Human Errors

by Nathan H. Lents

  These are just the functional problems with the human eye. Its physical design is riddled with all sorts of defects as well. Some of these contribute to the eye’s functional problems, while others are benign, if befuddling.

  One of the most famous examples of quirky design in nature is the retina of all vertebrates, from fish to mammals. The photoreceptor cells of vertebrate retinas appear to be installed backward—the wiring faces the light, while the photoreceptor faces inward, away from it. A photoreceptor cell looks something like a microphone; the “hot” end has the sound receiver, and the other end terminates in the cable that carries the signal to the amplifier. The human retina, located in the back of the eyeball, is designed such that all of the little “microphones” are facing the wrong way. The side with the cable faces forward, toward the light, while the hot end faces a blank wall of tissue.

  This is not an optimal design for obvious reasons. The photons of light must travel around the bulk of the photoreceptor cell in order to hit the receiver tucked in the back. When you’re speaking into the wrong end of a microphone, you can still make it work, provided that you turn the sensitivity of the microphone way up and you speak loudly, and the same principles apply for vision.

  Furthermore, light must travel through a thin film of tissue and blood vessels before reaching the photoreceptors, adding another layer of needless complexity to this already needlessly complicated system. To date, there are no workable hypotheses that explain why the vertebrate retina is wired backward. It seems to have been a random development that stuck because correcting it would be very difficult to pull off with sporadic mutations—the only tool evolution has in its toolkit.

  This reminds me of the time when I installed a piece of molding called a chair rail—molding that’s placed about halfway up a wall—in my house. It was my first attempt at woodworking, and it didn’t go as well as I had hoped. The long pieces of wood for a chair rail are not symmetrical; you have to choose which is the top surface and which is the bottom, and unlike with crown molding or baseboards, with chair rails, it’s not immediately obvious which is the top and which is the bottom. So I just picked the way I thought looked best and then set about installing it: doing all the measurements, making the cuts, staining the wood, hanging it, sinking the nails, applying wood putty to the seams and nail holes, and staining again. Finally, I was done. The first guest to see my handiwork immediately pointed out that I had installed the chair rail upside down. There was a correct top and bottom and I got it wrong.

  This is a good analogy to the backward installation of the retina because way back in the beginning, the light-sensing patch of tissue that would evolve into the retina could have faced in either direction with little functional difference for the organism. As the eye continued to evolve, however, the light sensors moved inside the cavity that would become the eyeball, and the backward nature of the installation became clear. But it was too late. At that point, what could be done? Flipping the whole structure around could never be achieved through a couple of mutations here and there, just like I could not simply flip my chair rail around; all the cuts and seams would be inverted. There was no way to correct my blunder without starting over completely, and there is no way to correct the backward installation of the vertebrate retina without starting over completely. So I kept the upside-down chair rail, and our ancestors kept their backward retinas.

  The photoreceptors of the cephalopod retina (top) are oriented toward the incoming light, while those of the vertebrate retina (bottom) are not. By the time this suboptimal design became disadvantageous to vertebrates, evolution was powerless to correct it.

  Interestingly, the retina of cephalopods—octopi and squid—is not inverted. The cephalopod eye and the vertebrate eye, while strikingly similar, evolved independently of each other. Nature “invented” the camera-like eye at least twice, once in vertebrates and once in cephalopods. (Insects, arachnids, and crustaceans have an entirely different type of eye.) During the evolution of the cephalopod eye, the retina took shape in a more logical way, with the photoreceptors facing toward the light. Vertebrates were not so lucky, and we are still suffering from the consequences of this evolutionary fluke; most ophthalmologists agree that the backward retina is what makes retinal detachment more common in vertebrates than in cephalopods.

  There is one more design quirk in the human eye that merits mention. Right smack in the middle of the retina, there is a structure called the optic disk; this is where the axons of the millions of photoreceptor cells all converge to form the optic nerve. Imagine the tiny cables from millions of tiny microphones all coming together into one bundle, a cable of cables to carry all the signals to the brain. (The visual center of the brain happens to be in the very back, as far from the eyes as it could possibly be!) The optic disk, located on the surface of the retina, occupies a small circle in which there are no photoreceptor cells. This creates a blind spot in each eye. We don’t notice these blind spots because having two eyes compensates for it; our brains fill in the pictures for us, but they are definitely there. You can find simple demonstrations of this on the Internet by searching for optic disk blind spot.

  The optic disk is a necessary structure insofar as the retinal axons must all converge at some point. A much better design would be to place it deeper in the back of the eye, tucked underneath the retina rather than smack on top of it. However, the backward placement of the retina makes the blind spot unavoidable, and all vertebrates have it. Cephalopods do not, because the right-side-in retina allows the easy placement of the disk behind an unbroken retina.

  Perhaps it would be too greedy to ask for hawks’ eyes, but couldn’t we aspire to those of an octopus at least?

  Nasal Sinuses That Drain Upward

  Just below the eyes, you’ll find another set of evolutionary errors: the nasal sinuses, a meandering collection of air- and fluid-filled cavities, some of them deep inside our heads.

