Life Cycle, Human
The human life cycle begins at fertilization, when an egg cell inside a woman and a sperm cell from a man fuse to form a one-celled zygote . Over the next few days, the single, large cell divides many times to form a hollow ball of smaller cells. On the sixth day after fertilization, this hollow ball burrows into the wall of the mother's uterus, or womb. The cells then form three layers that fold and bend into the more complex shape of an early embryo. Gradually, the cells begin to become different from one another, forming, for example, the nervous system and the circulatory system.
On the twenty-second day after fertilization, a simple tubelike heart begins to beat. The embryo has no other working organs: the first brain activity will not begin for five more months. But in just one more month, all the major organs will have formed in miniature, including tiny eyes and ears, liver, and kidneys. These organs do not work, but they are there. Once all the organs have formed, the individual is called a fetus. During the fetal period, all the organs begin to mature. Cells from the embryo and its mother also combine to form a placenta, an organ in the uterus that connects the embryo to the mother's blood supply.
Biologists count the days of development starting from fertilization, but medical doctors count from the first day from the last menstrual period, which is about two weeks before fertilization. So, where a biologist would say the embryo's heart begins beating at three weeks, a medical doctor would say the heart begins beating at five weeks. The total time from fertilization to birth is about thirty-eight weeks. At the end of the embryonic period (eight weeks), the embryo is about 30 millimeters (just over 1 inch) long. Between three months and nine months the fetus grows until it is about twenty times as long.
At birth, the muscles of the mother's uterus begin to contract and push the baby out through the vagina. This process is called "labor," because it is hard work and can take a long time. In the first stage, called dilation , the lower end of the uterus, called the cervix, opens to about the same diameter as the baby's head. Dilation takes from eight to twenty-four hours in a woman who has never given birth before. In the second stage, called expulsion, the baby is pushed out of the uterus, into the vagina, and out of the body. Expulsion takes about half an hour the first time a woman gives birth. In the third stage, the mother expels the placenta. A few hours later, her uterus begins to contract to a smaller size, and her breasts begin to synthesize milk.
Within a few minutes after the baby is born, it may begin to nurse. The mother and baby can nurse as many months as they like. Women in traditional cultures may nurse for several years, but most American women nurse for about six months. Human milk is better for babies than bottled formula or other alternatives. For example, human milk contains antibodies and immune cells that protect the infant from infections. Babies who eat solid foods too early seem to be more subject to allergies later in life.
During infancy, between birth and one year, the brain continues to develop and grow. In this respect, human infants differ from other primate infants, whose brains stop growing at birth. Indeed, the human brain continues to grow new neurons until the child is two years old.
Infants' bodies also grow and develop rapidly, though not as fast as the brain. A one-year-old human typically weighs three times what he or she did at birth, has several teeth, and has begun to walk. At about two years, most humans begin to speak in sentences. During childhood (one to thirteen years), humans develop their first set of teeth, lose them, and begin to develop a second, or adult, set of teeth.
Between eleven and thirteen, children enter puberty. After puberty, adolescent humans can produce viable eggs and sperm, and many girls can carry a baby to term. Girls and boys develop secondary sexual characteristics, including body hair, deeper voices (especially in boys), breasts (in girls), and larger external genitalia (in both girls and boys). Boys begin to produce fertile sperm for the first time. Girls begin a monthly cycle of ovulation (releasing eggs) and menstruation (shedding the uterine lining) that will continue until they are in their fifties.
The changes that adolescents go through are so dramatic that many biologists compare puberty to the metamorphosis that tadpoles go through when they become frogs. For example, before puberty, boys and girls have the same amount of muscle mass, bone mass, and body fat. After puberty, men have 1.5 times as much bone and muscle mass as women, and women have twice as much body fat as men. Changes in the brain and in behavior also occur. By their early to mid-twenties, humans have reached their adult size. The bones stop growing and the brain is fully mature.
Humans in their twenties are in their peak reproductive years. Women who reproduce at this time have the least-complicated pregnancies. For males, the late teens and twenties are a time of peak death rates from accidents and other misfortunes, most likely due to the behavioral effects of high testosterone levels.
After age thirty, human beings begin to age noticeably. Hormone levels decline, skin becomes thinner and less flexible, gray hair and wrinkles appear, muscle mass decreases, bones lose calcium, blood vessels stiffen, and brain cells begin to die. Starting around age thirty-five, humans may lose one hundred thousand brain cells per day. The ears, the eyes, and other sensory organs also become less sensitive. Women gradually stop ovulating and menstruating in their fifties, and men experience a slow decline in testosterone levels that is most often noticed in the fifties.
Why people age is not completely understood. But some aspects of aging result when cells can no longer divide and replace themselves as they die. Some cells also begin to lose their ability to repair mistakes in the DNA (deoxyribonucleic acid), which leads to abnormalities, including, sometimes, cancer. Another cause of aging may be destructive molecular fragments known as free radicals, which damage DNA, proteins , and lipids . The average American woman lives seventy-nine years, and the average man lives seventy-two years. But despite advances in health care and healthier lifestyles, few people live beyond age one hundred.
Browder, Leon W., Carol A. Erickson, and William R. Jeffery. Developmental Biology, 3rd ed. Philadelphia, PA: Harcourt College Publishing, 1991.
Moore, Keith L. The Developing Human, 3rd ed. Philadelphia, PA: W. B. Saunders, Co., 1982.
Saladin, Kenneth S. Anatomy and Physiology, 2nd ed. New York: McGraw-Hill, 2001.