Definition of Life

James LYONSW at UCONNVM.bitnet
Thu Feb 3 08:55:13 EST 1994


Douglas Adams    <adamsd.cerf.net> writes:

>I have to give a talk next week, for which it would be useful for me to
>know a couple of things:

>1  What is a good definition of Life? What are the criteria?
Complexity?
>Self-organisation? Reproduction? Competitive reproduction/Evolution?
When
>people argue over the Gaia hypothesis, (i.e. can the Earth be
considered a
>living organism) what are the criteria that apply?
>  What are some good examples of the most extreme forms it can take?
Like
>what is the simplest form, the most complex, the biggest, smallest,
most
>unlikely (by which I mean something that we would not expect to think
of as
>being alive). Colonies? Tree stands? (I think I read somewhere that a
>gigantic stand of trees has recently been discovered, but I don't have
>details)

>omitted...

Neat topic for this forum.  As an ecologist, I have often discussed this
topic with my students, with some rather useful insights as a result.

We can typically think of an object as "alive" or "with life" if it is
capable of ALL of the following AT SOME POINT IN ITS EXISTENCE:
- Growth
- Metabolism
- Reproduction

Two excpetions to these criteria might be viruses and viroids, which are
not capable or grwoth or metabolism at all - in fact, some biologists
prefer to think of them as non-living molcules of RNA wrapped up in
a protein coat.  Just about any intro to bio text (I think of Keeton)
will
have a discussion on this.

So if we consider the Gaia hypothesis, (i.e., that the biosphere
represents
a living organism of which all of life is but a part, does it meet the
criteria?

- Growth : in its history, the biosphere has gone from nonexistence
(before
life) to existence (after life - gee, that sounds funny), so one may
argue that the biosphere has grown.

- Metabolism : does the biosphere take packaged energy form its
environment
and break it down (catabolism) and the build other things from the
products
(anabolism)?  Remember that energy flow between compartments of the
biosphere
(ie, from individual-> individual; population -> population; community
->
community) is not catabolism. The energy which comes to the biosphere
from
its environment is (mosly) radiant energy - hardly packaged - so I
conclude no observation of metabolism is possible.
- Reproduction - we have just one planet in our solar system with life
(so far as we know).  However, some far-fetched ideas about
"terreforming"
Mars could eventualy bring about the spread of elements of this
biosphere
to a new one; such an occurence certainly might be observed to be
reproduction.  If this were to occur, we (humans) would be the gemmules
posited by LaMark for his theory of the inheritance of aquired
characteristics;
we would not necessarily be gametes.  Even so, the idea is so
far-fetched that
we can reject the Gaia hypothesis on the ground sof this criteria as
well.

The gigantic stand of tree (I think) is a stand of Populus which is
actually
a stand of genetically identical sprouts (ramets) of one individual
(genet).
That humongous fungus of recent acclaim might be worth mention, as well.


Perhaps other netters with more specifics on these two cases might
post a reply.  Good luck with your talk - have fun!

James



More information about the Bioforum mailing list