From: Spike (spike66@comcast.net)
Date: Thu Jul 24 2003 - 22:49:50 MDT
From: Robert J. Bradbury
Subject: RE: Global Carbon Cycle
On Thu, 24 Jul 2003, Spike wrote:
> But it does leave some questions, such as why was
> it so late in the game before a archaeo-carbon
> scavenger evolved?
Hmmm, spike you need to expand on this somewhat.
All bacteria are carbon scavengers...
Yes of course bacteria scavenge carbon from the
soil, and for that matter all animal life scavenges
carbon by devouring plant matter. But what I meant
by archaeo-carbon is that carbon which falls into
the cycle of peat to bituminous coal to anthracite
coal to diamond.
I would not be surprised if some lifeforms figured
out how to break down peat, but I know of none which
can break down bituminous, or the much harder anthracite.
... -- but the restrictions
would seem to be on the lack of sunlight for
photosynthesis...
No. There are thingies living at the bottom
of the sea which make their living breaking
down the chemical soup that spews from deep ocean
vents.
...or the lack of oxygen to oxidize
the reduced hydrocarbons (for energy sources) --...
Nyet. There was plenty of life on this planet
before there was free molecular oxygen.
...iffy premises to base the conclusion of "decline" on...
OK, do let us continue with your objections,
out of order.
> Carbon based life on this planet was already
> very much in decline before the last few thousand
> years when human started digging coal, [snip]
...I would question the use of the term "decline".
Isn't life always adapting to the resource base
that is available? There is no strict requirement
that we are currently aware of that carbon must be
the basis for "life". While it seems preferable
proving it would seem to be very difficult...
Ah, thank you very much for bringing this up
Robert. At this point I shall boldly claim that
carbon is the only material that is crucial to
all lifeforms, not just those on this planet.
Its chemical properties are unique in so many
ways, there simply is nothing like carbon, nothing
at all, not even close, not silicon or anything.
When I look at my CRC handbook of chemistry and
physics, and study all the compounds therein,
over half of them contain carbon, and all the
really interesting stuff is organic, with carbon
making all those wonderful rings and chains of
all different sizes and shapes. Carbon really
is my very favorite atom.
So are there any other candidates to be the basis
of a life form for a carbon starved planet, such
as the moon? First off, forget those elements
which have only unstable isotopes, since chains
could not last long if an atom were to transmute.
Scratch 24 elements, 81 left with at least one
nonradioactive isotope.
Five of those are out, the noble gases, not nearly
reactive enough. 76.
Metallic bonds tend to form giant molecules but
do not lend themselves to the creation of small
molecules, which would be necessary for living
tissue. Indeed covalent and bonds are critical in
all life functions we know of. Fifty eight of
the 76 remainders are metals, so we are left with
18 stable nonmetals. When I am classifying these
as non-metals, recall that I mean any material
with any non-metallic properties.
Break em down into classes of those which can form
one bond, two, three and four:
1 bond: hydrogen, fluorine, chlorine, bromine, iodine.
2 bonds: oxygen, sulfur, selenium, tellurium
3 bonds: boron, nitrogen, phosphorus, arsenic, antimony
4 bonds: carbon, silicon, germanium, tin.
The single bond atoms are pretty boring in the range
of stuff they can do. They form diatomic molecules.
Thats all. None are suitable for a basis for life.
13 left.
Oxygen doesn't form chains. Out. The other two-bond
elements can form chains and rings, but if they do,
they use up both of their bonds, so nothing else
interesting can happen. So out with sulfur, selenium
and tellurium based lifeforms.
Then there were nine. Nitrogen tends to form super-stable
diatomic molecules and do little else. Out. Of the
eight remaining elements, only silicon and carbon are truly
abundant in nature, with phosphorus a very distant third.
Silicon has a number of interesting properties, but silicon-
silicon bonds will not form when there is sufficient oxygen
or hydrogen about, which explains why we see no silicon-
silicon bonds anywhere in nature. The others remaining
on the list are also much more fond of oxygen than they are
each other, all except good old carbon.
A carbon-oxygen bond is stronger than a carbon-carbon bond,
but only slightly, 4 percent. So carbon *will* burn, if
heated sufficiently. Good thing for us. So carbon will
as soon form chains and rings with itself. Carbon is not
just the best, it really is the only.
I am deeply indebted to Isaac Asimov for the ideas in
this analysis. He wrote an excellent essay on it over thirty
years ago in Fact and Science Fiction, November 1972.
Nowthen, we can talk about silicon lifeforms in the sense
of uploaded minds running on a silicon-based computer,
but these kinds of lifeforms do not evolve directly from
non-living matter, but would rely upon carbon based life
to get them started and perhaps to keep them going. If
those carbon-based lifeforms are starved out early by
the carbon being trapped in coal beds and oil fields
before those same lifeforms figure out how to release
it, then it is the end of the road for life on that
planet. My claim is there simply is no substitute
for carbon, no way to evolve around a shortage of it,
no compensating for its scarcity. Carbon is the basic
building block for everything that breathes or
photosynthesizes, everywhere in the universe.
To me this is the best explanation for the apparent
radio silence in the galaxy: that in the overwhelming
majority of cases, life on nearly every habitable planet
comes in and goes out in a few billion years, never
figuring out how to either metabolize or burn coal.
Tech-capable intelligence is evidently a wacky anomaly.
We made it. We are the few, the proud. Everyone wins
as a result.
spike
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