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Re: Biological Origin of Earliest Fossils Substantiated



On Fri, 15 Mar 2002 20:17:15  
 David Marjanovic wrote:
>> which are dated to 3.5
>> Ga; these stromatolites are one of the oldest, if not the oldest, direct
>> evidences of life on Earth.  The oldest indirect evidence for life is the
>> banded iron deposits in Isua, Greenland, dated to 3.8 Ga.  I believe
>there's
>> also bundant Carbon 12 there, as well.
>
>Somewhere in Australia there's a place that's enriched in 12C (and, if I've
>correctly understood the paleobotany professor, 34S) and 3.85 Ga old (the
>number looks like a pun was intended). This is currently considered the
>oldest evidence for life.

As far as I know, the oldest specimens that have been rigorously shown to be 
fossils are 3.5 billion years old (plus or minus about five million years), and 
are from the Apex Chert of Australia.  The evidence of 3.85 billion year old 
life (if I remember correctly) comes from Greenland and only consists of 
indirect isotope chemistry (no actual fossils).  C12: C13 ratios played a major 
role in this.  Because photosynthetic organisms (bacteria and plants) rely on 
enzymes that "capture" carbon, the lighter C12 atom (as part of the lighter CO2 
molecule) is trapped easier.  This essentially means that limestone that 
supported a rich biomass is depleted in C12.  The normal ratio of C12:C13 is 
very high (I can't recall the exact number...something like 99:1).  Of course, 
as more C12 is used by photosynthetic organisms, that ratio decreases.  I've 
discussed this onlist before.  

In terms of Sulfur (like the S34 atom David mentioned), different analyses 
using S32:S34 ratios can ascertain what type of life was present when a certain 
sulfite rock layer was deposited.  Chemoautotrophs (sulfate reducing bacteria) 
make energy by adding hydrogen to sulfur atoms (usually cut from sulfate 
dissolved in sea water).  This liberates the gas hydrogen sulfide, which, when 
given off by living chemoautotrophs, is enriched in the lighter S32.  On the 
other hand, nonbiologic sulfur evaporates, such as gypsum, contain a higher 
amount of S34, the heavier isotope.  

Anyway, that's some basic geochemistry.  A bit off topic, perhaps, but still 
important.  Similar analyses have been used to determine biomass and ecological 
data for dinosaurs.  

And, since the Mars concretions/contaminants/"fossils" were mentioned, I have 
to say that I hope the new technology is used to study them.  Additional 
experiments cannot hurt.  

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
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