Humans are constantly changing. The body, mind and metabolism of a human being continuously move from one stage of the life cycle toward the next. Advancements in medical science, nutrition, sanitation and medicine have altered the length of time humans live. However, the stages of the human life cycle themselves remain the same. If a person lives long enough, she will pass through all of them.
In Utero
According to the Biology Reference Encyclopedia, sperm from a human male fertilizes an egg from a human female to form a zygote. This single-cell organism repeatedly divides, forming an embryo. Three weeks after conception a heartbeat begins in the embryo. After 8 weeks, all the major organs form and the embryo becomes a fetus. The first brainwave activity begins in the sixth month of pregnancy. A full-term pregnancy is 38 weeks.
Infancy
When a fetus is delivered, it is called an infant. Many consider this to be the beginning of life because it is the point at which the human can survive apart from its mother. Human infants are completely helpless at birth. They depend on other humans to provide shelter, protection and food. For the first 6 months to 1 year of an infant's life, this food is milk from the mother's breasts or formula. During this time infants grow rapidly. Most infants triple their birth weight, grow teeth, start talking and learn to walk.
Childhood
When an infant reaches 1 year of age, he is considered a child. This is another period of rapid growth, but the growth rate is not nearly as fast as that in the infant stage of the human life cycle. Human children learn how to communicate in language, solve problems and perform complex tasks. They grow a full set of teeth, the teeth fall out, and then permanent teeth grow in. The childhood stage lasts for approximately 10 to 12 years.
Adolescence
Most humans enter adolescence between the ages of 11 and 13. This is a time of rapid body changes as humans prepare for reproduction. Sexual organs develop and the differences between male and female become more pronounced. Men are larger with more muscle mass. Women have twice the body fat of men. Most female humans reach maturity and stop growing in their late teens or early 20s. This happens to most male humans in their early to mid-20s.
Adulthood
In modern times, this is the longest stage of the human life cycle. It is common for adulthood to last 50 years or more. Most people have and raise children, then watch their children have children during this time. After the age of 30, humans begin to age noticeably. Changes in hair, skin and muscle tone indicate the decline of the body. When humans are in their 50s, most women lose the ability to reproduce and most men experience a noticeable drop in testosterone levels.
Death
The human life cycle is considered at an end when there is no more brain activity. This is usually caused by organ failure when the brain can no longer be nourished by the body. In modern times, organ failure by natural causes typically happens in the 70s or 80s. Disease or trauma can cause premature death in humans at any time.
