The human life cycle begins at fertilization, then birth, and progresses through infancy, childhood, puberty, adulthood and aging, ending in death. Every stage is unique and encompasses changes in the human body.
Fertilization begins when a man's sperm cell enters a female's egg, creating a zygote. Inside the mother's uterus, cell division leads to the creation of a circulatory system, nervous system, heartbeat and organs. After the fetus grows and develops, it is ready for birth. The mother's body pushes the baby through the vagina, the birth process known as labor. When the baby is born, it is called an infant. Unlike many other mammals, the infant's brain continues to grow new neutrons until age 2. As children grow, they loose their baby teeth, which are replaced by their adult teeth. The next stage is puberty, which occurs from ages 11 to 13. During this time, the body drastically changes: body hair grows, genitalia becomes fully developed and girls begin their menstrual cycle, allowing them to carry a baby to full term. By the mid-20s, human brains and bones are fully developed. Humans are at their peak for reproduction during their 20s. Aging becomes noticeable after 30. Hormone levels begin to decline leading to the death of brain cells, looser skin, wrinkles, no more menstruation and lower testosterone. Aging means the cells stop dividing and replacing themselves as well as monitoring problems in DNA, leading to more abnormalities such as cancer. The average human female lives to around 79 years old and the males to around 72 years old.