stressed, you break into a sweat. Unlike the rest of the body, the palms don’t sweat in response to physical exertion or heat, but only from stress. Emotional sweating is what is measured in lie detector tests. Sweat glands come in two varieties: eccrine and apocrine.

is eccrine sweat in your feet—or more correctly the chemical breakdown by bacteria of the sweat in your feet—that accounts for their lush odor. Sweat on its own is actually odorless. It needs bacteria to create a smell. The two chemicals that account for the odor—isovaleric acid and methanediol—are also produced by bacterial actions on some cheeses, which is why feet and cheese can often smell so very alike. Your skin

make one’s hands safely clean after a medical examination requires thorough washing with soap and water for at least a full minute—a standard that is, in practical terms, all but unattainable for anyone dealing with lots of patients. It is a big part of the reason why every year some two million Americans pick up a serious infection in the hospital (and ninety thousand of them die of it). “The greatest difficulty,” Atul Gawande has written, “is getting clinicians like me to do the one thing that consistently halts the spread of infections: wash our hands.” A study at New York

another widely reported study, the Belly Button Biodiversity Project, conducted by researchers at North Carolina State University, sixty random Americans had their belly buttons swabbed to see what was lurking there microbially. The study found 2,368 species of bacteria, 1,458 of which were unknown to science. (That is an average of 24.3 new-to-science microbes in every navel.) The number of species per person varied from 29 to 107. One volunteer harbored a microbe that had never been recorded outside Japan—where he had never been. The problem with antibacterial soaps is that they kill good bacteria on your skin as well as bad. The same is true of hand sanitizers. In 2016, the Food and Drug Administration

all follicles

Little is understood about the process, but what is known is that a hormone called

dihydrotestosterone tends to go slightly haywire as we age, directing hair follicles on the head to shut down and more reserved ones in the nostrils and ears to spring to dismaying life. The one known cure for baldness is castration. Ironically, considering how easily some of us lose it, hair is pretty impervious to decay and has been known to last in graves for thousands of years. Perhaps the

Eighty percent of the air you breathe is nitrogen. It is the most abundant element in the atmosphere and it is vital to our existence, but it doesn’t interact with other elements. When you take a breath, the nitrogen in the air goes into your lungs and straight back out again, like an absentminded shopper who has wandered into the wrong store. For nitrogen to be useful to us, it must be converted into more sociable forms, like ammonia, and it is bacteria that do that job for us. Without their help, we would die. Indeed, we could never have existed. It is time to say thank you to your microbes. You are home to trillions and trillions of tiny living

you put all Earth’s microbes in one heap and all the other animal life in another, the microbe heap would be twenty-five times greater than the animal one. Make no mistake.

VIRUS, IN the immortal words of the British Nobel laureate Peter Medawar, is “a piece of bad news wrapped up in a protein.” Actually, a lot of viruses are not bad news at all, at least not to humans. Viruses are a little weird, not quite living but by no means dead. Outside living cells, they are just inert things. They don’t eat or breathe or do much of anything. They have no means of locomotion.

Like bacteria, they are incredibly successful. The herpes virus has endured for hundreds of millions of years and infects all kinds of animals—even oysters. They are also terribly small—much smaller than bacteria and too small to be seen under conventional microscopes. If you blew one up to the size of a tennis ball, a human would be five hundred miles high. A bacterium on the same scale would be about the size of a beach ball. In the modern sense of a very small microorganism, the term “virus” dates only from

used to be thought that all viruses cause disease—hence the Peter Medawar quotation—but we now know that most viruses infect only bacterial cells and have no effect on us at all. Of the hundreds of thousands of viruses reasonably supposed to exist, just 586 species are known to infect mammals, and of these only 263 affect humans. We know very little about most other, nonpathogenic viruses because only the ones that cause disease tend to get studied. In 1986,

Although it had been locked in permafrost for thirty thousand years, when injected into an amoeba, it sprang into action with the lustiness of youth. Luckily, P. sibericum proved not to infect humans, but who knows what else may be out there waiting to be uncovered? A rather more common manifestation of viral patience is seen in the varicella-zoster virus. This is the virus that gives you chicken pox when you are small, but then may sit inert in nerve cells for half a century or more before erupting in that horrid and painful indignity of old age known as shingles. It is usually described as a painful rash on the torso, but in fact shingles can pop up almost anywhere on the body surface. A friend of mine had

In a similar study at the University of Arizona, researchers infected the metal door handle to an office building and found it took only about four hours for the “virus” to spread through the entire building, infecting over half of employees and turning up on virtually every shared device like photocopiers and coffee machines. In the real world, such infestations can stay active for up to three days. Surprisingly, the least effective way to spread germs (according to yet another study) is kissing. It proved almost wholly ineffective among volunteers at the University of Wisconsin who had been successfully infected with cold virus. Sneezes and coughs weren’t much

only really reliable

way to transfer cold germs is physically by touch. A survey of subway trains in Boston found that metal poles are a fairly hostile environment for microbes. Where microbes thrive is in the fabrics on seats and on plastic handgrips. The most efficient method of transfer for germs, it seems, is a combination of folding money and nasal mucus. A study in Switzerland in 2008 found that flu virus can survive on paper money for two and a half weeks if...

wasn’t until 1959 that they were recognized as quite separate and given their own kingdom. They essentially divide into two groups—molds and yeasts. By and large fungi leave us alone. Only about three hundred out of several million species affect us at all, and most of those mycoses, as they are known, don’t make you really ill, but rather cause only mild discomfort or irritation, as with athlete’s foot, say. A few, however, are much nastier than that, and the number of nasty ones is growing. Candida albicans, the fungus behind thrush, until the 1950s was found only in the mouth and genitals, but now it sometimes invades the deeper body, where it can grow on the heart