Our ancestors began walking upright about six million years ago. It was one of the first physical changes as they diverged from other apes. It’s disappointing, although not altogether surprising, that human anatomy has not had time to catch up and complete this adaptation. However, at least we use all the bones that we have in our backs. As mentioned earlier, as humans evolved to stand upright, a couple of bones were added to the lower back. Apparently, evolution can duplicate bones when needed. It seems it’s not as good at deleting them when they’re no longer needed.
No Bones About It
Humans have way too many bones. This flaw isn’t unique to us. Nature is replete with animals that have bones they don’t need, joints that don’t flex, structures that aren’t attached to anything, and appendages that cause more problems than they’re worth. The reason for this is that embryonic development is extremely complicated. For a body to take shape, thousands of genes must be activated and deactivated in a precise order, perfectly coordinated in time and space. When a bone, for example, is no longer needed, deleting it is not as easy as flipping a switch. Hundreds—maybe thousands—of switches must be flipped, and they must be flipped in such a way as to not screw up the thousands of other structures that are also built with those same genes. Remember, too, that natural selection flips these switches randomly, like a chimpanzee at a typewriter. If we wait long enough, the chimp will write a sonnet, but the wait will be long indeed. For anatomy, the result is a whole lot of baggage lying around.
For humans, some of the most striking anatomical redundancies are found in our skeletons. Take the wrist. It’s a capable joint, no doubt about it. It can twist nearly 180 degrees in all rotational planes despite the vessels, nerves, and other cords that run from the arm to precise places in the hand. However, it is way more complicated than it needs to be. There are eight different bones in the wrist, not including the two bones of the forearm and the five bones of the hand. The small area that is just the wrist itself has eight fully formed and distinct bones tucked in there like a pile of rocks—which is about how useful they are to anyone.
Collectively, these wrist bones are helpful, but they don’t really do anything individually. They sort of sit there when you move your hand. Yes, they connect the arm bones to the hand bones through a complex system of ligaments and tendons, but this arrangement is incredibly complicated and redundant. Redundancy can be a desirable thing, as we saw with our poor Achilles tendon, but not in the case of bones. Having extra bones requires many more attachment points for tendons, ligaments, and muscles. Each one of those points of contact is a weakness, a potential for strain or (as happens with the ACL) a debilitating tear.
We have examples of superbly designed joints in our bodies; the shoulder and hip joints come to mind. Not the wrist, though. No sane engineer would design a joint with so many individual moving parts. It clutters up the space and restricts the range of motion. If the wrist were rationally designed, it would allow the hand a full range of motion so that the fingers could bend backward and lie along the top of the arm. But of course it can’t do that. The flexibility of the wrist joint is restricted by the many bones in there, not facilitated by them.
The seven bones of the human ankle (shown in white) are fixed in place relative to each other. No engineer would design a joint with so many separate parts, only to fix them together—yet incredibly, most humans manage just fine with this jumbled arrangement.
The human ankle suffers from the same clutter of bones that we find in the wrist. The ankle contains seven bones, most of them pointless. The ankle certainly has more to deal with than the wrist does, given that it is constantly bearing weight and is central to the locomotion of the entire body. But this is all the more reason why we would be better served by a simpler joint. Because many of the bones of the ankle do not move relative to one another, they would function better as a single, fused structure, their ligaments replaced with solid bone. Thus simplified, the ankle would be much stronger, and many of their current points of potential strain would be eliminated. There is a reason that twisted and sprained ankles are so common: the skeletal design of the ankle is a hodgepodge of parts that can do nothing except malfunction.
While the wrist bones and ankle bones are the most obnoxious examples of bones for which we have no use, there are others. For example, the tailbone.
The coccyx is the terminal part of the spinal column and consists of the last three (or four or five, depending on which you count) vertebrae fused together in a C-shaped structure. This section of bone has no function in humans. It doesn’t house or protect anything; the spinal cord, which vertebrae are designed to protect, terminates much higher than where the coccyx begins. It is vestigial—a remnant from our ancestors who had tails.
Nearly all vertebrates have tails, including most primates. The great apes are among the rare exceptions, but even apes begin their embryonic lives with prominent tails. That tail eventually shrinks, and by the twenty-first or twenty-second gestational week, its vestige has become the worthless coccyx. Attached to the coccyx there is even the tiny remnant of a muscle—the dorsal sacrococcygeal muscle—that could flex the tailbone if the bones weren’t fused. A pointless muscle for this pointless cluster of bones.
The coccyx does retain some connections to nearby musculature. It also bears much weight while you are in a reclined or seated position. But for the rare people whose tailbones are surgically removed because of injury or cancer, there are no long-term complications.
The human skull, like that of other vertebrates, is also a strange mishmash of bones that fuse together during childhood to form a single structure. The average human skull has twenty-two bones (some people have more!), with a lot of duplication. That is, the skull often has right-side and left-side versions of bones—a right and left jawbone fused together in the middle, for example, and a right and left top palate. There’s no clear reason for this redundancy. While it makes sense that arms are separate structures, the same can’t be said of the bones underneath the upper-lip area.
