The Body: A Guide for Occupants

No one knew what causes people to be male or female until scientist Nettie Stevens figured it out in 1905. Chromosomes had been discovered in the 1880s, their purpose a mystery. In 1891, a German researcher noticed an unusual chromosome in wasps, which he named “X.” Stevens, working with mealworms, found another chromosome that seemed to have a role in determining sex; she named it “Y.” 

Chromosomes are usually balled up in the cell nucleus and hard to study; the exact regions on the Y chromosome that determine sex were only found in 1990. Sexual differences aren’t required for life: Females of certain species, such as gecko lizards, simply produce egg clones of themselves that develop into new geckos. Most creatures, though, blend DNA from the female and the male, recombining to create genetic variations that make a species resilient against changes in the environment. 

Sexual activity is an awkward topic, and in surveys, most people tend to misreport their sexual activity. Sexual myths abound: It’s false, for example, that men think about sex every seven seconds; the number is closer to once an hour. 

There are numerous differences between male and female physiology aside from reproductive systems alone. Women have more subcutaneous fat, they suffer more from broken bones, autoimmune diseases, and Alzheimer’s—and they carry bags differently than men do. Women also experience heart attack symptoms differently, which can lead to misdiagnoses. Men are more likely to suffer or die from injury, infection, and suicide. Clinical studies tend to overlook women, who respond differently to medications and can suffer ill effects as a result. 

Human mitochondria, the cellular power generators, contain their own DNA, which is passed down solely through the female. If a woman has only male children, her mitochondrial DNA ends with her boys. The result, over time, has been a narrowing of mitochondrial genetic variety. All humans alive today can therefore be traced back to a single African female from 200,000 years ago. 

Doctors were for a long time squeamish about touching a woman’s body during an examination. The stethoscope was invented in 1816 partly so a doctor didn’t have to touch a female patient directly; women pointed to places on the bodies of dummies to show where they were having symptoms. Menopause wasn’t mentioned in medical journals until 1858, and vaginal exams almost never took place. 

Today most men are fairly ignorant of the anatomy of female genitalia. The outer opening, or vulva, includes skin flaps called labia, along with the clitoris, which has the most nerve endings of any part of a woman’s anatomy and extends several inches into the vagina. The vagina is the passageway between the vulva and the cervix; the cervix is the gatekeeper to the uterus, or womb, which normally weighs two ounces but during pregnancy can grow to two pounds while also encasing a gestating baby or two. 

Flanking the uterus are the ovaries, repositories of eggs and sex hormones. The ovaries connect to the uterus via oviducts, or Fallopian tubes, where eggs can be fertilized on their way to the womb. 

Men’s genitalia are somewhat simpler—basically penis, testes, and scrotum. The testes live in the scrotum, where they produce sperm, which is delivered by the penis. The scrotal sac contains 40 feet of tightly coiled small tubing, the epididymis, that connects, one to each testicle, and houses the sperm while they mature. A few other small organs, including the seminal vesicles and prostate, complete the production of semen. 

Human testicles live on the outside of the body, where they’re exposed to possible trauma, while ovaries dwell inside. Many animals have both within their body cavities; why human males have it otherwise is still unknown. 

Getting pregnant is harder than it appears. Only 3% of unprotected sexual couplings result in pregnancy, and 14% of couples in Western nations seek help in improving those odds. Since 1973, sperm counts in the West have fallen by up to 50%. Even so, a single ejaculation contains tens of millions more sperm than are needed to fertilize one egg, and women at puberty have 180,000 eggs at the ready inside their ovaries. 

A possible cause for declining fertility is exposure to endocrine disruptors, chemical byproducts of modern life. Also, Western women reach puberty two to three years earlier than their ancestors, and—especially the better educated ones—are having babies later, at around age 30. By age 35, 95% of a woman’s eggs have died off. Older men and women are less likely to conceive and more likely to generate birth defects. Identical twins are more likely from older parents; though only one in 250 births are identical twins, today they occur twice as often as in the past. 

Of the millions of sperm in the tablespoon of semen that is emitted during sex, only one will fertilize an egg, which then shuts itself off from the other sperm. The combined DNA forms a zygote, which in a week has subdivided into about 10 stem cells that begin to differentiate into all the cells that will make up the baby’s body. In three weeks, a heart begins to beat; at eight weeks, the embryo is considered a fetus; just past 100 days, the eyes begin to blink; at about 280 days, after 41 divisions that produce 2 trillion cells, the child is born. 

For centuries, doctors knew little about pregnancy; obstetrics wasn’t a medical school course until 1886. Around 1900, it became fashionable to have one’s ovaries removed for “menstrual cramps, back pain, vomiting, headaches, even chronic coughing” (295). These operations did no good and rendered women infertile. Pregnancy and childbirth were dangerous, and many women died. 

Puerperal fever arose in Europe in the mid-1600s, causing women who otherwise had healthy deliveries to take ill and die. Not until 1847 did a doctor figure out that the fever wouldn’t happen if doctors washed their hands before deliveries; his advice was ignored for decades. Many women refused to deliver in hospitals until well into the 20th century. The first cure for puerperal fever wasn’t hygiene but penicillin. 

Today, nearly all births are in hospitals, which are much cleaner than in the past. Death rates for mothers in Western nations are about 10 per 100,000, while the death rate for babies is one in 450 in France and one in 909 in Japan. The U.S. suffers death rates for mother and child among the highest in the industrialized world. 

Birth in some primates can take as little as a few minutes; for a first-time human mother, it can take 12 hours. Her pelvis, adapted to upright walking, is ill-designed for birth, and the human head at birth is too big for the birth canal. The bones of a fetus’s skull have not yet fused, making the head somewhat compressible, which helps but not enough to prevent the intense pain of labor and delivery. 

Between 5% and 10% of births require surgical delivery by Cesarean section, but one-third of American births, and 13% to 60% elsewhere, are delivered, for convenience, by Cesarean section. Cesarean babies aren’t bathed by the mother’s vaginal microbes during delivery and thereafter are at greater risk for diabetes, asthma, allergies, obesity, and celiac disease. These babies eventually acquire the correct microbes, but that first inoculation within the birth canal makes a critical difference; no one yet knows why. 

A mother’s milk contains 200 complex sugars her baby can’t digest, sugars meant exclusively to encourage the growth of certain gut microbes. The milk also contains antibodies useful to the infant’s developing immune system. Though mid-20th-century mothers were encouraged to bottle feed, artificial formulas proved inferior, which increased infant mortality. Today in the developed world, 80% of mothers breastfeed after birth, but that number after one year drops in the U.S. to 27% and, in Britain, less than 1%. 

From conception to age two, the “first thousand days” (302), everything that happens to a child has a critical effect on health outcomes later in life. Declining health habits in today’s young adults may cause their offspring to become the first generation to have shorter life spans than their parents.