- Photochemical dissociation - breakup of water molecules by ultraviolet
- Produced O2 levels approx. 1-2% current levels
- At these levels O3 (Ozone) can form to shield Earth surface from UV
- Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 - produced by cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere.
- Chemical Weathering - through oxidation of surface materials (early consumer)
- Animal Respiration (much later)
- Burning of Fossil Fuels (much, much later)
Step 6: Addition of Oxygen to Soil and then Atmosphere
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.
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
- Iron (Fe) is 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 evolution.
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 conditions.
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 BY old, but do not form today.
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.
Step 7:Prokaryotes to Eukaryotes
At some point over a billion years ago, prokaryotes evolve to eukaryote cells. Unlike prokaryotes, eukaryotes have a nucleus that contains the genetic material. Eukaryotes have a much more complex cell structure and can rapidly divide via mitosis. It is not well understood how these cells evolved but it is clear that eukaryotes developed to protect DNA from a rich oxygen environment. Eventually these eukaryotes learn to metabolize oxygen.
Step 8: Ozone Layer Development
Now the important question
to ask is why did it take so long for life to emerge (over two billion years) from the ocean?
The ozone layer had to form. The ocean protected early life from getting disassociated from
UV light. Until the ozone layer developed, all life existed in the ocean.
After the ground was saturated with oxygen and photosynthesis increased, more and more oxygen was deposited into the Earth's atmosphere. Ozone is formed via the following reaction:
O3
In order for this reaction to occur both monotomic oxygen (O) and diatomic oxygen (O2) must be present, in the vicinity of another molecule (usually nitrogen) which acts as a catalyst. In general, O is only found at the top of the Earth's atmosphere as it is formed by the UV photo-dissociation of O2. O2, however, is generally found at the bottom of the Earth's atmosphere (as it's produced by surface processes). When sufficient O2 was finally built up, the reaction could occur at a region of the atmosphere where the abundances of the three ingredients necessary to form ozone was optimal. This position is 10-20 km above the surface of the Earth. This region is known as the stratosphere which is the layer above the troposphere where most of the active weather occurs.
Step 9: Life moves to land
Once the ozone layer forms, there is now a significant protective layer which can allow organisms to move from the sea to the land. Shortly after species migrated from the sea to the land, species diversification exploded as there were many new ecological niches to fill. Through tenacity and resilience, evolving life on the land withstood tremendous disasters such as earthquakes, volcanoes, major ice ages, occasional asteroid impacts, and slowly intelligence evolved. So now we can crawl out on the Land - adapt, evolve, develop.