The left recurrent laryngeal nerve (RLN) loops under the aorta in all vertebrates. Therefore, the RLN of sauropod dinosaurs would have been incredibly long.
The result of this is that the RLN forms a long, unnecessary loop in the human neck and upper chest. While this may not seem like a huge deal, consider that all of the tetrapod vertebrates are stuck with the same anatomical arrangement inherited from a common ancestor, bony fishes. The RLN of the ostrich should need to travel only two to three centimeters to do its job, but instead, it travels nearly a full meter down the spinal cord, then a full meter back up the neck. The RLN of the giraffe can be up to five meters long! Of course, that is nothing compared with how long the RLN must have been in the apatosaurus, brachiosaurus, and other sauropods. Maybe we shouldn’t scorn our own relatively puny RLN after all.
Pains in the Neck
A runaway nerve is just one thing that is messed up with the human neck. Really, the whole neck is a bit of a disaster. For starters, it’s very poorly protected, especially compared to the protection other important areas get. Just above the neck, the brain is kept in a thick, rigid housing that can withstand a substantial degree of trauma. Below the neck, the heart and lungs are protected by a strong but flexible rib cage anchored by a flat chest plate that is similarly sturdy. Evolution went to a lot of trouble to protect the brain and the cardiopulmonary system but left the connections between them totally vulnerable. (It also failed to protect our visceral organs well, but that is a story for another day.)
It is very difficult for someone to do great harm to your brain or heart with his bare hands, but your neck can be snapped with one swift motion. This weakness is not unique to humans, but humans do have special problems. For example, the vertebrae that are so good at allowing smooth movement as we twist and turn our necks are also easily dislocated. The trachea—the tube through which fresh air gets to the lungs—rests just underneath a thin layer of skin in the front of the neck and can be pierced by even a dull point with little force. The human neck is just a glaring vulnerability.
An even more basic flaw in the neck is the fact that there is one tube, from the opening of the mouth until about halfway down the neck, that is common to both the digestive and respiratory systems. The throat conveys both food and air—what could possibly go wrong? While this, too, isn’t an exclusively human problem—the throat is a nearly universal structure in birds, mammals, and reptiles—that doesn’t make it less of a flaw. In fact, this universal poor design demonstrates the physical constraints that evolution has to work with. Mutations are good for making small incremental tweaks but they cannot be used to execute full-scale redesign. Most of the higher animals get their food and air through the same tube. Having totally separate anatomical structures for digestion and respiration would make much more sense in terms of hygiene, immune defense, and the general maintenance of these very different systems—but evolution came up with a different, less sensible solution for many animals, humans included.
For breathing, especially, our bodies are supremely underequipped. Air passes down a single tube in the throat that then divides into dozens of branches in the lungs. These branches terminate in tiny dead ends full of air sacs that allow the exchange of gases across a thin membrane. The pathway for expired air is precisely the reverse. Air comes in and out like ocean tides through all those branches, hence humans are termed tidal breathers. This is horribly inefficient because there is a great deal of stale air left in the lungs when fresh air is brought in. These mix, diluting the oxygen content of the air that actually reaches the lungs. This burden of stale air in the lungs limits oxygen delivery, and so we must overcome this by breathing deeper, particularly during moments of peak demand, like exercise.
To get an exaggerated sense of the extra work humans must do because of tidal breathing, try breathing through a tube or a hose. But don’t try it for too long, because if the tube is more than a few feet, you will slowly suffocate, no matter how deeply you breathe. If you have ever snorkeled, you have experienced the same exaggerated effects of tidal breathing. Even when floating quietly, making only gentle motions with their legs and arms, snorkelers must breathe deeply to remain comfortable. Every breath is a mixture of stale and fresh air. The longer the path, the more stale air left behind at the end of every breath.
There is a much better way to breathe. In many birds, the airway splits into two lanes of traffic before it reaches the respiratory sacs. Inbound air heads directly to the lungs, not mixing with stale air. Stale air is collected into an out spout and travels upward, joining with the trachea only high in the throat. One-way flow of traffic into the lungs ensures that each breath brings in mostly fresh air. This is a much more efficient design and allows birds to take far shallower breaths than we do to deliver the same amount of fresh air into their bloodstream. This is a critical improvement for birds, because flying demands massive quantities of oxygen.
Of course, the biggest danger in the human throat’s design is not suffocating, but choking. Nearly five thousand Americans choked to death in 2014, the majority of them choking on food. If we had separate openings for air and food, this would never happen. Cetaceans—whales and dolphins—have blowholes, a powerful innovation that provides a dedicated conduit for air. Many birds and reptiles also have a superior design for breathing in that their nostrils convey air directly to the lungs rather than merging with the throat. This is why snakes and some birds can continue to breathe even while they are slowly working on swallowing a huge meal. Humans and other mammals have no such apparatus; when we are swallowing, we have to stop breathing momentarily.
It also doesn’t help that the human instinctual physical reaction when startled is to gasp. That in and of itself is an example of poor design. What is the benefit of suddenly and forcefully pulling in a large breath of air when you are frightened or receive surprising news? There’s no upside to that, and if there is food or liquid in your mouth at that moment, it can create a big problem.
