Evolution Gone Wrong by Alex Bezzerides
The human body has many defects and fragilities. It can sometimes be surprising how natural selection permitted so many different problems to survive in the human genome. Most of humanity’s common faults make more sense when viewed in their historical evolutionary context.
Overcrowded teeth. Crooked teeth and wisdom tooth problems are the norm among modern humans rather than the exception. Primates have had 32 teeth for the past 30 million years or so, but human jaws are usually not big enough to accommodate all of them. Early ground-based apes evolved very strong jaws and very large molars to eat their tough plant-based diet. As hominids started cooking their food and eating more meat, powerful jaws became less important, and teeth and jaws both started to shrink over time. Tooth and jaw sizes are governed by completely different genes, so keeping the two in sync has always been a challenge. Hunter-gatherer humans maintained a fragile balance between jaw and tooth size, but this balance was shattered just over 10,000 years ago, when agriculture abruptly softened the human diet even further. Modern agricultural diets are so soft that human jaw bones and muscles don’t grow to their full potential, leading to undersized jaws and overcrowded teeth.
Wet eyes. Most of the evolution of the animal eye happened underwater among fish. The lens of the fish eye needs to be coated with water at all times, which wasn’t a problem for aquatic animals, but became a problem for later animals that migrated to land. Rather than reinvent the eye outright, land animals have made only incremental changes to the eye and have accommodated the wetness constraint with eyelids and tear ducts. In order to keep the eyes wet at all times, humans use up 300ml of fluid every day and blink around 14,000 times a day.
Bad color vision. Most humans have three types of cone cells in their eyes (sensitive to red, green, and blue). Most fish, birds, and reptiles have four types of cones and can thus perceive a much wider variety of colors than humans can. As a whole, mammals have poor color vision. Since most of the earliest mammals were nocturnal, night vision and rod cells were very important while color vision and cone cells were less important. Mammals lost two of the four types of cones and usually have just two cone types; primates evolved a third cone back so that they could better differentiate between shades of green. Most humans have trichromatic vision, but color-blind humans have bi-chromatic vision, in line with most other mammals. A very small number of women descended from color-blind men have tetra-chromatic vision, in line with non-mammal animals.
Near-sightedness. Near-sightedness was a rare problem for most of history, only becoming an epidemic in industrial times. Childhood-onset near-sightedness is strongly correlated with little time spent outdoors, which suggests that exposure to bright light is necessary for preventing the eyeball from growing too long front-to-back. The dim lights of modern life are putting human eyes in an environment they are poorly adapted to.
Vulnerability to choking. Animals put themselves in danger every time they eat because of the possibility of food lodging in a passage meant for air and getting stuck. Air bladders and lungs evolved in fish as an offshoot of the digestive system; the lungs and the digestive system are thus unnecessarily intertwined in all vertebrate animals. In order to perform complex speech, humans have unusually thin air passages and their voice boxes are positioned unusually low. These factors make humans especially vulnerable not only to choking, but also to snoring and sleep apnea. Natural selection has forgiven these faults in humans since the ability to speak has proven to be more beneficial than the downsides.
Bad knees. Knees are the joint in humans most vulnerable to recurring problems. Because of bipedalism, human knees must bear the weight that most other animals distribute across four legs. The lateral meniscus in human knees has evolved from the disc shape seen in other apes to a crescent shape. However, in a small percentage of cases, the usually-dormant genes for a disc meniscus get activated and the ape-like disc meniscus forms instead; such knees are usually asymptomatic but are more prone to medical issues. Many other parts of the knee have changed relatively little since bipedalism started; some ligaments, most notoriously the ACL, remain too fragile for the new stresses placed on them. The flip side of bipedalism is that human elbows are overpowered for their current role and chronic elbow problems are very rare.
Sore feet. Ground-based apes are an evolutionary offshoot of monkeys that lived primarily in trees - even now, human children still feel an instinctive itch to climb things. The feet of primates are optimized for grasping and climbing, but the rise of bipedalism about 4 million years ago forced the foot to quickly reinvent itself. To make themselves suitable as a shock absorber and weight bearer, hominid feet evolved forward-facing big toes, short digits, large heels, long Achilles tendons, and sole arches. Good shock absorbers should ideally have few moving parts, but human feet are still stuck with the 26 bones and countless ligaments of their ancestors. Most problems in human feet ultimately stem from them being a needlessly complicated structure. The feet remain a rapidly-evolving body part; there is lots of variation between individuals, especially in the height of the arches. Modern lifestyles have made foot pain worse since shoes are often too narrow and floor surfaces are often unnaturally firm.
Back pain. Back pain afflicts the majority of adults and is the leading cause of disability worldwide. Whereas other apes (as well as human babies) have C-shaped spines, human bipedalism required a vertical S-shaped spine. The bipedal spine is a fragile structure and the amount of curvature needs to be just right. Intervertebral discs in humans must not only fulfill their usual role in other vertebrates (keep the bones from grinding against each other), but they must also do it while bearing a lot of weight. The vertebrae are a rapidly-evolving body part with lots of variation between individual humans; humans with a more chimpanzee-like vertebral shape tend to have more back problems.
