<P> Though currently only known on Earth, life need not be restricted to it, and many scientists believe in the existence of extraterrestrial life . Artificial life is a computer simulation or man - made reconstruction of any aspect of life, which is often used to examine systems related to natural life . Death is the permanent termination of all biological functions which sustain an organism, and as such, is the end of its life . Extinction is the process by which an entire group or taxon, normally a species, dies out . Fossils are the preserved remains or traces of organisms . </P> <P> It is a challenge for scientists and philosophers to define life . This is partially because life is a process, not a substance . Any definition must be general enough to both encompass all known life and any unknown life that may be different from life on Earth . </P> <P> Since there is no unequivocal definition of life, most current definitions in biology are descriptive . Life is considered a characteristic of something that exhibits all or most of the following traits: </P> <Ol> <Li> Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature </Li> <Li> Organization: being structurally composed of one or more cells--the basic units of life </Li> <Li> Metabolism: transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life . </Li> <Li> Growth: maintenance of a higher rate of anabolism than catabolism . A growing organism increases in size in all of its parts, rather than simply accumulating matter . </Li> <Li> Adaptation: the ability to change over time in response to the environment . This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors . </Li> <Li> Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms . A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis . </Li> <Li> Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms . </Li> </Ol>

What is the number of years of good health and function during an organism's life