The Body, page 2
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.
2 THE OUTSIDE: SKIN AND HAIR
Beauty is only skin deep, but ugly goes clean to the bone.
—DOROTHY PARKER
I
IT MAY BE slightly surprising to think it, but our skin is our largest organ, and possibly the most versatile. It keeps our insides in and bad things out. It cushions blows. It gives us our sense of touch, bringing us pleasure and warmth and pain and nearly everything else that makes us vital. It produces melanin to shield us from the sun’s rays. It repairs itself when we abuse it. It accounts for such beauty as we can muster. It looks after us.
The formal name for the skin is the cutaneous system. Its size is about two square meters (approximately twenty 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. It is thinnest on the eyelids (just one-thousandth of an inch thick) and thickest on the heels of our hands and feet. Unlike a heart or a kidney, skin never fails. “Our seams don’t burst, we don’t spontaneously sprout leaks,” says Nina Jablonski, professor of anthropology at Penn State University, who is the doyenne of all things cutaneous.
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.
Skin flakes are properly called squamae (meaning “scales”). We each trail behind us about a pound of dust every year. If 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.
Beneath the epidermis is the more fertile dermis, where reside all the skin’s active systems—blood and lymph vessels, nerve fibers, the roots of hair follicles, the glandular reservoirs of sweat and sebum. Beneath that, and not technically part of the skin, is a subcutaneous layer where fat is stored. Though it may not be part of the cutaneous system, it’s an important part of your body because it stores energy, provides insulation, and attaches the skin to the body beneath.
Nobody knows for sure how many holes you have in your skin, but you are pretty seriously perforated. Most estimates suggest you have somewhere in the region of two to five million hair follicles and perhaps twice that number of sweat glands. The follicles do double duty: they sprout hairs and secrete sebum (from sebaceous glands), which mixes with sweat to form an oily layer on the surface. This helps to keep skin supple and to make it inhospitable for many foreign organisms. 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. It appears to be related to the Greek acme, denoting a high and admirable achievement, which a faceful of pimples most assuredly is not. How the two became twinned is not at all clear. The term first appeared in English in 1743 in a British medical dictionary.
Also packed into the dermis are a variety of receptors that keep us literally in touch with the world. If a breeze plays lightly on your cheek, it is your Meissner’s corpuscles that let you know.* When you put your hand on a hot plate, your Ruffini corpuscles cry out. Merkel cells respond to constant pressure, Pacinian corpuscles to vibration.
Meissner’s corpuscles are everyone’s favorites. They detect light touch and are particularly abundant in our erogenous zones and other areas of heightened sensitivity: fingertips, lips, tongue, clitoris, penis, and so on. They are named after a German anatomist, Georg Meissner, who is credited with discovering them in 1852, though his colleague Rudolf Wagner claimed that he in fact was the discoverer. The two men fell out over the matter, proving that there is no detail in science too small for animosity.
All are exquisitely fine-tuned to let you feel the world. A Pacinian corpuscle can detect a movement as slight as 0.00001 millimeter, which is practically no movement at all. More than this, they don’t even require contact with the material they are interpreting. As David J. Linden points out in Touch, if you sink a spade into gravel or sand, you can feel the difference between them even though all you are touching is the spade. 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 are much better than men at tactile sensitivity with fingers, but possibly just because they have smaller hands and thus a more dense network of sensors. 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.
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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.”
A tall, elegant woman with silvery hair cut short, Jablonski works in a very tidy office on the fourth floor of the anthropology building on the Penn State campus, but her interest in skin came about almost thirty years ago when she was a young primatologist and paleobiologist at the University of Western Australia in Perth. While preparing a lecture on the differences between primate skin color and human skin color, she realized there was surprisingly little information on the subject and embarked on what has become a lifelong study. “What began as a small, fairly innocent project ended up taking over a big part of my professional life,” she says. In 2006, she produced the highly regarded Skin: A Natural History and followed that six years later with Living Color: The Biological and Social Meaning of Skin Color.
Skin color turned out to be more scientifically complicated than anyone imagined. “Over 120 genes are involved in pigmentation in mammals,” says Jablonski, “so it is really hard to unpack it all.” What we can say is this: skin gets its color from a variety of pigments, of which the most important by far is a molecule formally called eumelanin but known universally as melanin. It is one of the oldest molecules in biology and is found throughout the living world. It doesn’t just color skin. It gives birds the color of their feathers, fish the texture and luminescence of their scales, squid the purply blackness of their ink. It is even involved in making fruits go brown. In us, it also colors our hair. Its production slows dramatically as we age, which is why older people’s hair tends to turn gray.
“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 they moved into northern latitudes. It therefore became a great advantage to have lighter skin, to synthesize extra vitamin D.
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 name for sunburn is erythema. Pregnant women frequently undergo a darkening of the nipples and areolae, and sometimes of other parts of the body such as the abdomen and face, as a result of increased production of melanin. The process is known as melasma, but its purpose is not understood. 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 of people end up in places where sun levels and skin tones don’t get along at all. 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 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.
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As people evolved lighter skin, they also developed lighter-colored eyes and hair—but only pretty recently. 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.
