Definition of life
Name: Chris E Lee
Does anyone know what is currently the most accepted idea
on the characteristics needed for any kind of life?
I assume since no one has answered your question in nearly a
month that no one has a sufficient answer for you, so I'll take
a crack at it. As far as I know, most of the accepted criteria
for determining whether something is "alive" can also, at
least individually, be applied to non-living things -- it's
kind of difficult to say. The main points I can think of at
the moment are:
MOTION -- does it seem to move under its own power? Does it move
with some discernible purpose? (Toward food, away from heat, etc)
REPRODUCTION -- does it have some way of making more of itself,
either through sexual reproduction or by budding or fissioning in
CONSUMPTION -- does it eat or drink? Does it take in nutrients
in one way or another in order to survive, grow, and eventually
GROWTH -- does the organism develop over time, increase in
complexity, until it reaches a mature stage?
STIMULUS RESPONSE -- does the organism respond to external
stimuli, i.e. has a nervous system of some sort to detect
To qualify as a living thing, an organism must in one way or
another meet each of those criteria. After all, crystals grow
in solution, and take on more material from the surrounding
solution in order to do so, but do not respond neurologically
if you poke them with a pin. Of course, you don't often see
mature Ponderosa pines strolling down Fifth Avenue either, so
the criteria are open to interpretation. Plants move through
growth, except in special cases like the Venus flytrap; most
plants follow the sun through a complex system which floods the
side of the plant shaded from the sun with water, swelling the
shaded side and causing the plant to lean toward the sun.
Even when all the criteria are met, it may be difficult to
determine if something is alive or not. Take a virus. It is
a strand of either DNA or RNA, and cannot move on its own
power. Yet when it attaches itself to a receptive host, it
inserts itself into the cell and forces the host to make more
of the virus, a clear reproductive plan. It utilizes the host's
cellular processes to do so, in a sense taking in "nutrients"
in order to survive and multiply. In some cases, exterior
conditions cause the virus to integrate itself into the host DNA,
in order to hide until conditions are better to reproduce, showing
a response to external stimuli. Is it alive?
Please read more about this at your local library.
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Update: June 2012