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Name: jconner
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

What is the criteria for living versus non-living? Is life functions (growth, reproduction, etc.) sufficient or must structure also (cells) be included?

Yes, textbooks are vague as no satisfactory answer has been found. This is probably because we have not tried very hard as life on earth is easy to recognize. As we begin to study outer space, we may need a better definition so we can recognize the very different forms of life that may exist elsewhere. All life forms on earth have deoxyribonucleic acids or ribonucleic acids. These molecules store then information necessary to build the next generation, so as to make more nucleic acid. One could ask whether organisms are merely the means to making more nucleic acids, in the form of genes. Many people have included the criterion of cellular structure in the definition of life but I have never known why. All life depends on other life forms for their reproduction, but viruses are particularly needy. Philosophers have taken some stabs at more general definitions of life, unlike most biologists who are usually satisfied to be able to recognize earth life when they see it. One definition I like is that living things are those that require energy to reproduce; and this reproduction is accomplished with some errors (ie. mutations). These errors distinguish life from inanimate crystals that can grow on themselves. This answer addresses how to define a living species, but ignore the questions of how to determine if any one organism is alive at any given time. That is a different question and one of degree.

Jim Murray

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