Human Errors Read online

  Contents

  * * *

  Title Page

  Contents

  Copyright

  Dedication

  Introduction: Behold the Blunders of Nature

  Pointless Bones and Other Anatomical Errors

  Our Needy Diet

  Junk in the Genome

  Homo sterilis

  Why God Invented Doctors

  A Species of Suckers

  Epilogue: The Future of Humanity

  Acknowledgments

  Notes

  Index

  About the Author

  Connect with HMH

  Footnotes

  Copyright © 2018 by Nathan H. Lents

  All rights reserved

  For information about permission to reproduce selections from this book, write to [email protected] or to Permissions, Houghton Mifflin Harcourt Publishing Company, 3 Park Avenue, 19th Floor, New York, New York 10016.

  hmhco.com

  Library of Congress Cataloging-in-Publication Data

  Names: Lents, Nathan H., author.

  Title: Human errors : a panorama of our glitches, from pointless bones to broken genes / Nathan H. Lents.

  Description: Boston : Houghton Mifflin Harcourt, 2018. | Includes bibliographical references and index.

  Identifiers: LCCN 2017046396 (print) | LCCN 2017057190 (ebook) | ISBN 9781328974693 (hardback) | ISBN 9781328974679 (ebook)

  Subjects: LCSH: Human physiology. | Human evolution. | BISAC: SCIENCE / Life Sciences / Human Anatomy & Physiology. | SCIENCE / Life Sciences / Evolution. | SCIENCE / Life Sciences / Biology / General.

  Classification: LCC QP34.5 .L467 2018 (print) | LCC QP34.5 (ebook) | DDC 612—dc23

  LC record available at https://lccn.loc.gov/2017046396

  Illustrations by Donald Ganley/© Houghton Mifflin Harcourt

  Charts © Houghton Mifflin Harcourt

  Cover design by Christopher Moisan

  Cover illustration by Harry Campbell

  Author photograph © Arpi Pap at Pap Studio

  v1.0418

  Now, there’s a topic you know a lot about!

  —My mother upon learning I was writing a book about human flaws

  Introduction:

  Behold the Blunders of Nature

  Here is a story you’ve heard numerous times: Behold the incredible beauty, complexity, and greatness of the human body and its many systems, organs, and tissues! It seems that the deeper into our bodies we look, the more beauty we find. Like the layers of an onion, the cells and molecules that make up the human body have seemingly infinite levels of complexity. Human beings enjoy a rich world of the mind, perform astoundingly complex physical tasks, digest food material and then commingle it with their own matter and energy, effortlessly turn genes on and off, and every now and then produce whole new individuals in “endless forms most beautiful.”

  Somehow, all of these processes come together to create the wondrous complexity of human life while allowing us to remain oblivious to the underlying mechanisms. A perfectly ordinary human can sit down and play “Piano Man” without ever having to think about the cells and the muscles of her hands, the nerves in her arms, or the brain centers where the information to play the piece are stored. A second human can sit and listen to the song without ever bothering to contemplate the vibrations of his eardrum, the conductance of nerve impulses to his auditory processing center, or the memory recall that allows him to belt out the refrain, however imperfectly. The song itself was composed by another such human (albeit an exceptional one), a person who I dare to suggest had little appreciation for the genes, proteins, and neurons that were hard at work as he did so.

  Despite the fact that we often take them for granted, the capabilities of the human body are simply wondrous—miraculous even. So why not write a book about that?

  Because you’ve heard about it many times. Those books have already been written. If you want a book about the glorious intricacy of the human body, you are in luck—simply walk into any medical library, and you will find tens of thousands of volumes. If you count biomedical journals, where new discoveries are announced, the number of accolades to the greatness of the human form rises into the tens of millions. There is no shortage of words and pages dedicated to how well the body usually works.

  This is not that story. This is a story of our many flaws, from head to toe.

  As it turns out, our flaws are extremely interesting and informative. By exploring human shortcomings, we can peer into our past. Each and every flaw discussed in this book tells a story about our species’ evolutionary history. Every cell, every protein, and every letter in our DNA code has been subjected to the harshness of natural selection over the fullness of evolutionary time. All of that time and all of that selection has resulted in a body form that is fantastically robust, strong, resilient, clever, and mostly successful in the great rat race of life. But it is not perfect.

  We have retinas that face backward, the stump of a tail, and way too many bones in our wrists. We must find vitamins and nutrients in our diets that other animals simply make for themselves. We are poorly equipped to survive in the climates in which we now live. We have nerves that take bizarre paths, muscles that attach to nothing, and lymph nodes that do more harm than good. Our genomes are filled with genes that don’t work, chromosomes that break, and viral carcasses from past infections. We have brains that play tricks on us, cognitive biases and prejudices, and a tendency to kill one another in large numbers. Millions of us can’t even reproduce successfully without a whole lot of help from modern science.

  Our flaws illuminate not only our evolutionary past but also our present and future. Everyone knows that it is impossible to understand current events in a specific country without understanding the history of that country and how the modern state came to be. The same is true for our bodies, our genes, and our minds. In order to fully grasp any aspect of the human experience, we must understand how it took shape. To appreciate why we are the way that we are, we must first appreciate what we once were. To twist the old saying a bit, we can’t understand where we are now if we don’t know where we came from.