Even though all mammals can get foreign bodies lodged in their tracheae, humans are particularly prone to choking because of some very recent evolutionary changes in our species’ neck anatomy. In other apes, the larynx is substantially lower in the neck than ours. This design allows for a longer throat, giving more room for the muscles involved in swallowing to do their work. In all mammals, during swallowing, a flap of cartilage (called the epiglottis) must slap down over the opening of the trachea to cover it so food heads to the stomach, not the lungs. Of course, this usually works just fine, but not always, and recently the human voice box has been drifting upward, shortening the throat and tightening the space in which the delicate dance of swallowing is done.
Most scientists believe that the larynx has migrated high in the neck of the modern human to enhance vocalizations. With shallower throats, humans are able to contort their soft palates in ways that other apes cannot, giving us a far richer toolkit for making sounds. Indeed, many of the vowel sounds found throughout world languages today are made possible only by our species’ unique throat. There is even one specific sound, the throat click (made by the tight puckering of the back of the throat), that only humans can make and that is a standard part of many sub-Saharan African languages. While it’s a bit too strong to say that our throats evolved purely or mostly to enable this click sound, it was one of a variety of vocalizations that were made possible by the gradual elevation of the voice box.
But these unique vocal powers came at a cost. The rise in the larynx meant the squishing of the throat, causing swallowing to be a much more error-prone affair. For babies, swallowing can be truly hazardous because there is just not much room in their tiny throats to accomplish the complicated and coordinated muscle contractions that this basic act involves. Anyone who has cared for infants or toddlers knows that they choke on their food and drink constantly, but that doesn’t happen much in other young animals.
Swallowing is a good example of the limits of Darwinian evolution. The human throat
is simply too complex for a random mutation—the basic mechanism of evolution—to undo its fundamental defects. We have to resign ourselves to the absurdity of taking in air and food through the same pipe.
A different evolutionary dynamic helps to explain the next design flaw, which also concerns one of the most elementary human activities: moving around on two feet. Here, the issue isn’t that evolution can’t solve the problem but that it simply hasn’t—at least, not yet. The problem is due to incomplete adaptation. Nowhere is this clearer than in the human knee.
Knuckle-Walkers
Whereas other primates move about on all four limbs, humans walk on two legs; this is called bipedalism. If you watch gorillas, chimps, and orangutans when they’re not swinging from trees, they amble about using their feet and their knuckles. Sure, they can stand up on two legs and clumsily walk that way for short stretches, but it is not comfortable for them, and they’re not good at it. Human anatomy, however, has evolved to support our species’ standing upright, mostly by way of changes in the legs, pelvis, and vertebral column. We move much faster this way, and it’s inefficient to move around on four limbs. We must have perfected the bipedal posture by now, right?
Not so much. The anatomical adaptation to upright walking never quite finished in humans. We have several defects that are the result of this failure to complete the process. For example, the intestines and other visceral organs are held together with thin sheets of connective tissue called mesenteries. Mesenteries are elastic and act to keep the gut loosely in place. However, these thin sheets are not suspended from the top of the abdominal cavity, as would make sense for a bipedal posture. Instead, they are attached to the back of the abdominal cavity, like they are in the other apes. That makes good sense for our quadrupedal cousins, but it’s a poor design for us and causes occasional problems.
People who sit upright with little movement for long periods strain these mesenteries, which can then tear, requiring surgery. This defect hasn’t yet been corrected by evolution because the selective pressure to fix it is quite low; before truck driving and desk jobs rolled around, torn mesenteries were probably quite rare. Still, this is poor design, leading to an unnecessary convolution of connective tissue in our abdomens.
The bones and ligaments of the human knee, with the kneecap (patella) removed to reveal the anterior cruciate ligament (ACL). Our incomplete adaptation to bipedalism has forced this relatively skinny ligament to endure much more strain than it is designed for, which is why humans—athletes especially—suffer torn ACLs so often.
There are more serious examples as well. Have you heard of the anterior cruciate ligament? If you are a sports fan, you have; the tearing of this ligament (referred to as the ACL) is one of the most frequent sports injuries. Probably most common in football, torn ACLs also occur in baseball, soccer, basketball, track and field, gymnastics, tennis—basically all high-impact, fast-paced sports. Located in the middle of the knee, the ACL connects the femur (thighbone) to the tibia (shinbone) and is located underneath the patella (kneecap), deep inside the joint. It does most of the work of holding the upper leg and lower leg together.
The natural postures of a standing ape and a standing human. Because of our erect bipedal posture, humans rely on our leg bones to bear most of our weight when standing and walking. Apes, on the other hand, often employ a bent-leg posture, which recruits muscles to share the burden.
