Moving Towards Life
How did chemisty
and oceans produce this?
There is still a problem:
- Not much is available for anerobic
processes to use.
- But hey, there is all this that we are floating in so let's do
So, chlorophyll evolves to more complex forms so that it can capture
and store enough UV energy to breaking the hydrogen-oxygen bond in
Now we have a mechanism to deliver Oxygen to the atmosphere via the
aerobic photosynthesis of blue-green algae. But, the oxygen does not
initially go into the earth's atmosphere:
Addition of O2 to the Atmosphere:
Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels
in the atmosphere? Revisit the oxygen cycle:
Photochemical dissociation - breakup of water molecules by
Produced O2 levels approx. 1-2% current levels
At these levels O3 (Ozone) can form to shield Earth surface
Photosynthesis - CO2 + H2O + sunlight = organic
compounds + O2 - produced by cyanobacteria, and eventually higher
plants - supplied the rest of O2 to atmosphere. Thus plant populations
Throughout the Archean there was little to no free oxygen in the atmosphere
(<1% of presence levels). What little was produced by cyanobacteria,
was probably consumed by the weathering process. Once rocks at the surface
were sufficiently oxidized, more oxygen could remain free in the atmosphere.
Chemical Weathering - through oxidation of surface materials (early
Animal Respiration (much later)
Burning of Fossil Fuels (much, much later)
During the Proterozoic the amount of free O2 in the
atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria,
which increase in abundance in the fossil record 2.3 Ga. Present levels
of O2 were probably not achieved until ~400 Ma.
Evidence from the Rock Record
Conclusion - amount of O2 in the atmosphere has increased with
time, but does not reach its current atmospheric level until 400-600 Million years ago.
Iron (Fe) i s extremely reactive with oxygen. If we look at the oxidation
state of Fe in the rock record, we can infer a great deal about atmospheric
Archean - Find occurrence of minerals that only form in non-oxidizing environments
in Archean sediments: Pyrite (Fools gold; FeS2), Uraninite (UO2).
These minerals are easily dissolved out of rocks under present atmospheric
Banded Iron Formation (BIF) - Deep water deposits in which
layers of iron-rich minerals alternate with iron-poor layers, primarily
chert. Iron minerals include iron oxide, iron carbonate, iron silicate,
iron sulfide. BIF's are a major source of iron ore, b/c they contain magnetite
(Fe3O4) which has a higher iron-to-oxygen ratio than
hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not form
Red beds (continental siliciclastic deposits) are never found
in rocks older than 2.3 B. y., but are common during Phanerozoic time.
Red beds are red because of the highly oxidized mineral hematite (Fe2O3),
that probably forms secondarily by oxidation of other Fe minerals that
have accumulated in the sediment.
Biological Evidence for lack of early oxygen
Chemical building blocks of life could not have formed in the presence
of atmospheric oxygen. Chemical reactions that yield amino acids are inhibited
by presence of very small amounts of oxygen.
Oxygen prevents growth of the most primitive living bacteria such as photosynthetic
bacteria, methane-producing bacteria and bacteria that derive energy from
fermentation. Conclustion - Since today's most primitive life forms are
anaerobic, the first forms of cellular life probably had similar metabolisms.
Today these anaerobic life forms are restricted to anoxic (low oxygen)
habitats such as swamps, ponds, and lagoons.
So we have to do all of this before we have oxygen in the atmosphere
as well as an ozone layer:
So now we can crawl out on the Land!