Menstruation. Menstruation comes with significant blood loss and puts women at risk of iron-deficiency anemia. Only about 5% of mammals menstruate, mainly primates, elephant shrews, and a few species of bats; most other mammals reabsorb their uterine matter back into their bodies if a period doesn’t lead to fertilization. In mammals that menstruate, uterine cells undergo the full decidualization process every single period, whereas most mammals only do it in response to fertilization; the thick decidualized matter cannot be reabsorbed and must be shed. Preemptive decidualization protects mothers from having embryos burrow themselves too deeply, a necessary defense in species like humans where embryos are particularly aggressive. Early decidualization also makes it easier for mothers to quickly identify and miscarry defective embryos, a useful ability in species like humans where pregnancy and parenting is costly. Modern life has made menstruation a greater inconvenience; since modern women are pregnant a much smaller fraction of the time than hunter-gatherer women, they menstruate far more times per lifetime.
Infertility. Many human couples fail to conceive despite trying very hard. Unlike most other female mammals, human women don’t give off any clear physical signals when they are ovulating; they are also willing to have sex at all points in their cycle rather than only at peak fertility. This means that both men and women can never be sure when a woman is most fertile and can rarely be sure which specific sexual encounter caused a conception. Whereas most male mammals are deadbeat dads, this ambiguity incentivizes human men to be a constant presence in their partners’ lives so that they can get more sexual chances. Whereas many male mammals try to kill children that their female sexual partners made with other males, human men can be deterred from infanticide by the possibility that a child may in fact be theirs. Ambiguous female sexuality led to pro-social changes in the human species, but the flip side is that it is difficult to identify the optimal timing for sex.
Dangling testicles. The testicles of human men dangle outside the body rather than inside as in many other mammals, leaving them exposed and injury-prone. The temperature of the scrotum is cooler than inside the body, which helps to keep sperm in a fresh and low-energy state (this may also be why humans prefer having sex at night, when temperatures are cooler). By mammal standards, humans have large penises and testicles for their body size, though they are smaller than close relatives like chimpanzees and bonobos. Large male genitals tend to evolve in species with promiscuous sexual behavior; if a male’s sperm is likely to directly compete with the sperm of other males, it is advantageous to produce more sperm, keep the sperm fresh for longer, thrust them more deeply, and have a penis shape that can scrape other males’ sperm out.
Birth defects. Humans miscarry an unusually large fraction of their embryos and frequently have children with chromosomal abnormalities. In mammals, embryos must produce sufficient quantities of a hormone called HCG (the chemical that pregnancy tests check for) to signal to the mother that implantation has succeeded. In mammals that give birth to large litters, producing HCG is a collective responsibility of the whole litter, but since humans typically have only one child at a time, the job falls on one individual. The early human embryo is therefore under pressure to divide extremely quickly after conception; such rapid cell division increases the risk of DNA-copying errors.
Illnesses during pregnancy. Because of their large brains, human fetuses have uniquely high demands for oxygen and energy, which puts uniquely high stress on pregnant mothers. About 60% of a human fetus’s energy goes to its brain; for most mammals this figure is about 20%. Human embryos burrow right up against a woman’s bloodstream, which gives them lots of leeway to hormonally manipulate their mothers. During the vulnerable first trimester, a mother’s immune system is put into overdrive, causing symptoms such as nausea, vomiting, and food aversions. Pregnancy food aversions are usually for high-bacteria foods such as meat and vegetables, which suggests that morning sickness is a fetus’s way of guarding itself against bacterial infections. To secure themselves more sugar and oxygen, human fetuses increase blood pressure and insulin resistance in their mothers. Whereas increases in blood pressure and blood sugar during pregnancy are common in mammals, only in humans are these increases large enough to become dangerous.
Long and dangerous childbirth. Because of the large heads of human babies, human labor is uniquely long, difficult, and dangerous. Human labor averages around 14 hours for a first birth and 7 hours for subsequent births; other primates average around 2 hours. The human cervix must expand to a width of 10cm, versus just 3cm for a chimpanzee mother. Losing 500-1000ml of blood during childbirth is typical, but women have evolved to compensate by having more blood in their bodies than usual during pregnancy. Childbirth has gotten worse in industrial times; since calories are more abundant, human babies are being born bigger than ever before.
Helpless newborns. Compared to other primates, human newborns are underdeveloped and helpless. Humans are born with their brains at 30% of their final adult size, whereas this figure for other primates is 40-50%; this is a very big difference as far as independence is concerned. Even if women had wider hips, they would still be unable to carry their fetuses for longer than they currently do. Even at 30%, a baby’s brain and body are already so energy-hungry that they push a woman’s metabolism to its theoretical limits.
Painful breastfeeding. New mothers are often caught by surprise by how awkward and painful breastfeeding can be; pain causes many mothers to abandon the practice too early. Most mammal babies are not helpless and can help maneuver themselves into good breastfeeding positions. In humans, this responsibility falls entirely on the mother. Breastfeeding pain tends to get better with time as babies become stronger and as mothers improve their technique.