The ACL is vulnerable to tearing in humans because our upright, bipedal posture forces it to endure much more strain than it is designed to. In quadrupeds, the strain of running and jumping is spread among four limbs, and the limb muscles absorb most of it. Once our ancestors transitioned to bipedalism, however, the strain was spread over two legs instead of four. This was too much for the muscles by themselves, so our bodies recruited the leg bones to help with the strain. The result was that human legs became straightened so that the bones, rather than the muscles, could bear most of the impact. Compare a standing human with a standing ape: a human’s legs are fairly straight, while an ape’s legs are bowlegged and usually bent.
This straight-leg arrangement works out okay for normal walking and running. But for sudden shifts in direction or momentum—when you’re running and then stop short or when you make a sharp turn at high speed—the knees must bear the force of this sudden, intense strain. Sometimes, the ACL is simply not strong enough to hold the leg bones together as they twist or pull away from each other, and it tears.
To make matters worse, we as a species are getting heavier, so it’s even harder for the ACL to withstand the strain put on it during those sudden shifts. This is especially true for athletes, who now weigh more than ever before and who also make lots of sudden high-speed weight shifts. You may have noticed that ACL injuries have become more common in the world of professional sports as athletes get ever larger.
Short of losing weight, we can’t do much about this problem. It is not possible to isolate the ACL and strengthen it with exercise. It is what it is. Repeated strain doesn’t make it stronger; it makes it weaker. As if that weren’t bad enough, when the ACL is torn, it must be repaired surgically. Knee surgery demands a long recovery and rehabilitation period because ligaments are not very vascular—that is, they are fed by very few blood vessels and have very few of the cells that normally do the work of healing and rebuilding tissues. This is why ACL tears are among the most feared injuries in professional sports. A torn ACL usually means a full season lost.
The Achilles tendon holds another story about our imperfect evolution. No other nonskeletal structure underwent as dramatic a change during our species’ transition to upright walking than this very conspicuous tendon. As our ancestors gradually shifted their weight from the balls of their feet to their heels, the Achilles tendon—which connects the calf muscle to the heel of the foot—found itself with much more work to do. A dynamic sinew, it responded well and is now the most visible feature of the human ankle. It has expanded dramatically to take on its demanding new role, and it reacts to both endurance exercise and strength training by becoming stronger still. The Achilles tendon is a workhorse.
However, by taking on most of the strain of the ankle joint, the Achilles tendon has become the Achilles’ heel of the entire joint, if you’ll pardon the cliché. Injuries to the Achilles tendon are another one of the most frequent sports injuries, and there is no built-in redundancy for this tendon as there is in other joints. To add to the problems, the tendon is exposed prominently on the back of the leg, unprotected.
If the tendon is injured, even walking is impossible. The poorness of this design can be summed up in the observation that the function of the entire joint rests on the actions of its most vulnerable part. A modern mechanical engineer would never design a joint with such an obvious liability.
The knees and ankles aren’t the only structures that underwent redesigns as our ancestors started to walk upright. The back also had to adjust. Ironically, as posture straightened, the back had to become curvier, particularly the lower back, which took on a pretty sharp concave shape in order to help transmit upper-body weight to the pelvis and legs evenly. Evolution even added bones to the lower back to allow for the sharper curve. Because of this curve, however, the lower back has to flex when you stand erect for long periods and it can fatigue. Lower-back pain is a common complaint among those whose jobs require them to stand in one place for hours.
Lower-back fatigue is mild compared to other problems we can have with our backs, and some of them are caused directly by design flaws. All vertebrates have disks of cartilage that lubricate the joints between the vertebrae in the spinal column. These disks are solid but compressible to absorb shock and strain. They have the consistency of firm rubber and allow the spine to be flexible while remaining strong. In humans, though, these disks can “slip” because they are not inserted in a way that makes sense given our species’ upright posture.
In all vertebrates except us, the spinal disks are positioned in line with the nor
mal posture of that animal. For example, the spinal columns of fish endure completely different kinds of strain than the spinal columns of mammals. The fish uses its backbone to stiffen its body and then pulls against it in a side-to-side motion in order to swim. But fish don’t have to worry much about gravity and shock absorption since they are suspended in water. Mammals, however, must use limbs to hold their body weight, and those limbs must attach to the spinal column. Different mammals have different postures and so require different strategies for weight distribution via the spine. In almost all of the tremendously diverse spinal columns found in nature, the spinal disks have adapted to the posture and gait of the animal. But not in ours.
A herniated disk of cartilage in the human spinal column. As our ancestors adopted a more upright posture, the lumbar area of the vertebral column became sharply curved. The disks of cartilage between each vertebra are not optimally placed for this upright, curved posture; as a result, they sometimes “slip,” leading to this painful condition.
In humans, the vertebral disks are in an arrangement that is optimal for knuckle-draggers, not upright walkers. They still do a decent job of lubricating and supporting the spine, but they are much more prone to being pushed out of position than the vertebral disks of other animals. They are structured to resist gravity by pulling the vertebral joints toward the chest, as if humans were on all fours. With our upright posture, however, gravity often pulls them backward or downward, not toward the chest. Over time, this uneven pressure creates protuberances in the cartilage. This is known as a spinal disk herniation or, more commonly, a “slipped disk.” Spinal disk herniation is nearly unheard of in any primate species but us.
Human Errors Page 3