LIFE

the state of a material complex or individual characterized by the capacity to perform certain functional activities, including metabolism, growth, reproduction, and some form of responsiveness and adaptation. Life is further characterized by the presence of complex transformations of organic molecules and by the organization of such molecules into the successively larger units of protoplasm, cells, organs, and organisms. The profusion of life on Earth has been studied in great detail, and a number of general principles have been revealed. Foremost among them is the principle of evolution by natural selectionthe stepwise adaptation of organisms to their environment with increasing precision by small random mutations, or changes, in their hereditary materialwhich is the feature that distinguishes living from non-living matter. This article treats first the varieties of definitions of life and then covers, in some detail, the similarities and differences among organisms on Earth. It deals with the problem of the origin of life on Earth and concludes with a consideration of the possibility of life beyond the Earth. the state of a material complex or individual characterized by the capacity to perform certain functional activities, including metabolism, growth, reproduction, and some form of responsiveness and adaptation. Life is further characterized by the presence of complex transformations of organic molecules, and by the organization of such molecules into the successively larger units of protoplasm, cells, organs, and organisms. Metabolism involves a living system's continual exchange of some of its materials with its surroundings, principally in the process of building up or destroying its protoplasm. Growth involves an increase in the size of an organism, which in turn is usually due to its having a higher metabolic rate of synthesis of protoplasm than a rate of breakdown of that matter. Reproduction, perhaps the most striking hallmark of life, involves at its most basic level the division of one cell into two cells. This process is ultimately mediated by the hereditary information contained in large molecules, called genes, that are composed of nucleic acids. The reproduction of genes themselves passes the instructions for the various characteristics of an organism on to the next generation. Responsiveness and adaptation represent the ability of an organism to change in response to alterations in its environment. These two qualities are fundamental determinants of the process of natural selection, through which the hereditary characteristics of organisms evolve over long periods of time and many generations. Opinion concerning the essential nature of life has been historically divided between vitalist and mechanistic concepts. Vitalism asserts the existence of some vital force that separates living from nonliving matter and forms life's underlying principle. Mechanism, on the other hand, asserts that the wondrous phenomena of life are merely processes and transformations obeying elementary chemical and physical laws, and that a living system is ultimately reducible to its constituent atoms and molecules, and nothing more. Procaryotic bacteria and blue-green algae are the earliest known forms of life on Earth. Fossils of these organisms, discovered in the Fig Tree chert of the Transvaal, have been dated at 3.5 billion years old. This suggests that living systems appeared within a few hundred million years after the origin of their host planet, since the Earth itself is thought to be about 4.6 billion years old. Hypotheses of the origin of life range from religious concepts of life's creation out of inanimate matter by a divine agency to the more scientifically acceptable theory that life began on the early Earth by a series of progressive chemical reactions. According to this hypothesis, which has been supported by recent laboratory experimentation, inorganic compounds such as methane, ammonia, and water vapour that were plentiful in early times on the Earth formed into simple organic molecules with the aid of atmospheric electric discharges and ultraviolet radiation as energy sources. The question of how the resulting simple amino acids eventually became the much more highly organized and self-replicating systems known as life is much more difficult to answer, however, and is still far from being resolved. Additional reading Definitions of life and life on Earth are explored in Lawrence J. Henderson, The Fitness of the Environment: An Inquiry into the Biological Significance of the Properties of Matter (1913, reissued 1970); Philip Handler (ed.), Biology and the Future of Man (1970); Franklin M. Harold, The Vital Force: A Study of Bioenergetics (1986); Lynn Margulis and Dorion Sagan, Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors (1986, reprinted 1991), and What Is Life? (1995), valuable treatments of the microbial basis of life; Renato Dulbecco, The Design of Life (1987), a study of life at the molecular level; and a special issue of Scientific American (Oct. 1994), devoted to life in the universe.The origin of life is discussed in many books. Frances Crick, Life Itself: Its Origin and Nature (1981), posits an extraterrestrial origin for life on Earth. Robert Shapiro, Origins: A Skeptic's Guide to the Creation of Life on Earth (1986), critiques assorted theories on the origin of life. Andrew Scott, The Creation of Life: Past, Future, Alien (1986), provides a brief, readable account. An original cosmological argument for the origin of life is proposed in Eugene F. Mallove, The Quickening Universe: Cosmic Evolution and Human Destiny (1987). Sidney W. Fox, The Emergence of Life: Darwinian Evolution from the Inside (1988), outlines the author's research and philosophy. A.G. Cairns-Smith, Seven Clues to the Origin of Life: A Scientific Detective Story (1990), approaches the topic from the standpoint of thermodynamic laws and clues from the chemical operation of known cells. Also of interest are Freeman Dyson, Origins of Life (1985); George Greenstein, The Symbiotic Universe: Life and Mind in the Cosmos (1988); and Christian De Duve, A Blueprint for a Cell: The Nature and Origin of Life (1991), and Vital Dust: Life as a Cosmic Imperative (1995).Studies on extraterrestrial life include A.G.W. Cameron (ed.), Interstellar Communication (1963); I.S. Shklovskii and Carl Sagan, Intelligent Life in the Universe (1966; originally published in Russian, 1962); Elie A. Shneour and Eric A. Ottesen (compilers), Extraterrestrial Life: An Anthology and Bibliography (1966); Colin S. Pittendrigh, Wolf Vishniac, and J.P.T. Pearman (eds.), Biology and the Exploration of Mars (1966); Walter Sullivan, We Are Not Alone: The Continuing Search for Extraterrestrial Intelligence, rev. ed. (1993); Samuel Glasstone, The Book of Mars (1968); Carl Sagan and Jerome Agel, The Cosmic Connection (1973, reissued 1989); Edward Regis, Jr. (ed.), Extraterrestrials: Science and Alien Intelligence (1985), a collection of essays, some advocating and others dissenting from the concept of life on other planets; Emmanuel Davoust, The Cosmic Water Hole (1991; originally published in French, 1988), highlighting European research efforts; Frank Drake and Dava Sobel, Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence (1992), an account of Drake's research; Paul Davies, Are We Alone?: Philosophical Implications of the Discovery of Extraterrestrial Life (1995), essays that both support and refute the idea; and Ben Zuckerman and Michael H. Hart (eds.), ExtraterrestrialsWhere are They?, 2nd ed. (1995), a collection of essays, most of which are skeptical of the existence of extraterrestrial life. Histories of the theories of extraterrestrial life are found in Steven J. Dick, Plurality of Worlds: The Origins of the Extraterrestrial Life Debate from Democritus to Kant (1982); and Michael J. Crowe, The Extraterrestrial Life Debate, 17501900: The Idea of a Plurality of Worlds from Kant to Lowell (1986). Thomas R. McDonough, The Search for Extraterrestrial Intelligence: Listening for Life in the Cosmos (1987); and Joseph A. Angelo, Jr., The Extraterrestrial Encyclopedia: Our Search for Life in Outer Space, rev. and updated (1991), examine the technologies used in the quest for extraterrestrial life. Carl Sagan The Editors of the Encyclopdia Britannica