As with the duplicated bones in our skulls, there is no real reason to have paired bones in our forearms and lower legs. The upper arm has one bone, but the lower arm has two. Same for the leg; the thigh has one bone, but the shin has two. Yes, the two bones in the lower arm do allow for a twisting motion, but that’s not the case in the lower leg. You cannot twist your leg below the knee without breaking something. Even in the forearm, having two parallel bones is not the only way to make a joint that can twist. In fact, having two bones ensures that the twisting cannot possibly exceed 180 degrees since the bones unavoidably knock into each other when you twist them. For comparison, the shoulders and hips do the task of twisting even better than the elbow, and they do it without the two-bone arrangement. No robot arm will ever be designed to imitate our nonsensical bone structure.
Human anatomy is beautiful, no doubt about it. We are very well adapted to our environment, but we are not perfectly adapted. Little imperfections exist. It’s possible that, if our ancestors had lived the hunter-gatherer life for a longer time before moving into the modern era of vaccines and surgery, evolution would have continued to perfect human anatomy. However, that environment, like all environments, was so dynamic that evolution would simply have substituted our current imperfections for others. Evolution is a continual process—never quite complete. Evolution and adaptation are more like running on a treadmill than running on a track: we must keep adapting in order to avoid extinction, but it can feel like we never really get anywhere.
Coda: A Dolphin with Hind Fins
Although we humans have our share of superfluous bones, there are many other animals with even more blatant examples of vestigial structures and extra bones. For example, there are some species of snakes that have tiny vestiges of a pelvis, though their limbs were lost eons ago. These useless snake pelvises don’t attach to anything and perform no function. Then again, they don’t really harm the animal; if they did, natural selection wou
ld have completed their removal from the body plan. Most whales also have the internal remnants of a pelvis—the quiet whispers of their legged ancestors that wandered back into the ocean more than forty million years ago. When these ancestors returned to marine life, their forelimbs gradually evolved into pectoral fins. Their hind limbs, however, simply regressed into nothing.
The “hind fins” of the dolphin named AO-4 (right), compared to a typical dolphin (left). These tiny but otherwise well-formed fins likely represent a spontaneous mutation that undid a prior one that caused the disappearance of the hind fins. Such “spontaneous revertants” offer a rare glimpse into how adaptations emerge through random mutations.
In 2006, Japanese fishermen caught a dolphin that had tiny hind fins, for lack of a better term. This dolphin, later named AO-4, was a rare find and was sent to the Taiji Whale Museum for display and further study.
The discovery of a dolphin with a tiny but perfectly formed pair of hind fins reveals the power of single mutation during development. In this case, a random mutation just happened to undo a previous mutation. Obviously, these are rare events—rather like lightning hitting the same place twice—but they are powerfully informative when we do find them. As of this writing, there has been no reported discovery of the precise mutation responsible for AO-4, but the scientific hunt continues.
It appears that the hind fins of dolphins didn’t slowly regress in tiny increments down to nothing. Rather, a single mutation was able to take the last dramatic step and cause them to disappear entirely. Similar kinds of “high-impact” mutations were almost certainly responsible for the duplication of vertebrae in our species’ lower back when we needed more of them for upright posture. Don’t believe me? Humans are born every day with extra fingers or toes, perfectly formed and functional. If having twelve fingers had conferred a big advantage some time in our evolutionary past, you can bet that everyone would have twelve fingers now. Genes important for embryonic development have far-reaching effects and so mutations in just the right spot can make large anatomical rearrangements. These rearrangements are random and therefore usually result in harmful birth defects, but when we’re talking about evolutionary timescales, events that seem unimaginably rare are possible.
Mutations like AO-4’s lift the evolutionary veil that normally conceals an animal’s past life. The mutation-driven tweaks and tugs of evolution can sometimes be undone, with dramatic results. Because we are constantly reminded of the slow and steady pace of evolution, we don’t normally think of it as dramatic. The dolphin AO-4 reminds us that, at times, it can be.
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Our Needy Diet
Why humans, unlike other animals, require vitamins C and B12 in their diets; why almost half of all children and pregnant women are anemic despite getting plenty of iron; why we are all doomed to calcium deficiency; and more
A casual stroll through a bookstore or library will reveal shelves upon shelves of books about food and eating. There are books on the history of cooking, books on exotic and ancient foods, cookbooks—and, of course, advice guides and manuals for fad diets.
We are constantly reminded of all the various things we need to eat. You must eat enough vegetables. Don’t forget the fruits. A balanced breakfast is important. Remember to get lots of fiber. Meat and nuts are important for protein. Be sure to get omega-3 fatty acids. Dairy is important for calcium. Leafy greens are vital for magnesium and B vitamins. You cannot stay healthy by eating the same thing all the time. You should maintain a diverse diet in order to get all the various nutrients that your body needs.
