All organisms develop in one way or another, and humans are no exception. This transformation in the physical structure of a plant or animal and in the way it functions usually occurs in stages over the course of the organism's life. In the early stages, development typically improves an organism's ability to survive -- its ability to find and use available resources. Later in life, another phase of development enables an organism to reproduce.

Human offspring are born nearly helpless, even compared to the young of other mammals. For example, most rodent offspring, called kits, spend just a few weeks under their mother's care. Bear cubs stay with their mother for about two years, growing, learning, and developing the skills they'll need to survive on their own. In contrast, depending on the culture in which they grow up, human offspring take anywhere from about 15 to 21 years or more to fully gain independence from their parents.

Prior to independence, human children go through a particularly dramatic stage of development. This stage, called puberty, will transform them, at least physically, into adults. Hormones drive this phase of development, producing physiological changes known as the secondary sex characteristics. The hormone testosterone promotes muscle development, body hair growth, and sperm maturation in boys. Estrogen and progesterone promote breast development, ovulation, and menstrual cycles in girls.

Sexual development in humans often continues for several years. By the end of this process, a child has become an adult in the sense that he or she is able to conceive and reproduce offspring with another person of the opposite sex.

For many animals, development continues long after sexual maturity has been reached. This continued development does not involve significant physical changes, however. Instead, changes take place inside the brain. The human brain in particular continues to gain information and make connections throughout one's life. In fact, often people's brains remain remarkably active, even when the body has begun to deteriorate.