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, especially 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 ...