"The first cells must have been anaerobic because there was no oxygen in
the atmosphere. In addition, they were probably thermophilic
("heat-loving") and fermentative. Rocks as old as 3.5 billion years old
have yielded prokaryotic fossils. Specifically, some rocks from Australia
called the Warrawoona series give evidence of bacterial communities
organized into structures called stromatolites. Fossils like these have
subsequently been found all over the world. These mats of bacteria still
form today in a few locales (for example, Shark Bay Australia). Bacteria
are the only life forms found in the rocks for a long, long time
--eukaryotes (protists) appear about 1.5 billion years ago and fungi-like
things appear about 900 million years ago (0.9 billion years ago). "
This is from:
http://www.talkorigins.org/faqs/faq-intro-to-biology.html
which I strongly recommend everyone on this list read (if you haven't already).
Susan
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Life is short, but it's also very wide.
http://www.telepath.com/susanb/