And then there are the supplements. Now, most scientists consider the supplement industry a sham (I’m looking at you, herbal supplements), but many of these pills and powders do contain essential vitamins and minerals of which we simply must consume a minimum amount in order to be healthy. Some people’s diets don’t give them everything they need, and even people who get everything they need can’t always absorb it properly. So sometimes, we need a little boost. That’s why we’re always being told to drink milk, for instance; it gives us the calcium that we need but can’t produce in sufficient quantities ourselves.
Now compare our demanding diet with the diet of the cows that produce that milk. Cows can survive on pretty much nothing but grass. They live long and perfectly healthy lives and produce delicious milk and rich meat. How can these cows thrive without a delicate mix of legumes, fruits, fiber, meat, and dairy like humans are told to eat?
Forget cows; look at your own cats or dogs. Consider how simple their diets are. Most dog food is nothing more than meat and rice. No vegetables. No fruits. No supplemental vitamins. Dogs do just fine on this diet and, if not overfed, can live long and healthy lives.
How do these animals do it? Simple: they are better designed for eating.
Humans have more dietary requirements than almost any other animal in the world. Our bodies fail to make many of the things that other animals’ do. Since we don’t make certain necessary nutrients, we have to consume them in our diet or we die. This chapter tells the story of all the things we need to have in our diets simply because our lackluster bodies can’t make them for us, substances as basic as, say, vitamins.
The Scurvy of Humanity
Vitamins are what is known as essential micronutrients, a category of molecules and ions that we must get from our diets or we will suffer and die. (Other essential micronutrients are minerals, fatty acids, and amino acids.) Vitamins are among the largest molecules that cells need to survive.
Major Dietary Vitamins and Their Deficiencies
Vitamin
Alias
Deficiency
A
Retinol
Vitamin A deficiency
B1
Thiamine
Beriberi
B2
Riboflavin
Ariboflavinosis
B3
Niacin
Pellagra
C
Ascorbic acid
Scurvy
D
Cholecalciferol
Rickets, Osteoporosis
Major dietary vitamins and the conditions that result from deficiency. Because humans adapted to thrive on a highly varied diet, we now need a highly varied diet in order to obtain all the micronutrients that we no longer synthesize in sufficient quantities for ourselves.
Most vitamins assist other molecules to facilitate key chemical reactions inside our bodies. For example, vitamin C assists at least eight enzymes, including three that are necessary for the synthesis of collagen. Even though we have these enzymes, they cannot make collagen without vitamin C. When the enzymes can’t work, we get sick.
Vitamin C is called essential not because it is important but because we must get it from our diets. All vitamins are important, crucial even, to human health, but those that are essential are the ones that we cannot make ourselves and therefore must ingest.
In addition to vitamin C, there are other essential vitamins that perform important functions in the body. The B vitamins, for example, aid in the extraction of energy from food. Vitamin D helps us absorb and use calcium. Vitamin A is crucial for the functioning of the retina, and vitamin E has a wide variety of roles throughout the body, including protecting tissues from free radicals, harmful byproducts of chemical reactions.
The one thing that this diverse family of molecules has in common is that our bodies cannot make them. This is what makes vitamins A, B, C, D, and E different than, say, vitamin K or vitamin Q. If you haven’t heard of those vitamins, it’s because they are not essential, in the dietary sense of the word. They are just as important as other vitamins, but we don’t need to get them from our food because we make them ourselves.
When people can’t produce a specific vitamin and can’t get it through their food, their health can really, really suffer. Again, vitamin C offers a useful example.
Schoolchildren in the United States often begin their study of American history by learning about the
fifteenth- and sixteenth-century Europeans who explored this continent. I distinctly remember a story my classmates and I were told about how sailors carried potatoes or limes on their long voyages in order to prevent scurvy. As we now know, this horrible disease is caused by a deficiency in vitamin C. Without it, we cannot make collagen, an essential component of something called the extracellular matrix, or ECM. The ECM is like a microskeleton that runs through all of our organs and tissues, giving them shape and structure. Without vitamin C, the ECM gets weak, tissues lose integrity, bones become brittle, we bleed from various orifices, and our bodies basically fall apart. Scurvy is a dystopian novel written by the human body.
So how can dogs live on meat and rice, neither of which has any vitamin C whatsoever, and not develop scurvy? They make their own. In fact, nearly all animals on the planet make plenty of their own vitamin C, usually in their livers, and thus have no need for it in their diets. Humans and other primates are nearly alone in the need for dietary vitamin C (although guinea pigs and fruit bats have this problem as well). This is because, somewhere in our evolutionary past, human livers actually lost the ability to make this micronutrient.
The physical appearance of scurvy. This horrific disease is caused by a deficiency of vitamin C—an essential micronutrient that human ancestors were able to make for themselves but that humans now must obtain through diet.
Human Errors Page 4