Re: Why do many hold that the early atmosphere was reducing without oxygen

   Well, I cannot be sure, this is a little out of my field of expertise,
though an area of great fascination.

    If you are looking for exact details on abiogenesis and potential
biogeochemical interactions I would contact the researchers that are doing
the primary investigations.  These can be found by a literature search on a
university database, or often via the web.  From what I have picked up on 
the
newer evidences:

    The first formation of the organic molecules may have been around
deep-sea vents.  This region is A) often a very anoxic environ, and B) is
where we find the most "primitive" or least derived form of archaebacteria
today.

   Stanley Miller:
"We really don't know what the Earth was like three or four billion years
ago. So there are all sorts of theories and speculations. The major
uncertainty concerns what the atmosphere was like. This is major area of
dispute."

We may not know if the atmosphere was reducing, but based on the other
planets in the solar system, it is presumed to have been reducing, at least
during pre-biotic earth.  I believe that is one other reason that some
researchers favor a deep-sea vent hypothesis for abiogenesis.  It helps
remove the atmosphere out of the equation momentarily.  The thing about
looking back in time that far, it becomes more and more difficult to test,
hence the strong debate amongst researchers.  Ahhh...the beauty of science!

Here are two great links that can lend much more insight than I ever could!
http://www.accessexcellence.org/WN/NM/miller.html
http://www.talkorigins.org/faqs/faq-abiogenesis.html

Hope this helps.

Admin note:
Since the argument that life evolved from a reducing atmosphere, which must have 
existed for life to evolve from is a bit circular, many scientists have turned to 
the geological data to determine, independently, the state of the probiotic 
atmosphere, and have come to the same conclusions:

http://www.sciencemag.org/cgi/content/abstract/288/5466/658
http://link.springer-ny.com/link/service/journals/00239/bibs/47n2p127.html
http://georef.cos.com/cgi-bin/getRec?un=1993-025370