Life is the most complex state of organization of matter known in the Universe. Living systems achieve this state of organization by using energy that they extract from their environment. Many physical systems do this, but a living system is distinguished in that it uses the energy to convert the material taken from its surroundings into a form characteristic of itself. This process is known as METABOLISM. It may be argued that metabolism is a sufficient condition for life for any individual organism. But in order to have a LIFE-FROM, that is a collection of similar living organisms, the individuals must also possess the ability to grow and reproduce. According to this definition of life, a virus is not alive (it is not a metabolic system), nor can a collection of crystals be considered alive. A more precise definition of life is not really possible. According to the most widely-held view, life did not appear spontaneously but there was a transition from the non-living to the living. Just where in this metamorphosis life began can only be a matter of semantics.

Although speculation on the nature and origin of life has a long history in theology and philosophy, the basic scientific viewpoint which has set the tone for subsequent debates was put forward in the 1920s by A.I.Oparin in the Soviet Union, and J.B.S.Haldane in Britain. Haldane enumerated the broad categories into which theories for the origin of life would have to fall. Firstly, one might assert that life had no origin, but is an integral part of the Universe that has been propagated from star to star by some means. Such a point of view is quite definitely ruled out by current ideas in cosmology, where all the evidence points to the Universe having originated some tens of billions of years in our past. Secondly, there are theories which assert that life is the consequence of some super- natural event. The very nature of such hypotheses puts them beyond scientific investigation. Finally, we may suppose that life evolved as a result of chemical processes. There are two theories that fall into this third category. There is the theory that although life on Earth is a spontaneous result of achieving a certain level of chemical complexity, the chain of events is so long, that the origin of life is very improbable. On this view, the Earth may be the only I abode of life in the entire Universe since elsewhere the conditions were never all just right. The other theory simply asserts that life” will appear automatically once a certain level of chemical complexity is reached. This is the view towards which a majority of scientists today would lean. It is a good scientific hypothesis since it is possible to perform laboratory experiments which may shed light on the origin of life. A consequence of this hypothesis is that life will appear wherever terrestrial conditions are present. Accordingly, a clear understanding of the development of life on Earth is of direct relevance to the question of life elsewhere, and the strategy adopted from here on will be to find the answer to what happened on Earth. We draw on astronomical, geological, biological and chemical evidence. We argue that the first astronomical, geological and chemical conditions that existed on the primitive Earth were also likely to have been realized elsewhere. The question of the origin of life elsewhere in the Universe will then be reduced to a speculation concerning the probability of making a transition from the non-living world of complex organic systems to the living.

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