Defining "life" is a very difficult task, and scientists don’t all agree on a common list of the characteristics of life. some of the other characteristics that you may discover and are often listed in textbooks include those bulleted below. many of these traits are not limited to living things. for example, fire uses energy, grows, and can reproduce, but it is not considered alive in part because it cannot evolve; its traits are necessary, but not sufficient, for life. nasa scientist bruce jakosky, in his book the search for life on other planets, provides a generally accepted definition of something being “alive” if it 1) utilizes energy from some source to drive chemical reactions, 2) is capable of reproduction, and 3) can undergo evolution. all living things (organisms) • use energy (metabolism) • maintain a stable internal environment (homeostasis) • detect and respond to select external stimuli • show growth and development; that is, specialization of cells or structures. even unicellular organisms show a tiny amount of growth, and single cells repair and use materials from the environment to replace internal structures as needed. • reproduce • have some form of nucleic acid as the hereditary molecule • show adaptation or change over time • are made of one or more cells are viruses alive? this debate rages among scientists. here we see the action of a virus as it enters a human cell. consider the passage detailing the characteristics of life and this illustration. cite evidence to defend the point of view that viruses are alive.