Free oxygen is fatal to abiogenesis scenarios such as those that Stanley
Miller experimented with. Evidence indicates that the early earth had
significant oxygen.
There is a variety of evidence that the early atmosphere did not have
significant oxygen (Turner 1981).
Banded iron formations are layers of hematite
(Fe2O3) and other iron oxides deposited in
the ocean 2.5 to 1.8 billion years ago. The conventional
interpretation is that oxygen was introduced into the atmosphere
for the first time in significant quantities beginning about 2.5
billion years ago when photosynthesis evolved. This caused the free
iron dissolved in the ocean water to oxidize and precipitate. Thus,
the banded iron formations mark the transition from an early earth
with little free oxygen and much dissolved iron in water to present
conditions with lots of free oxygen and little dissolved iron.
In rocks older than the banded iron formations, uranite and pyrite
exist as detrital grains, or sedimentary grains that were rolling
around in stream beds and beaches. These minerals are not stable
for long periods in the present high-oxygen conditions.
"Red beds," which are terrestrial sediments with lots of iron
oxides, need an oxygen atmosphere to form. They are not found in
rocks older than about 2.3 billion years, but they become
increasingly common afterward.
Sulfur isotope signatures of ancient sediments show that oxidative
weathering was very low 2.4 billion years ago (Farquhar et
al. 2000).
The dominant scientific view is that the early atmosphere had 0.1
percent oxygen or less (Copley 2001).
Free oxygen in the atmosphere today is mainly the result of
photosynthesis. Before photosynthetic plants and bacteria appeared, we
would expect little oxygen in the atmosphere for lack of a source.
The oldest fossils (over a billion years older than the transition to
an oxygen atmosphere) were bacteria; we do not find fossils of fish,
clams, or other organisms that need oxygen in the oldest sediments.
