Growth, Development, and Aging

From the time the zygote is formed, the organism begins to grow. First, it increases in cell number and then in cell size. Once enough cells have been formed, they differentiate into the three different germ layers (ectoderm, endoderm, and mesoderm). Each of these layers will then give rise to the tissues, organs, and organ systems needed for the organism to survive to adulthood. Growth and development does not just happen during this early time—it continues throughout the organism's life span through a process called aging. Aging is the gradual changes in development and behavior that occur during an organism's life. It involves all of the physical, behavioral, emotional, and developmental changes that occur over an organism's life.

Most animals go through various stages as they age and develop, and the amount of time it takes to move from one stage to the next varies. For example, in humans the embryo becomes a fetus at the end of the first 90 days. At that point, the fetus's heart is beating and most of the organs have begun to form. The fetus continues to grow and increases in size, and the organs develop over the remainder of the gestation period (about 240 days in total). The final stages of development in humans include a large amount of growth and a maturation of the organs and systems. Growth continues after the baby is born and through adulthood.

Development over the life of an animal varies greatly by species. Members of the phylum Arthropoda (including insects and shellfish) show characteristic changes in their development. However, unlike humans, most of these developmental changes can be seen once the egg hatches. In mammals, birds, and reptiles, most of the formation and differentiation of cells and tissues happens while the embryo is still inside the egg or the mother. However, many species, such as chickens, show visible changes as they age (e.g., the fluffy chick molts its first feathers and develops new feathers as it becomes an adult). As animals age, their bodies go through gradual changes and become less resilient. For example, in humans, wounds are slower to heal, skin becomes wrinkled, and hair turns gray. There are several hypotheses as to why aging happens. The age program theory suggests that living things are genetically programmed to die. There is a specific biological timeline that living things follow with a beginning, middle, and end, and this timeline is predestined. The limit theory of aging suggests body cells can only divide about 50 times after birth. Once they stop dividing, the body's organs start to fail, as the organ's cells are no longer replaced. A corollary to this is the telomere shortening theory. Telomeres are the protective end caps of the chromosomes, which get shorter with each cell division. When they become too short, additional divisions cannot take place. The immunological theory of aging suggests there is a predetermined decline in immune functioning in the body, and when a certain age is reached (such as the elderly in humans) the body loses its ability to fight infection. Individuals who cannot fight off infection eventually die.

Whatever the cause, all living things will eventually succumb to old age. Different species have different life spans, which can range from a few hours in mayflies to over 200 years in the Galapagos tortoise. While aging may be predetermined, it is important to remember there are many environmental factors that can influence how old an organism lives to be. For example, baby sea turtles are left on their own to find their way from the nest in the sand to the water after hatching. The trip for these young is often met with much danger, as predatory birds wait for the baby turtles to emerge. Baby horses born in the wild must walk within an hour after birth in order to keep up with the herd. If they do not, they may be left behind and will die. Additionally, infectious disease, injury, and other environmental factors may lead to premature death.