is life and how can you tell if something is alive?
people have thought about the answers and most agree that there
are seven characteristics or features of living things that make
them different from non-living things. To be alive, all
biology textbooks often offer the following seven characteristics
to define life:
respected biologist Edward Mayr elaborated on this in his book,
The Growth of Biological Thought (1982).
He suggested the following eight points necessary for consideration
in recognizing living systems and entities. In simplified
form his points were as follows:
of Mayr's ideas seem a little subjective and are still rather narrow
in definition when thinking in universal terms. They relate
to life on Earth and that, we assume, has originated from a single
probably Earth-bound source or "event". Mayr's ideas lack
the vision of trying to put life into some sort of context that
takes account of the whole Universe rather than our parochial planet.
things are complex and can adapt in an organized way.
chemistry of life is made up from a unique set of large molecules.
living organism is individual, and there is much variability
things evolve in a non-predetermined way, but a record of their
evolution is maintained in their genetic code.
things are classified by their genetic evolution.
living things have evolved by natural selection.
processes of life cannot be easily predicted.
things are best recognized by qualitative rather than quantitative
the light of recent discoveries, the very different environmental
conditions found in space as compared with Earth, it is necessary
to broaden and change some of the definitions.
must be or have:
organized pattern: STRUCTURE
store and pass on instructions for same: REPRODUCTION
ability to convert energy: METABOLISM
ability to change structure: GROWTH
ability to change into new and different entities: EVOLUTION
to interact with surroundings: IRRITABILITY
certain robustness to changes in the environment: RESILIENCE
be alive an organism must possess all seven of these features.
have some of these abilities, and fire others, but neither could
be considered as living. Robots can be built to have yet more
of the features of life, but they cannot grow physically, evolve
or reproduce, so fail the life test on three counts.
Wrote The Book of Life? Picking Up Where D'Arcy Thompson Left Off
What Exactly Is It?- Discussion with Dr Stanley Miller
Sagan Criteria for Life Revisited
Seattle Times, Today's Top Stories: Microbe unlike life as we know it
1999 Satellite Events Enterprises Inc.