The Body: A Guide for Occupants

by Bill Bryson

the only thing special about the elements that make you is that they make you. That is the miracle of life.

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.8 Think of it: there is enough of you to leave the solar system. You are in the most literal sense cosmic.

DNA exists for just one purpose – to create more DNA. Your DNA is simply an instruction manual for making you.

DNA is extremely stable. It can last for tens of thousands of years.

DNA passes on information with extraordinary fidelity. It makes only about one error per every billion letters copied. Still, that works out at 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.

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.

And how do we celebrate the glory of our existence? Well, for most of us by exercising minimally and eating maximally. 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.

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

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 formal name for the skin is the cutaneous system. Its size is about two square metres (approximately 20 square feet) and all told your skin will weigh somewhere in the region of ten to fifteen pounds, though much depends, naturally, on how tall you are and how much buttock and belly it needs to stretch across.

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.

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.

‘People act as if skin colour 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 colour, 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 colour of their skin.’

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.

When a human body adapts to altered circumstances, the process is known as phenotypic plasticity. We alter our skin colour all the time – when we tan or burn beneath a bright sun or blush from embarrassment.

In 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 (almost three kilos) 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 per cent of people globally are estimated to be vitamin D deficient for at least part of the year.16 In northern climes, it may be as much as 90 per cent.

As people evolved lighter skin, they also developed lighter-coloured eyes and hair – but only pretty recently.17 Lighter-coloured eyes and hair evolved somewhere around the Baltic Sea a...

Hair is unique to mammals. Like the underlying skin, it serves a multitude of purposes: it provides warmth, cushioning and camouflage, shields the body from ultraviolet light, and allows members of a group to signal to each other that they are angry or aroused.

is not easy to think of a way that armpit hair enriches human existence. One line of supposition is that secondary hair is used to trap or disperse (depending on theory) sexual scents, or pheromones. The one problem with this theory is that humans don’t seem to have pheromones.

The idea that all fingerprints are unique is actually a supposition. No one can say for absolute certain that no one else has fingerprints to match yours. All that can be said is that no one has yet found two sets of fingerprints that precisely match.

‘The loss of most of our body hair and the gain of the ability to dissipate excess body heat through eccrine sweating helped to make possible the dramatic enlargement of our most temperature-sensitive organ, the brain.’39 That, she says, is how sweat helped to make you brainy.

It 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 odour. Sweat on its own is actually odourless. It needs bacteria to create a smell. The two chemicals that account for the odour – 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.44

You have about 100,000 microbes per square centimetre of your skin, and they are not easily eradicated. According to one study, the number of bacteria on you actually rises after a bath or shower because they are flushed out from nooks and crannies.45 But even when you try scrupulously to sanitize yourself, it isn’t easy.

Itching (the medical term for the condition is pruritus) is confined to the outer layer of skin and a few moist outposts – eyes, throat, nose, and anus primarily. No matter how else you suffer, you will never have an itchy spleen.

A single parent bacterium could in theory produce a mass of offspring greater than the weight of the Earth in less than two days.5 In three days, its progeny would exceed the mass of the observable universe.6 Clearly that could never happen, but they are with us already in numbers beyond imagining.

If you put all the 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

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.

Colds unquestionably are more frequent in winter than in summer, but that may only be because we spend more time indoors then and are more exposed to others’ leakages and exhalations.

Surprisingly, the least effective way to spread germs (according to yet another study) is kissing.

The only really reliable way to transfer cold germs is physically by touch.

metal poles are a fairly hostile environment for microbes. Where microbes thrive is in the fabrics on seats and on plastic handgrips.

Every bit of penicillin made since that day is descended from that single random cantaloupe.

Fleming warned that microbes could easily evolve resistance to antibiotics if they were carelessly used. Seldom has a Nobel speech been more prescient. IV

Your brain is you. Everything else is just plumbing and scaffolding.

hypothalamus, amygdala, hippocampus, telencephalon, septum pellucidum, habenular commissure, entorhinal cortex and a dozen or so othersfn2 – which collectively are known as the limbic system (from the Latin limbus, meaning ‘peripheral’).

The basal ganglia, for instance, play an important part in movement, language and thought, but it is only when they degenerate and lead to Parkinson’s disease that they normally attract attention to themselves.

Thinking is our most vital and miraculous talent, yet in a profound physiological sense we don’t really know what thinking is.

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 one-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. The brain tricks you in a lot of ways for your own good.

the brain manufactures all the components that make up our senses. It is a strange, non-intuitive fact of existence that photons of light have no colour, sound waves no sound, olfactory molecules no odour.

‘while we have the overwhelming impression that the greenness of the trees and the blueness of the sky are streaming through our eyes as through an open window, yet the particles of light impacting on the retina are colourless, just as the waves of sound impacting on the eardrum are silent and scent molecules have no smell.16 They are all invisible, weightless, subatomic particles of matter travelling through space.’ All the richness of life is created inside your head. What you see is not what is, but what your brain tells you it is, and that’s not the same thing at all.

colour isn’t a fixed reality but a perception.

Memory storage is idiosyncratic and strangely disjointed. The mind breaks each memory into its component parts – names, faces, locations, contexts, how a thing feels to the touch, even whether it is living or dead – and sends the parts to different places, then calls them back and reassembles them when the whole is needed again.19 A single fleeting thought or recollection can fire up a million or more neurons scattered across the brain.20 Moreover, these fragments of memory move around over time, migrating from one part of the cortex to another, for reasons entirely unknown.21 It’s no wonder we get details muddled.

The brain takes a long time to form completely. The wiring in a teenager’s brain is only about 80 per cent completed (which may not come as a great surprise to the parents of teenagers).30 Although most of the growth of the brain occurs in the first two years and is 95 per cent finished by the age of ten, the synapses aren’t fully wired until a young person is in his or her mid to late twenties. That means that the teenage years effectively extend well into adulthood. In the meantime, the person in question will almost certainly have more impulsive, less reflective behaviour than his elders, and will also be more susceptible to the effects of alcohol.

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.

The inescapable fact is that the brain is an unnerving place as well as a marvellous one. There seems to be an almost limitless number of curious or bizarre syndromes and conditions associated with neural disorders. Anton-Babinski syndrome, for instance, is a condition in which people are blind but refuse to believe it. In Riddoch syndrome, victims cannot see objects unless they are in motion. Capgras syndrome is a condition in which sufferers become convinced that those they know well are imposters.45 In Klüver-Bucy syndrome the victim develops urges to eat and fornicate indiscriminately (to the understandable dismay of loved ones).46 Perhaps the most bizarre of all is Cotard delusion, in which the sufferer believes he is dead and cannot be convinced otherwise.

Whole regions of the brain are devoted solely to recognizing faces. We are exquisitely sensitive to the subtlest alterations of mood or expression, even if we are not always conscious of them.

Ekman concluded that six expressions are universal: fear, anger, surprise, pleasure, disgust and sorrow.

The most universal expression of all is a smile, which is rather a nice thought. No society has ever been found that doesn’t respond to smiles in the same way.

True smiles are brief – between two-thirds of a second and four seconds. That’s why a held smile begins to look menacing. A true smile is the one expression that we cannot fake. As the French anatomist G.-B. Duchenne de Boulogne noticed as long ago as 1862, a genuine, spontaneous smile involves the contraction of the orbicularis oculi muscle in each eye and we have no independent control over...