Life cycle

The human life cycle is similar to that of other placental mammals. The fertilized egg divides inside the female's uterus to become an embryo, which over a period of thirty-eight weeks (9 months) of gestation becomes a human fetus. After this span of time, the fully-grown fetus is birthed from the woman's body and breathes independently as an infant for the first time. At this point, most modern cultures recognize the baby as a person entitled to the full protection of the law, though some jurisdictions extend personhood earlier to human fetuses while they remain in the uterus.

Compared with other species, human childbirth is dangerous. Painful labors lasting twenty-four hours or more are not uncommon and often leads to the death of the mother, or the child.[38] This is because of both the relatively large fetal head circumference (for housing the brain) and the mother's relatively narrow pelvis (a trait required for successful bipedalism, by way of natural selection).[39][40] The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain relatively hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times more common than in developed countries
In developed countries, infants are typically 3 – 4 kg (6 – 9 pounds) in weight and 50 – 60 cm (20 – 24 inches) in height at birth.[43] However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions.[44] Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Females continue to develop physically until around the age of 18, whereas male development continues until around age 21. The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, adulthood and old age. The lengths of these stages, however, have varied across cultures and time periods. Compared to other primates, humans experience an unusually rapid growth spurt during adolescence, where the body grows 25% in size. Chimpanzees, for example, grow only 14%.[45]
There are significant differences in life expectancy around the world. The developed world is generally aging, with the median age around 40 years (highest in Monaco at 45.1 years). In the developing world the median age is between 15 and 20 years. Life expectancy at birth in Hong Kong, China is 84.8 years for a female and 78.9 for a male, while in Swaziland, primarily because of AIDS, it is 31.3 years for both sexes.[46] While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older.[47] The number of centenarians (humans of age 100 years or older) in the world was estimated by the United Nations at 210,000 in 2002.[48] At least one person, Jeanne Calment, is known to have reached the age of 122 years; higher ages have been claimed but they are not well substantiated. Worldwide, there are 81 men aged 60 or older for every 100 women of that age group, and among the oldest, there are 53 men for every 100 women.

Humans are unique in the widespread onset of female menopause during the latter stage of life. Menopause is believed to have arisen due to the Grandmother hypothesis, in which it is in the mother's reproductive interest to forgo the risks of death from childbirth at older ages in exchange for investing in the viability of her already living offspring.[49]

The philosophical questions of when human personhood begins and whether it persists after death are the subject of considerable debate. The prospect of death causes unease or fear for most humans, distinct from the immediate awareness of a threat. Burial ceremonies are characteristic of human societies, often accompanied by beliefs in an afterlife or immortality.