  Most of the human design flaws that I describe in this book fall into one of three categories. First, there are aspects of our design that evolved in a different world than the one we now inhabit. Evolution is messy and takes time. Our species’ tendency to gain weight easily and lose weight only with difficulty made very good sense in the Pleistocene savannas of Central Africa but not so much in a twenty-first-century developed nation.

  The next category of flaws includes those of incomplete adaptation. For example, the human knee is the product of a redesign that took place as our ancestors gradually shifted from a quadrupedal posture and an arboreal lifestyle to a bipedal posture and a mostly terrestrial lifestyle. Most of the various components of the knee adapted very well to the new demands placed on this crucial joint, but not all of the kinks were worked out. We are almost fully adapted to upright walking—but not quite.

  The third category features those human defects that are due to nothing more than the limits of evolution. All species are stuck with the bodies that they have and they can advance only through the tiniest changes, which occur randomly and rarely. We inherited structures that are horrendously inefficient but impossible to change. This is why our throats convey both food and air through the same tiny space and why our ankles have seven pointless bones sloshing around. Fixing either of those poor designs would require much more than one-at-a-time mutations could ever accomplish.

  A good example of the tremendous constraints of evolution even during episodes of great innovation is the vertebrate wing. Wings have been invented in many separate lineages. The wings of bats, birds, and pte
rosaurs all evolved separately and therefore have big structural differences. However, in all of those cases, the wing evolved from a forelimb. Those animals lost many uses of their forelimbs in order to get wings. Neither birds nor bats can grasp things very well. They have to crudely use their feet and mouths to manipulate objects. It would have been far better for those animals to grow wholly new wings while retaining their forelimbs, but evolution rarely works that way. For an animal with a complex body plan, growing new limbs is not an option, but slowly reshaping existing limbs is. Evolution is a constant game of tradeoffs. Most innovations come with a cost.

  Evolutionary innovations are as varied as they are pricey. They range from copying errors in the blueprints inside each cell to glaring design defects in the assembly of bones, tissues, and organs. In this book, I will address each of these categories of errors in turn, looking at whole sets of flaws that share the same general themes and that, taken together, tell an incredible story about how evolution works, what happens when it doesn’t, and the high price our species has paid for these adaptations over the millennia.

  Human anatomy is a clumsy hodgepodge of adaptations and maladaptations. We have pointless bones and muscles, underwhelming senses, and joints that don’t quite keep us upright. Then there’s our diet. Whereas most animals do just fine eating the same thing day in and day out, we humans have to have ridiculously varied meals in order to get all the nutrients we need. Most of the contents of our genomes are completely useless, and occasionally they are actually harmful. (We even carry around thousands of dead viruses tucked in the DNA of every one of our cells, and we spend our lifetimes dutifully replicating these carcasses.) And there are yet other, even more astounding imperfections: We are incredibly inefficient at our ultimate goal of making more of ourselves, and we have immune systems that attack our own bodies, just one of many design-related diseases. Even what is arguably our crowning evolutionary achievement—the powerful human brain—is filled with defects that lead individuals to make extremely poor choices in their daily lives, sometimes at the expense of their very existence.

  But as strange as it may sound, there is beauty in our imperfections. How boring would our lives be if each of us were a purely rational, perfect specimen? Our flaws are what make us who we are. Our individuality comes from tiny variations in our genetic and epigenetic codes, and much of this diversity arises from the haphazard insults of mutation. Mutations, like lightning strikes, are random and often destructive, but they are also, somehow, the source of all human greatness. The flaws discussed in this book are scars of battles won in the great struggle for survival. We are the unlikely survivors of this endless evolutionary conflict, the products of four billion years of dogged perseverance in the face of great odds. The history of our flaws is a war story unto itself. Gather round and listen.

  1

  Pointless Bones and Other Anatomical Errors

  Why the human retina is installed backward; why one of our mucous drains is located near the top of a sinus cavity; why our knees are so bad; why the disks of cartilage between our vertebrae sometimes “slip”; and more

  We love to admire physical excellence. We can’t get enough of massive bodybuilders, graceful ballerinas, Olympic sprinters, shapely swimsuit models, and hardy decathletes. In addition to its innate beauty, the human body is also dynamic and resilient. The carefully orchestrated functions of the heart, lungs, glands, and GI tract are truly impressive, and we continue to discover the elaborate intricacies through which the body maintains its health despite the onslaught of a changing environment. Any discussion of the shortcomings of our physical form must first begin with an acknowledgment that the beauty and capability of the human body far outshines the few odd quirks here and there.

  But quirks there definitely are. Lurking in our anatomy are some odd arrangements, inefficient designs, and even outright defects. Mostly, these are fairly neutral; they don’t hinder our ability to live and thrive. If they did, evolution would have handled them by now. But some are not neutral, and each has an interesting tale to tell.