  Many people don’t appreciate just how much open space there is in the skull. When you inhale through your narrow nostrils, the flow of air branches into four pairs of large chambers tucked in the bones of your face; this is where the air comes in contact with mucous membranes. The mucous membranes are highly folded patches of wet and sticky tissue designed to catch dust and other particles, including bacteria and viruses, so that they do not reach your lungs. In addition to trapping particulates, the sinuses are also useful for warming and humidifying the air you breathe.

  The mucous membranes in the nasal sinuses produce a slow and steady flow of sticky mucus. This mucus is swept away by tiny, pulsating, hairlike structures called cilia. (Picture a miniature version of the hairs on your arms constantly swirling in order to push sticky water along your skin.) Inside your head, mucus drains into several spots and is ultimately swallowed and sent to the stomach—the safest place to put the mucus, since the bacteria and viruses it contains can be dissolved and digested by the acid there. The sinus passages, when working properly, keep the mucus flowing, which clears the bacteria and viruses before they can cause infections and prevents mucus from gumming up the whole system.

  Of course, the whole system does get gummed up sometimes, and that can lead to a sinus infection. Bacteria that are not swept along fast enough can set up camp and establish an infectious colony that may spread throughout the sinuses and beyond. Mucus, normally thin and mostly clear, becomes thick, viscous, and dark green when you have an infection. Most infections are not serious, but they aren’t fun either.

  Have you ever noticed that dogs, cats, and other animals don’t seem to have head colds nearly as often as humans do? Most humans suffer between two and five head colds (also called upper respiratory infections) per year, and some of them are accompanied by full-blown sinus infections. In the six years that I have had my dog, however, I haven’t noticed a single episode of a runny or stuffy nose, watery eyes, coughing, or repeated sneezing. He’s never even had a fever that I know of. Sure, dogs can get sinus infections, and the most
common symptom is the easily recognized runny nose. But this is a rare occurrence for them. Most dogs will go their whole lives with no major episodes of infection in their nasal sinuses.*

  Wild animals are similarly free of nasal symptoms. Sinus infections are possible but rare in nonhuman animals, although they are a little more common in primates than in other mammals. Why do humans have it so bad?

  There are a variety of reasons for why we’re so susceptible to sinus infections, but one of them is that the mucous drainage system is not particularly well designed. Specifically, one of the important drainage-collection pipes is installed near the top of the largest pair of cavities, the maxillary sinuses, located underneath the upper cheeks. Putting the drainage-collection point high within these sinuses is not a good idea because of this pesky thing called gravity. While the sinuses behind the forehead and around the eyes can drain downward, the largest and lowest two cavities must drain upward. Sure, there are cilia to help propel the mucus up, but wouldn’t it be easier to have the drainage below the sinuses rather than above them? What kind of plumber would put a drainpipe anywhere but at the bottom of a chamber?

  This poor plumbing is not without consequence. When the mucus becomes thicker, things get sticky, both figuratively and literally. Mucus thickens when it carries a heavy load of dust, pollen, or other particulates or antigens; when the air is cold or dry; or when a bacterial infection is fighting to take hold. During these times, the cilia have much more work to do to get the sludgy mucus to the collection point. If only we had gravity to help with the drainage, like other animals do! Instead, our cilia must work against gravity and the increased viscosity of the thick mucus. They simply can’t keep up and this leads to the nasal symptoms of the common cold. This is also why colds and allergies occasionally trigger secondary bacterial sinus infections; as the mucus pools, bacteria can fester.

  The human maxillary sinus cavity. Because the mucous collection duct is located at the top of the chamber, gravity cannot help with drainage. This is part of the reason why colds and sinus infections are so common in humans but unheard of in other animals.

  The poor location of the drainpipes in the maxillary sinuses also helps to explain why some people with colds and sinus infections can briefly find relief by lying down. When they don’t have to work against gravity, the cilia in the maxillary sinuses can propel some of the thick mucus toward the collecting duct, which relieves some of the pressure. This is no cure, however, and the respite is only temporary. Once a bacterial infection takes hold, drainage alone can no longer combat it, and the bacteria must be defeated by the immune system. In some people, mucous drainage is so poor that only nasal surgery can bring relief from near-constant sinus infections.

  But why is the drainage system at the top of the maxillary sinuses instead of below? The evolutionary history of the human face holds the answer. As primates evolved from earlier mammals, the nasal features underwent a radical change in structure and function. In many mammals, smell is the single most important sense, and the structure of the entire snout was designed to optimize this sense. This is why most mammals have elongated snouts: to accommodate huge air-filled cavities chock-full of odor receptors. As our primate ancestors evolved, however, there was less reliance on smell and more reliance on vision, touch, and cognitive abilities. Accordingly, the snout regressed, and the nasal cavities got smushed into a more compact face.

