Before we are able to discover life elsewhere in the universe, we must first know what we are looking for. There is no universally accepted definition of life, with ideas on the nature of life changing over the years. Different biologists favour different definitions. A physiological definition sees life as fulfilling a number of functions, such as feeding, growing, metabolising, excreting, reproducing, moving and responding to stimuli. We have already seen how the phenomenon of cryptobiosis challenges this physiological definition of life (see Chapter 3).
A metabolic definition considers an organism to have a distinct boundary (such as a membrane) which separates it from its non-living environment, but there is an exchange of materials with its surroundings, enabling the organism to maintain its structure by the consumption of energy. Again, cryptobiosis challenges this definition and it could also apply to a clearly non-living entity such as a candle flame.
A thermodynamic definition of life sees organisms as being in contradiction of the second law of thermodynamics. This states that the amount of disorder (entropy) is always increasing, with the universe (as a whole) moving towards a state of disorder and randomness. Organisms, at first sight, appear to defy this by maintaining order in the face of increasing disorder. Living things, however, are not closed systems. They maintain their structure by consuming energy from their surroundings - by absorbing sunlight, for example. There is thus a net increase in entropy (a decrease in order) as a result of the burning of the sun's fuel. Nevertheless, by absorbing energy, organisms maintain their structure, ultimately at the expense of an increase in the entropy of their surroundings.
Many biologists favour a definition of life which includes an evolutionary and a genetic or informational component. Living things include the instructions for their structure and functioning within genes which consist of nucleic acids (DNA and RNA) that can reproduce this information and pass it on to the organism's offspring. The replication of genes is, however, not always free of errors. These errors (mutations) are often fatal, but, occasionally, produce new structures or new metabolic processes which are advantageous to the organism. An organism that possesses such a favourable mutation is more likely to survive and reproduce than one which does not. Organisms are thus likely to accumulate favourable mutations and to become better adapted to survive and reproduce in their environment (or to become able to exploit new ones). This is, of course, the process of evolution and Darwinian evolution is seen by many as being central to our understanding of life. A recent definition of life which is frequently quoted was proposed by Gerald Joyce from the Scripps Research Institute in San Diego: 'Life is a self-sustaining chemical system capable of undergoing Darwinian evolution'.
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