Human Errors: A Panorama of Our Glitches, from Pointless Bones to Broken Genes

by Nathan H. Lents

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.

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.

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.

How did we lose the ability to make vitamin C? Well, it turns out that we do have all of the genes that are necessary for vitamin C synthesis, but one of them is broken, mutated to the point of being nonfunctional. The broken gene, known as GULO, codes for an enzyme that is responsible for a key step in the manufacture of vitamin C. Somewhere in the ancestors of primates, the GULO gene suffered a mutation, rendering it inoperable, and then random mutation continued, littering the gene with tiny errors. As if to mock the uselessness of pieces of DNA like this, scientists call them pseudogenes.

Without enough dietary vitamin D or enough sunlight, young humans can develop a disease called rickets, and older humans can develop osteoporosis. Rickets is extremely painful and leads to weak bones that break easily and heal slowly and, in severe cases, to stunted growth and skeletal deformities.

we need vitamin D to help absorb calcium from food. We could eat all the calcium in the world, and none of it would be absorbed without sufficient vitamin D. (This is why vitamin D is commonly added to milk: it helps our bodies absorb the calcium that the milk contains.)

in order to get sufficient vitamin D in our diets, we need to eat at least some fish, meat, or eggs.

The domestication of animals for meat and eggs (roughly five thousand years ago in the Middle East and at different points elsewhere) mostly solved the problem of rickets.

Humans cannot make their own vitamin B12, and, since plants have no need for this vitamin, they do not produce it, so the only dietary sources of it are in meat, dairy, seafood, arthropods, other animal-derived foods, and vitamin supplements. Vegans, take note: you need these pills. But what about vegetarian animals? There are many animals that eat only plants, but if plants don’t have any B12 and all animals need B12 to survive, how do cows, sheep, horses, and the thousands of other herbivorous animals avoid anemia? The answer is that they make it—or, rather, the bacteria in their large intestines make B12 for them.

For reasons that are not always understood, nutrients are not spread evenly throughout a plant. For example, the skins of potatoes and apples are where most of their vitamins A and C are located, so peeling them can rob them of most of their nutrients.

Animals are the exact opposite of self-sufficient. They must constantly eat other living things in order to survive. They can eat plants, algae, or plankton, or they can eat other animals that eat them. Either way, animals must get all of their energy from organic molecules made by other living things since they cannot harvest solar energy themselves.

Even if we have plenty of vitamin D, we still aren’t very good at absorbing calcium, and we get worse and worse at it as we age. While infants can absorb a respectable 60 percent of the calcium they consume, adults can hope to absorb only around 20 percent, and by retirement age, that drops to 10 percent or even lower. Our intestines are so bad at extracting calcium from food that our bodies are forced to extract it from our bones instead—a strategy with devastating consequences. Without constant calcium and vitamin D supplementation, most people would develop the brittle bones of osteoporosis in their golden years.

The most commonly known role of iron is in the functioning of hemoglobin, the protein that transports oxygen throughout our bodies. Red blood cells are absolutely packed with this protein, each molecule of which needs four iron atoms. In fact, the iron atoms in hemoglobin are what give it its characteristic red color (which means your blood and the surface of Mars have more in common than you might think). Iron is also vital for other crucial functions, including the harvesting of energy from food. Despite the fact that there is plenty of iron in our bodies, our environment, our earth, and our solar system, deficiencies in iron are among the most common diet-related ailments in humans. In fact, according to the Centers for Disease Control and Prevention (known as the CDC) and the World Health Organization (WHO), iron deficiency is the single most common nutritional deficiency in the United States and worldwide. That iron deficiency is pandemic in a world filled with iron is paradoxical, to say the least.

Plant- and animal-derived iron are structurally different things. In animals, iron is generally found in blood and muscle tissue, and it’s easy enough to process; humans usually have little trouble extracting iron from a nice hunk of steak. The iron in plants, however, is embedded in protein complexes that are much harder for the human gut to rip apart, and so they remain in the gastrointestinal tract and end up as waste, making iron consumption another concern for vegetarians. In this respect, humans are worse off than most animals. The majority of the creatures on earth are mostly or completely vegetarian, yet their intestines do just fine in processing iron.

Vegetarians use this trick to boost their iron absorption. By combining sources of iron with sources of vitamin C, they can ensure that their bodies are better able to absorb both. A large dose of vitamin C can increase iron absorption sixfold.

The hard truth is that humans in the developed world are surrounded with high-calorie foods that they are ill equipped to resist. For most of our species’ history, this just hasn’t been something anyone needed to worry about. Until the past couple of hundred years, most people didn’t have access to diets rich in meat and sweets. It was the industrial revolution that began to bring rich diets to the masses. Before that, stoutness in a man and plumpness in a woman were signs of wealth, power, and privilege, and the commoners were, like animals in the wild, prone to constant hunger. Overeating was a fine strategy when it wasn’t possible to do very often. But when people can pack it in three or four times a day, day after day, their feeble willpower doesn’t stand a chance of moderating intake to prevent unhealthy weight gain. Human psychology is no match for human physiology. This is why people too often treat every meal as if it were the last one before a long winter, as if they’re gorging themselves in anticipation of barely being able to find any food at all.