  The evolutionary rearrangement of the face continued as apes evolved from monkeys. The Asian apes—gibbons and orangutans—simply ditched the upper set of cavities altogether; their lower sinuses are smaller and drain in the direction of gravity. The African apes—chimpanzees, gorillas, and humans—all share the same type of sinuses. However, in the other apes, the sinuses are larger and more cavernous, and they are joined to each other by wide openings, which facilitates unrestricted flow of both air and mucus. Not so with humans.

  Nowhere are there more differences between humans and nonhuman primates than in the facial bones and skull. Humans have much smaller brows, smaller dental ridges, and flatter, more compact faces. In addition, our sinus cavities are smaller and disconnected from one another, and the drainage ducts are much skinnier. Evolutionarily speaking, humans gained nothing by having those drainage pathways squeezed into narrow tubes. This was likely a side effect of making room for our big brains.

  This rearrangement produced a suboptimal design that has left us more susceptible to colds and painful sinus infections than perhaps any other animal. But as far as poor design goes, this evolutionary mishap is nothing compared to what lurks just a bit farther down in the body: a nerve that should drive straight from the brain to the neck but instead takes a few dangerous detours along the way.

  A Runaway Nerve

  The human nervous system is astonishingly intricate and important. Our brains are highly developed, and our nerves make those brains functional.

  Nerves are bundles of tiny individually wrapped cables called axons that convey impulses from the brain to the body (or, for sensory nerves, from the body to the brain). For example, there are motor neurons that live near the top of the brain that send their long axons out of the brain, down the spinal cord, out of the lumbar region, and down the legs to their targets in the big toes. A long route, for sure, but a direct one. There is a web of cranial and spinal nerves that carry their axons from the brain to every muscle, gland, and organ in the body.

  Evolution has left us with some very bizarre defects in that system. Consider just one example, the awkwardly named recurrent laryngeal nerve (RLN). (There are actually a pair of these nerves, one on the left and one on the right, as there are for most nerves in the human body, but for simplicity’s sake, we’ll talk about just the left one.)

  The axons found in the RLN originate near the top of the brain and connect to the muscles of the larynx (also known as the voice box). These muscles, under the direction of the nerve, are what allow us to make and control audible sounds when we speak, hum, and sing.

  You would think that axons that begin in the brain and end in the upper throat would travel a short distance: through the spinal cord, into the throat, and to the larynx. The whole thing could be just a few centimeters long.

  The left vagus nerve and some of the nerves that branch off from it—including the recurrent laryngeal nerve (RLN). Its circuitous route through the chest and neck is an evolutionary throwback to our early vertebrate ancestors, in which a straight path from the brain to the gills went very near to the heart.

  Nope. The axons of the RLN are packaged within a more famous nerve, the vagus. It travels down the spinal cord all the way to the upper chest. From there, the sub-bundle of axons known as the RLN exit the spinal cord a little below the shoulder blade. The left RLN then loops under the aorta and travels back up to the neck, where it reaches the larynx.

  The RLN is more than three times longer than it has to be. It winds through muscles and tissue that it need not. It is one of the nerves that heart surgeons must be very careful with, given how it intertwines with the great vessels from the heart.

  This anatomical oddity has been recognized as far back as the time of the ancient Greek physician Galen. Is there a functional reason for this circuitous route? Almost certainly not. In fact, there is another nerve, the superior laryngeal nerve, that also innervates the larynx and travels the exact route that we would predict. This sub-bundle, which also branches from the larger vagus bundle, leaves the spinal cord just underneath the brain stem and travels the short distance to the larynx. Nice and easy.

  So why does the RLN travel this long, lonely road? Once again, the answer is in ancient evolutionary history. This nerve originated in ancient fish, and all modern vertebrates have it. In fish, the nerve connects the brain to the gills, which were the ancestors of the larynx. However, fish don’t really have necks, their brains are tiny, they don’t have lungs, and their hearts are more like muscular hoses than pumps like ours. Thus, a fish’s central circulatory system, located mo
stly in the space behind its gills, is quite different than a human’s.

  In fish, the nerve makes the short trip from the spinal cord to the gills in a predictable and efficient route. Along the way, however, it weaves through some of the major vessels that exit the fish heart, the equivalent of the branching aortas of mammals. This weaving makes sense in fish anatomy and allows for a compact and simple arrangement of nerves and vessels in a very tight space. But it also paved the way for an anatomical absurdity that would begin to develop as fish evolved into the tetrapods that would eventually give rise to humans.

  During the course of vertebrate evolution, the heart began to move farther back as the body form took on a distinct chest and neck. From fish to amphibians to reptiles to mammals, the heart inched farther and farther away from the brain. But the gills did not. The anatomical position of the human larynx relative to the brain is not that different from the relative position of fish gills to the brain. The RLN should not have been affected by the changing position of the heart—except that it was intertwined with the vessels. The RLN got stuck and was forced to grow into a large loop structure in order to travel from the brain to the neck. Apparently, there was no easy way for evolution to reprogram the embryonic development of this nerve so as to untangle it from the aorta.