The Body: A Guide for Occupants

The most remarkable part of all is your DNA. You have a metre of it packed into every cell, and so many cells that if you formed all the DNA in your body into a single fine 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.

Just sitting quietly, doing nothing at all, your brain churns through more information in thirty seconds than the Hubble Space Telescope has processed in thirty years. A morsel of cortex one cubic millimeter in size—about the size of a grain of sand—could hold two thousand terabytes of information, enough to store all the movies ever made, trailers included, or about 1.2 billion copies of this book.

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.

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.

All that is really going in your mouth is texture and chemicals. It is your brain that reads these scentless, flavorless molecules and vivifies them for your pleasure. Your brownie is sheet music. It is your brain that makes it a symphony.

One of the most memorably unexpected events I experienced in the course of doing this book came in a dissection room at the University of Nottingham in England when a professor and surgeon named Ben Ollivere (about whom much more in due course) gently incised and peeled back a sliver 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 of character when all it is is a reaction to sunlight. 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.

For each visual input, it takes a tiny but perceptible amount of time—about two hundred milliseconds, one-fifth of a second—for the information to travel along the optic nerves and into the brain to be processed and interpreted. One-fifth of a second is not a trivial span of time when a rapid response is required—to step back from an oncoming car, say, or to avoid a blow to the head. To help us deal better with this fractional lag, the brain does a truly extraordinary thing: it continuously forecasts what the world will be like a fifth of a second from now, and that is what it gives us as the present. That means that we never see the world as it is at this very instant, but rather as it will be a fraction of a moment in the future. We spend our whole lives, in other words, living in a world that doesn’t quite exist yet.

Make no mistake. This is a planet of microbes. We are here at their pleasure. They don’t need us at all. We’d be dead in a day without them.

An interesting thing about touch is that the brain doesn’t just tell you how something feels, but how it ought to feel. That’s why the caress of a lover feels wonderful, but the same touch by a stranger would feel creepy or horrible. It’s also why it is so hard to tickle yourself.

The great paradox of the brain is that everything you know about the world is provided to you by an organ that has itself never seen that world. The brain exists in silence and darkness, like a dungeoned prisoner. It has no pain receptors, literally no feelings. It has never felt warm sunshine or a soft breeze. To your brain, the world is just a stream of electrical pulses, like taps of Morse code. And out of this bare and neutral information it creates for you—quite literally creates—a vibrant, three-dimensional, sensually engaging universe. Your brain is you. Everything else is just plumbing and scaffolding.

In breathing, as in everything in life, the numbers are staggering – indeed fantastical. Every time you breathe, you exhale some 25 sextillion (that’s 2.5 × 1022) molecules of oxygen – so many that with a day’s breathing you will in all likelihood inhale at least one molecule from the breaths of every person who has ever lived.1 And every person who lives from now until the sun burns out will from time to time breathe in a bit of you. At the atomic level, we are in a sense eternal.

Every day, it has been estimated, between one and five of your cells turn cancerous, and your immune system captures and kills them.

Almost three-quarters of the forty million antibiotic prescriptions written each year in the United States are for conditions that cannot be cured with antibiotics.

Just being kind, for instance. A study in New Zealand of diabetic patients in 2016 found that the proportion suffering severe complications was 40 per cent lower among patients treated by doctors rated high for compassion. As one observer put it, that is ‘comparable to the benefits seen with the most intensive medical therapy for diabetes’.

An increase of only a degree or so in body temperature has been shown to slow the replication rate of viruses by a factor of two hundred—an astonishing increase in self-defense from only a very modest rise in warmth.

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.

The history of epilepsy can be summarised as 4,000 years of ignorance, superstition and stigma followed by 100 years of knowledge, superstition and stigma.

Most of us have experienced that abrupt feeling of falling while asleep known as a hypnic or myoclonic jerk. No one knows why we have this sensation. One theory is that it goes back to the days when we slept in trees and had to take care not to fall off. The jerk may be a kind of fire drill. That may seem far-fetched, but it is a curious fact, when you think about it, that no matter how profoundly unconscious we get, or how restless, we almost never fall out of bed, even unfamiliar beds in hotels and the like. We may be dead to the world, but some sentry within us keeps track of where the bed’s edge is and won’t let us roll over it (except in unusually drunk or fevered circumstances). Some part of us, it seems, pays heed to the outside world, even for the heaviest sleepers.

Dreaming may simply be a by-product of this nightly cerebral housecleaning. As the brain clears wastes and consolidates memories, neural circuits fire randomly, briefly throwing up fragmentary images, a bit like someone jumping between television channels when looking for something to watch. Confronted with this incoherent flow of memories, anxieties, fantasies, suppressed emotions, and the like, the brain possibly tries to make a sensible narrative out of it all, or possibly, because it is itself resting, doesn’t try at all, and just lets the incoherent pulses flow past. That may explain why we generally don’t remember dreams much despite their intensity—because they are not actually meaningful or important.

We have body clocks not just in the brain but all over—in our pancreas, liver, heart, kidneys, fatty tissue, muscle, virtually everywhere—and these operate to their own timetables, dictating when hormones are released or organs are busiest or most relaxed. Your reflexes, for instance, are at their sharpest in mid-afternoon, while blood pressure peaks toward evening. Men tend to pump more testosterone early in the morning than later in the day.