  Over millions of generations, human bodies morphed tremendously. Most of our species’ various anatomical structures were transformed in that metamorphosis, but a few were left behind and exist now purely as anachronisms, the whispers of days long gone. For instance, the human arm and the bird wing perform totally different functions but have striking structural similarities in the scaffolding of their bones. That’s no coincidence. All quadruped vertebrates have the same basic skeletal chassis, modified as much as possible for each animal’s unique lifestyle and habitat.

  Through the random acts of mutation and the pruning of natural selection, the human body has taken shape, but it’s not a perfect process. A close inspection of our mostly beautiful and impressive bodies reveals mistakes that got caught in one of evolution’s blind spots—sometimes literally.

  I Can’t See Clearly Now

  The human eye is a good example of how evolution can produce a clunky design that nonetheless results in a well-performing anatomical product. The human eye is indeed a marvel, but if it had been designed from scratch, it’s hard to imagine it would look anything like it does now. Inside the human eye is the long legacy of how light-sensing slowly and incrementally developed in the animal lineage.

  Before we consider the puzzling physical design of the eye, let me make one thing clear: The human eye is fraught with functional problems as well. For instance, many of the people who are reading this book right now are doing so only with the aid of modern technology. In the United States and Europe, 30 to 40 percent of the population have myopia (nearsightedness) and require assistance from glasses or contact lenses. Without them, their eyes do not focus light properly, and they cannot make out objects that are more than a few feet away. The rate of myopia increases to more than 70 percent of the population in Asian countries. Nearsightedness is not caused by injury. It’s a design defect; the eyeball is simply too long. Images focus sharply before they reach the back of the eye and then fall out of focus again by the time they finally land on the retina.

  Humans can also be farsighted. There are two separate conditions that cause this, each resulting from a different design flaw. In one, hyperopia, the eyeballs are too short, and the light fails to focus before hitting the retina. This is the anatomical opposite of myopia. The second condition, presbyopia, is age-related farsightedness caused by the progressive loss of flexibility of the lens of the eye, the failure of the muscles to pull on the lens and focus light properly, or both. Presbyopia, which literally translates as “old-man sight,” begins to set in around age forty. By the age of sixty, virtually everyone has difficulty making out close objects. I’m thirty-nine, and I have noticed that I hold books and newspapers farther and farther from my face each year. The time for bifocals is nigh.

  Add to these common eye issues others such as glaucoma, cataracts, and retinal detachment (just to name a few), and a pattern begins to emerge. Our species is supposed to be the most highly evolved on the planet, but our eyes are rather lacking. The vast majority of people will suffer significant loss of visual function in their lifetimes, and for many of them, it starts even before puberty.

  I got glasses after my first eye exam, when I was in the second grade. Who knows how long I had actually needed them? My vision isn’t just a little blurry. It’s terrible—somewhere around 20/400. Had I been born before, say, the 1600s, I would probably have gone through life unable to do anything that required me to see farther than arm’s length. In prehistory, I would have been worthless as a hunter—or a gatherer, for that matter. It’s unclear if and how poor vision affected the reproductive success of our forebears, but the rampant nature of poor vision in modern humans argues that excellent vision was not strictly required to succeed at least in the most recent past. There must have been ways that early humans with poor vision could have thrived.

  Human vision is even more pitiable when compared with the excellent vision of most birds, espe
cially birds of prey such as eagles and condors. Their visual acuity at great distances puts even the sharpest human eyes to shame. Many birds can also see a broader range of wavelengths than we can, including ultraviolet light. In fact, migrating birds detect the North and South Poles with their eyes. Some birds literally see the Earth’s magnetic field. Many birds also have an additional translucent eyelid that allows them to look directly into the sun at length without damaging their retinas. Any human attempting to do the same would most likely suffer permanent blindness.

  And that’s just human vision during the day. Human night vision is, at best, only so-so, and for some of us it is very poor. Compare ours with cats’, whose night vision is legendary. So sensitive are cats’ eyes that they can detect a single photon of light in a completely dark environment. (For reference, in a small, brightly lit room, there are about one hundred billion photons bouncing around at any given moment.) While some photoreceptors in human retinal cells are apparently able to respond to single photons, these receptors cannot overcome background signaling in the eye, which leaves humans functionally incapable of sensing just one photon and thus unable to perform the sorts of visual feats that cats pull off so easily. For a human to achieve conscious perception of the faintest possible flash of light, she needs five or ten photons delivered in rapid succession, so cats’ vision is substantially better than humans’ in dim conditions. Furthermore, human visual acuity and image resolution in dim light is far worse than that of cats, dogs, birds, and many other animals. You might be able to see more colors than dogs can, but they can see at night more clearly than you.

  Speaking of color vision, not all humans have that either. Somewhere around 6 percent of males have some form of colorblindness. (It’s not nearly as common in females because the screwed-up genes that lead to colorblindness are almost always recessive and on the X chromosome. Because females have two X chromosomes, they have a backup if they inherit one bum copy.) Around seven billion people live on this planet, so that means that at least a quarter of a billion humans cannot appreciate the same palette of colors that the rest of the species can. That’s almost the population of the United States.