It gets worse. As recent studies have shown, our bodies adjust our metabolic rates so we gain weight easily and lose weight only with difficulty. Those who have struggled with their weight will tell you that weeks of dieting and exercise often result in negligible weight loss, while a weekend calorie binge can pile on a few pounds almost instantly. Thus, obesity and type 2 diabetes are the quintessential evolutionary mismatch diseases, conditions that directly result from humans living in a very different environment than the one they evolved in.*

Probably the most well-known and widespread example of a genetic disease that has frustrated humans for ages is sickle cell disease, or SCD. Three hundred thousand babies are born with this condition every year. In

Like many genetic diseases, sickle cell disease is recessive. This means that you need to inherit two copies of the mutant allele, one from each parent, in order to develop the disease. If you inherit only one copy, you will not be affected in any noticeable way—although you will be a carrier, able to pass the gene to your children, and they may develop the disease if the other parent also gives them a bad copy. If two carriers of SCD procreate, approximately one-fourth of their children will develop the disease, even though both parents appear to be healthy. For this reason, recessive traits sometimes give the appearance of skipping generations.

Around 8 percent of the DNA inside every single cell of your body consists of remnants of past viral infections, nearly a hundred thousand viral carcasses in all. Humans share some of these carcasses with cousins as distant as birds and reptiles, meaning that the viral infections that originally created them took place many hundreds of millions of years ago and these viral genomes have been passed along, silently and pointlessly, ever since.

This was one of Darwin’s key insights. He noticed that all organisms seem to reproduce constantly and in great numbers and yet their populations remained pretty much the same size. This meant that life was a challenge that most individuals failed. The only way a species has any chance to survive and compete is by making a lot of babies. Some make fewer babies than others but care for them better, while others make tons of offspring but don’t care for them at all. But for all species, prolific reproduction is a key goal of an individual’s life, if not the key goal. We all have an inborn drive to make more of ourselves. It’s the only way a species survives.

Estimates vary based on geographic location and precisely how the term infertility is defined, but most studies report that somewhere between 7 to 12 percent of couples trying to conceive have faced persistent difficulties. Fertility problems are equally common in women and men, and in around 25 percent of cases, both partners find that they have reproductive problems.

lateness of male puberty, which is later even than female puberty,

Human sperm cells, you see, can’t turn left. This is due to the corkscrew nature of their propulsion system; rather than snapping their whiplike tails back and forth and side to side, sperm cells rotate their tails around in a corkscrew motion, like the way you would move your index finger to draw a circle in the air. Because most sperm whip their tails in a right-handed spiral, the rotation pushes them forward and toward the right and they end up swimming in ever-widening circles. This means that it can take as long as three days to reach the egg waiting in the fallopian tube to be fertilized. Very few of the original number get anywhere near their goal. This is one reason why human males produce sperm in such large numbers. You need about two hundred million of them to start with in order to get just one to its destination.

Even when each potential parent produces and releases healthy sperm or egg cells, there is no guarantee they’ll be able to create a pregnancy. First of all, insemination has to be timed very carefully with ovulation or it won’t be successful. In a typical twenty-eight-day menstrual cycle, the fertile period is just three days long in the very best of scenarios, with twenty-four to thirty-six hours being the more common window of opportunity. This means that even perfectly fertile couples usually have to try for months before the female conceives.

basically all other female mammals, including the other female apes, who conspicuously advertise when they are at the fertile point of their estrous cycle. To be sure, other animals have plenty of sex outside the fertile period, underscoring the many nonprocreative functions of sex, such as strengthening a pair bond.

Why is concealed ovulation peculiar to Homo sapiens? There may be adaptive purposes for the concealment; if a man cannot be sure when a woman is ovulating, he cannot be certain that a child is his own unless he sticks with one female constantly. If ovulation were obvious, an alpha male could simply have sex with every ovulating female, spreading his genes widely but not sticking around to invest in the offspring. Thus, concealed ovulation has led humans to form more long-lasting pair bonds and enhanced paternal investment in the offspring. But here, too, a feature of our bodies is also a bug—concealed ovulation adds tremendously to the inefficiency of human reproduction. Other animals know exactly when the fertile period of the estrous cycle is. Humans have to guess.

As of 2014, all but one of the major developed countries had an infant mortality rate below 0.5 percent. The one exception is the United States, which, at 0.58 percent, has a higher infant mortality rate than Cuba, Croatia, Macau, and New Caledonia. (This is due in large part to two particular practices by American doctors: the frequent medical induction of labor, which artificially accelerates the natural process of childbirth, and the overuse of cesarean sections. The reason C-sections are performed so often in the United States? Lawyers. Doctors fear being sued on the off chance that a C-section was needed but wasn’t performed. But tragically, these invasive abdominal surgeries often involve many fatal complications of their own.) By contrast, the infant mortality rate in Japan is 0.20 percent, and in Monaco, it’s 0.18 percent. These are relatively low risks, to be sure, but birth is still one of the riskiest moments of our lives. And in regions where medical practice is far from modern, there is still a high rate of infant mortality—a testament to how far from perfect the human reproductive system is. In Afghanistan, for instance, the United Nations estimates the current infant mortality rate at 11.5 percent. In Mali, it is 10.2 percent.

Part of why humans are so out of step with other mammals when it comes to childbirth is because human infants are simply born too early. This is due to our species’ massive craniums and the females’ relatively narrow hips. Human gestation time is similar to that of chimps and gorillas, even though humans’ much larger brains require more time and cognitive development in order to reach their full potential. However, the size of the female pelvis limits how large the fetus’s head can grow while still in utero. If it grows too large, there is no way to get it out, and both baby and mother can perish. The compromise is that fetal gestation is cut short, and human babies are born way before they are ready.