The nucleus accumbens, a region of the forebrain associated with pleasure, grows to its largest size in one’s teenage years. At the same time, the body produces more dopamine, the neurotransmitter that conveys pleasure, than it ever will again. That is why the sensations you feel as a teenager are more intense than at any other time of life. But it also means that seeking pleasure is an occupational hazard for teenagers. The leading cause of deaths among teenagers is accidents—and the leading cause of accidents is simply being with other teenagers. When more than one teenager is in a car, for instance, the risk of an accident multiplies by 400 percent.

Sperm pass on none of their mitochondria during conception, so all mitochondrial information is transferred from generation to generation through mothers alone. Such a system means that there will be many extinctions along the way. A woman endows all her children with her mitochondria, but only her daughters have the mechanism to pass it onward to future generations. So if a woman has only sons or no children at all—and that happens quite often, of course—her personal mitochondrial line will die with her. All her descendants will still have mitochondria, but it will come from other mothers on other genetic lines. In consequence, the human mitochondrial pool shrinks a little with every generation because of these localized extinctions. Over time, the mitochondrial pool for humans has shrunk so much that, almost unbelievably but rather wonderfully, we are all now descended from a single mitochondrial ancestor—a woman who lived in Africa about 200,000 years ago. You might have heard her referred to as Mitochondrial Eve. She is, in a sense, mother of us all.

In 2011, an interesting milestone in human history was passed. For the first time, more people globally died from non-communicable diseases like heart failure, stroke and diabetes than from all infectious diseases combined.1 We live in an age in which we are killed, more often than not, by lifestyle. We are in effect choosing how we shall die, albeit without much reflection or insight.

Our eyes contain two types of photoreceptors for vision—rods, which help us see in dim conditions but provide no color, and cones, which work when the light is bright and divide the world up into three colors: blue, green, and red. People who are “color-blind” normally lack one of the three types of cones, so they don’t see all the colors, just some of them. People who have no cones at all, and are genuinely color-blind, are called achromatopes. Their main problem isn’t that their world is pallid but that they really struggle to cope with bright light and can be literally blinded by daylight. Because we were once nocturnal, our ancestors gave up some color acuity—that is, sacrificed cones for rods—to gain better night vision. Much later, primates re-evolved the ability to see reds and oranges, the better to identify ripe fruit, but we still have just three kinds of color receptors compared with four for birds, fish, and reptiles. It’s a humbling fact, but virtually all nonmammalian creatures live in a visually richer world than we do.

Bowel transit time, as it is known in the trade, is a very personal thing and varies widely between individuals, and in fact within individuals depending on how active they are on a given day and what and how much they have been eating. Men and women evince a surprising amount of difference in this regard. For a man, the average journey time from mouth to anus is fifty-five hours. For a woman, typically, it is more like seventy-two. Food lingers inside a woman for nearly a full day longer, with what consequences, if any, we do not know.Roughly speaking, however, each meal you eat spends about four to six hours in the stomach, a further six to eight hours in the small intestine, where all that is nutritious (or fattening) is stripped away and dispatched to the rest of the body to be used or, alas, stored, and up to three days in the colon, which is essentially a large fermentation tank where billions and billions of bacteria pick over whatever the rest of the intestines couldn’t manage—fiber mostly. That’s why you are constantly told to eat more fiber: because it keeps your gut microbes happy and at the same time, for reasons not well understood, reduces the risk of heart disease, diabetes, bowel cancer, and indeed death of all types.

Race is one millimeter deep. Intrepidly attending the dissection of a corpse", Bryson quotes the surgeon who pulled back a minute layer of skin and said: “That’s all that race is – a sliver of epidermis.” As we spread across the world, some people are thought to have evolved lighter skin in order to glean vitamin D from weaker sunlight. Throughout human history, people have “de-pigmented” and “re-pigmented” to suit their environment.Biologically, skin colour is just “a reaction to sunlight”, Bryson quotes the anthropologist Nina Jablonski as saying. She adds: “And yet look how many people have been enslaved or hated or lynched or deprived of fundamental rights through history because of the colour of their skin.

The skin consists of an inner layer called the dermis and an outer epidermis. The outermost surface of the epidermis, 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 remorselessly we turn to dust.

Where America really differs from other countries is in the colossal costs of its health care. An angiogram, a survey by The New York Times found, costs an average of $914 in the United States, $35 in Canada. Insulin costs about six times as much in America as it does in Europe. The average hip replacement costs $40,364 in America, almost six times the cost in Spain, while an MRI scan in the United States is, at $1,121, four times more than in the Netherlands. The entire system is notoriously unwieldy and cost-heavy. America has about 800,000 practicing physicians but needs twice that number of people to administer its payments system. The inescapable conclusion is that higher spending in America doesn’t necessarily result in better medicine, just higher costs.

Even more appallingly, in the United States 80 percent of antibiotics are fed to farm animals, mostly to fatten them. Fruit growers can also use antibiotics to combat bacterial infections in their crops. In consequence, most Americans consume secondhand antibiotics in their food (including even some foods labeled as organic) without knowing it. Sweden banned the agricultural use of antibiotics in 1986. The European Union followed in 1999. In 1977, the Food and Drug Administration ordered a halt to the use of antibiotics for purposes of fattening farm animals, but backed off when there was an outcry from agricultural interests and the congressional leaders who supported them.

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.