Life Cycle

All living organisms have a life cycle beginning with the initiation of life and continuing until death. Scientists generally consider a life cycle to encompass the changes that an organism experiences from the beginning of a specific developmental stage until inception of the initial developmental stage in the next generation. The stages an organism will experience and the length of the life cycle varies widely across species. Species survive only when sufficient offspring complete the life cycle to continue the species. Such cycles vary from minutes to multiple years.

The time to complete a life cycle is generally correlated to the size of the organism. Bacteria complete their life cycle in 30 minutes by dividing to form another generation. Higher organisms take much longer. The giant sequoia, for example, produces its first fertile seeds after 60 years. Some plants and animals die after reproducing, with reproduction symbolizing the end of the life cycle. Other animals, notably humans, live for many years after their reproductive cycle is complete.

Most simple organisms complete their life cycle in one generation with an initial splitting in two of an adult, the growth of the new organisms, and upon reaching maturity, the division of each one into two new individuals. Higher animals can also have a single generation with the fusion of male and female sex cells to create the embryo and the growth of the offspring to reproductive maturity when they produce sex cells that create new offspring. Plant life cycles, however, are usually mul-tigenerational. A spore will germinate and grow into a gametophyte. At maturity, the gametophyte forms gametes. with fertilization, the gametes develop into sporophytes. Once reproductive maturity is reached, the sporophytes produce spores and a new life cycle begins. This process is frequently referred to as “alternation of generations” and occurs in some fungi and protists as well.

The time the young animals spend developing prior to being born (the gestation period) or hatched varies across species. Small marine animals frequently produce large numbers of tiny eggs that hatch at a very early developmental stage, while others produce fewer, larger eggs with the young exiting at a more advanced developmental stage. The smaller young will need longer to develop before becoming adults, while the more fully developed young move quickly into adulthood. This effect is common in higher organisms as well. Generally the more developed the young, the fewer produced in one reproductive cycle, as there is more chance of the young surviving. Larger animals take longer to bear their young, as the young must be ready for the hardships of life. Elephants have the longest pregnancy period of any mammal at 22 months. Smaller animals have shorter gestation [Page 786]periods. For example, the domestic cat has a pregnancy term of approximately 9 weeks.