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